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INTERREG III A, FLOOD1 The role of groundwater in flooding events

Characterization by Magnetic Resonance Sounding of the East-Isley and Beche Park Wood experimental

sites, Pang Catchment, West Berkshire, England

Final Report

BRGM/RP-55257-FR May 2007

INTERREG III A, FLOOD1 The role of groundwater in flooding events

Characterization by Magnetic Resonance Sounding of the East-Isley and Beche Park Wood experimental sites, Pang

Catchment, West Berkshire, England

Final Report

BRGM/RP-55257-FR May 2007

Girard J-F., Baltassat J-M.

Vérificateur :

Original signé par :

Nom : M.L. Noyer

Date :

Signature :

Approbateur :

Original signé par :

Nom : H. Modaressi

Date :

Signature :

Le système de management de la qualité du BRGM est certifié AFAQ ISO 9001:2000.

Keywords : Geophysics, Magnetic Resonance Sounding, chalk, unsaturated zone, Pang catchment, West Berkshire, England.

In bibliography, this report should be cited as follows: Girard J.F., Baltassat J.M. (2007) – INTERREG III A, FLOOD1, The role of groundwater in flooding event : Characterization by Magnetic Resonance Sounding of East-Isley and Beche Park Wood experimental sites, Pang Catchment, West Berkshire, England BRGM Report RP-55257-FR, 79 p., 20 fig., 5 tabl., 8 ann.. © BRGM, 2007. No part of this document may be reproduced without the prior permission of BRGM.

FLOOD1 project – MRS characterisation of experimental sites in England

BRGM/RP-55257-FR – Final report -3 -

Synopsis

Within the framework of the FLOOD1 project two sites were investigated from 2nd to 13th of May 2006 using Magnetic Resonance Sounding in the Pang Catchment, West Berkshire, England. These experimental sites, East Isley and Beche Park Wood, had been instrumented in the framework, respectively of the FLOOD1 and LOCAR projects and benefit from in situ suction and water content measurements which are useful for comparison and calibration of MRS measurements.

The proposed methodology is based on the comparison of MRS parameters measured from the surface and hydraulic characteristics. In order to obtain the necessary wide range of values to establish a significant correlation, comparison should be performed at contrasted unsaturated conditions. For reliable comparisons, experimental sites instrumented for monitoring of water content and suction are required.

From 2nd to 13th of May 2006, a field survey was performed at the two English sites. MRS parameters were measured using the commercial NumisPLUS equipment and the new Numisproto, currently under development for better recording of short relaxation time signals like the MRS signal in chalk. EM noise conditions are relatively good on these sites and make them suitable for methodological investigations and for determining MRS parameters (water content, W and relaxation time constants T1 and T2*) with acceptable uncertainty.

The main result concerning non-destructive monitoring of the chalk vadoze zone variation is that the relaxation times, T1 and T2* measured at Beche Park Wood are globally lower than those measured at East Isley. A parallel is made between these contrasted MRS observations and the higher suction values measured at Beche Park Wood as compared to those at East Isley.

MRS water content measurements at both sites are similar. Water content calculated from the Numisproto data is higher than when obtained from the Numisplus. This is consistent with the fact that the early time response is better recorded, and the initial amplitude (that is directly linked with water content) is then more reliably evaluated.

None of the experimental results negate the possible and expected relationship between MRS relaxation times and suction. Despite very contrasted pressure conditions between EI and BW, the relationship is not clearly demonstrated as some observations remain inconsistent and modelling shows that, in conditions of unsaturated chalk, the distinction of varying relaxation time is at the sensitivity limit of the method.

The results presented in this report were obtained in the framework of the INTERREG III A Flood1 project in partnership with the British Geological Survey and the University of Brighton. The BRGM financial partners are the EU through the ERDF funds, the MEDD (Ministère de l’Ecologie et du Développement Durable) through the DIREN Picardie and two local end-users the Conseil Régional de Picardie and the Conseil Général de la Somme.

Acknowledgements are presented to Brian Adams (Flood1 project manager for BGS) who kindly reviewed the draft and helped to improve the clarity of this document.



Index

1. Introduction....................................................................................................................9

1.1. GENERAL VIEW ............................................................................................... 9

1.2. HYDROGEOLOGICAL SETTING.................................................................... 11

1.3. OBJECTIVES .................................................................................................. 14

2. Work carried out ..........................................................................................................15

2.1. PROPOSED METHODOLOGY ....................................................................... 15

2.2. MEASUREMENTS CARRIED OUT................................................................. 16

2.3. WORKFLOW OF ACQUISITION AND PROCESSING .................................... 18

3. MRS Methodology in chalk .........................................................................................19

3.1. DISCUSSION ON SPECIFIC CHARACTERISTICS OF MRS MEASUREMENTS IN CHALK ............................................................................................................ 19

3.2. REPROCESSING RAW FID1 AND FID2: T2* ESTIMATION ON FID2 AND T1 FIT. 23

3.3. ARE WE ABLE TO DISCRIMINATE T1 RELAXATION TIMES WITHIN THE RANGE 40-100 MS ? ......................................................................................................... 23

4. Results and interpretation of field survey .................................................................25

4.1. COMPARISON OF MRS RESPONSE AT THE EAST ISLEY (EI) AND BECHE PARK WOOD (BW) SITES ........................................................................................ 25

4.2. METHODOLOGICAL PROBLEMS ENCOUNTERED...................................... 31

4.3. SYNTHESIS OF OBSERVATIONS ON THE ENGLISH SITES ....................... 32



4.4. COMPARISON WITH THE FRENCH EXPERIMENTAL SITE OF WARLOY-BAILLON (WB) 33

4.5. INTERPRETATIONS....................................................................................... 33

5. Conclusion ...................................................................................................................37

6. References ...................................................................................................................39



List of figures

Figure 1 – Location of experimental sites.................................................................................... 10

Figure 2 – Lithostratigraphic level of the chalk at the different studied sites............................... 11

Figure 3 – Location of MRS investigations at Beche Park Wood. Different loops were used as transmitter and/or receiver : 75 m square side (red), 56 m square side (grey), 37 m square side (blue), 8-square 37.5 m side (pink)and 8-square 19 m side (red) ............................................... 12

Figure 4 – Location of MRS investigations at East-Isley. Different loops were used as transmitter and/or receiver : 8-square 37.5 m side (red)and 8-square 19 m side (rose)............................... 12

Figure 5 –Hydrogeological conditions on the English and French Warloy-Baillon experimental sites...................................................................................................................................................... 14

Figure 6 – Conventional MRS sequence of measurement.......................................................... 15

Figure 7 – Standard result sheet of MRS measurements. .......................................................... 18

Figure 8 – Schematic MRS sequence of measurement. After the pulse a dead-time is applied before recording the signal: the 12 ms (proto) and 40 ms (plus) dead-times do not have the same impact when the MRS signal is long or short (as in chalk) ..................................................................... 19

Figure 9 – Frequency response of a bandpass filter centered on 2kHz with a +/-80 Hz bandpass (above). Impulse response of the same bandpass filter in the time domain (axis in second, below)...................................................................................................................................................... 21

Figure 10 – Effect of bandpass filter on MRS data : 100ms (blue) and 10 ms (red) decay time signals; Above : raw synthetic data; Below : filtered data. The longest signal is less affected. .. 22

Figure 11 – Modelling FID2 amplitudes when varying delays and relaxation times corresponding to those observed in chalk FID2(∆t)=100 *(1-exp(∆t/T1) ................................................................ 24

Figure 12 – Initial amplitude (left) and apparent T2* (right) fit on FID1 (blue) and FID2 (green) curves (full record 0-200 ms). Same configuration of Q sounding at East Isley (above) and Beche Park Wood (below)............................................................................................................................... 26

Figure 13 – East Isley site : EI0506E Q sounding data and inversion results (eight square 37.5m, Numis

plus). .................................................................................................................................... 27

Figure 14 – Beche Park Wood site : BW0506H Q sounding data and inversion results (eight square 37.5m, Numis

plus). ........................................................................................................................ 27

Figure 15 – East Isley site : EI0506A Q sounding data and inversion results (TX eight shape 37.5m RX sq19mx2, Numis

proto). ............................................................................................................ 28

Figure 16 – East Isley site, EI0506A Q sounding: Initial amplitude (left) and apparent T2* (right) fit on FID1 (blue) and FID2 (green) curves (calculated on full record 0-200 ms). T2* for low pulses are not significant because of low S/N..................................................................................................... 28

Figure 17 – Beche Park Wood site : BW0506A Q sounding data and results (TX/RX : square 75m; Numis

plus) ..................................................................................................................................... 29

Figure 18 – Beche Park Wood site, BW0506A Q sounding : Initial amplitude (left) and apparent T2* (right) fit on FID1 (blue) and FID2 (green) curves (full record 0-200 ms).................................... 29

Figure 19 – Beche Park Wood site : BW0506D Q sounding data and inversion results (TX: square 75m, RX square 37m, Numis

proto). ............................................................................................... 30



Figure 20 – Beche Park Wood site, BW0506D : Initial amplitude (left) and apparent T2* (right) fit on FID1 (blue) and FID2 (green) curves (full record 0-200 ms). ...................................................... 30

List of tables

Table 1 : Noise level observed with NUMISPlus and a square8-shape antenna of 37.5 m side.16

Table 2- Soundings are listed with their main characteristics in chronological order; pulse = pulse duration in ms for P1/P2, full delay ∆t = deadtime + FID1 length + delay D1 + pulse2 in ms, Equipment = NUMIS

plus or NUMIS

proto....................................................................................... 17

Table 3 : Comparison of water content estimations. ................................................................... 31

Table 4 – Summary of observations on English exprimental sites.............................................. 34

Table 5 – Comparison with the french experimental site of Warloy-Baillon. ............................... 35

List of appendices

Appendix 1 Plan of instrumentation at the Beche Park Wood site ..................................... 41

Appendix 2 Variable Q sounding at East Isley ...................................................................... 43

Appendix 3 Variable Q sounding at Beche Park Wood ........................................................ 47

Appendix 4 Variable delay at East Isley ................................................................................. 51

Appendix 5 Variable delay at Beche Park Wood ................................................................... 55

Appendix 6 Re-processing of Q sounding at East Isley : Mean FID1 and FID2 amplitude integrated over the full record and for late times................................................................... 57

Appendix 7 Re-processing of Q sounding at Beche Park Wood : Mean FID1 and FID2 amplitude integrated over the full record and for late times................................................. 61

Appendix 8 Artefact tests – frequency spectra ..................................................................... 65



1. Introduction

1.1. GENERAL VIEW

Within the framework of the FLOOD1 project, the capability of geophysical methods to characterize the hydric parameters of the chalk unsaturated zone has been tested. The objective was the development of an innovative non-invasive methodology for monitoring the chalk unsaturated zone from the surface (thus lowering the cost of future monitoring and limiting the impact on the environment).

The interest of surface geophysical methods lies in the fact they are non destructive methods since they don’t require the drilling of boreholes to be operated. Our effort is mainly focused on Magnetic Resonance Sounding (MRS) which is particularly promising because its response is directly linked to the electromagnetic properties of the water molecules.

The proposed methodology is based on the comparison of MRS parameters measured from the surface and hydraulic characteristics such as negative pressure (suction) and water content obtained from other methods (in situ boreholes measurement or laboratory measurements on samples). These comparisons, if significant, could lead to the establishment of empirical relationship linking MRS and hydraulic parameters in the Chalk vadose zone. If the tests were successful MRS could be a non-destructive tool to evaluate the hydraulic parameters in the Chalk vadose zone directly from the surface. In order to obtain the necessary wide range of values to establish a significant correlation, comparison at contrasted unsaturated conditions is required. For this purpose, MRS investigations were planned in France at the Warloy-Baillon experimental site, at different seasons when different unsaturated conditions are expected. In parallel, MRS measurements were also planned in different English experimental sites where contrasted hydric conditions are expected from site to site and in comparison with Warloy-Baillon.

For reliable comparisons, experimental sites instrumented for monitoring water content and suction were required. As the water content is not likely to vary in a significant manner relatively to the MRS sensitivity across the whole unsaturated profile, sites equipped with tensiometers which measure the suction through the whole profile are preferred. The East Isley site (developed in the framework of the FLOOD1 project) and the Beche Park Wood site (developed in the framework of the LOCAR project) were chosen for their extensive instrumentation and their low electromagnetic noise conditions in order to provide favourable conditions for MRS experimentation.

A third site was initially envisaged near Stonehenge site which is not specifically equipped but where the water table is monitored through numerous boreholes along a straight profile. The interest here was to study the variation of the unsaturated zone along a profile where the water table varies significantly and where its level is reliably monitored. Our objective was to demonstrate that along such a profile, MRS parameters in the unsaturated zone vary significantly in a manner which is consistent with the expected suction variation. Since the access to the Stonehenge site proved to be difficult to obtain and to maintain for multiple experiments, this site was abandoned.



MRS parameters are currently measured using NumisPLUS equipment which has limited efficiency for characterising a rapidly decaying signal such as encountered in the Chalk vadoze zone. Because of an instrumental dead time, there is a lack of observation at the beginning of the measuring window, and short relaxation time cannot be correctly measured and analysed. With the collaboration of IRD (Institute for Research and Development) and Iris Instrument (Manufacturer of MRS instruments) a new piece of equipment, with a shorter dead time is currently under development for a better recording of short relaxation time signals with a resultant for a better determination of water content. Both instruments (the NumisPLUS and a prototype Numisproto) were used during the present survey.

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NewburyNewbury

AbingdonAbingdon

Henley-on-ThamesHenley-on-Thames

WokinghamWokingham

East Isley

Beche Park

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1°15'0"O

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51°20'0"N 51°20'0"N

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51°35'0"N 51°35'0"N

51°40'0"N 51°40'0"N

51°45'0"N 51°45'0"N

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Wallingford

Figure 1 – Location of experimental sites.



1.2. HYDROGEOLOGICAL SETTING

The East Isley and Beche Park Wood experimental sites are south west of Wallingford (where the BGS and CEH offices are located) and south of Oxford in the county of Berkshire (Figure 1). They are both located within the Pang hydrological catchment, part of the Thames basin developed in the Chalk aquifer.

The geology at both sites is composed of chalk, which at the Beche Park Wood site occurs below 2 m of clay-with-flints. The lithostratigraphy is however different at each English site and different from the French Warloy-Baillon site as shown in Figure 2.

50 m

0 m

depth

40 m

0 m

depth

100 m

0 m

depth

Warloy-Baillon

EastIsley

Beche ParkWood

Seafordchalk

Leweschalk

New Pit chalk

Figure 2 – Lithostratigraphic level of the chalk at the different studied sites.

At each site, MRS measurements were performed as close as possible to the boreholes (Figure 3). At Beche Park Wood, the MRS loop was set up around the main borehole (PL28A) and around the main instrumentation that was set up within the framework of the LOCAR project. Instrumentation, in Beche Park Wood, follows a scheme detailed in annex 1 and is composed of:

- ETEN equitensiometers at depths from 2 to 10 m,

- Pressure transducer tensiometers

- 10 m tubes (4 m to 9 m deep) for neutron probe measurements which are regularly performed every two weeks since 2001.



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'''

99999999999999999999999990 200 m0 200 m0 200 m0 200 m0 200 m0 200 m0 200 m0 200 m0 200 m

PL28APL28APL28APL28APL28APL28APL28APL28APL28A

Figure 3 – Location of MRS investigations at Beche Park Wood. Different loops were used as transmitter and/or receiver : 75 m square side (red), 56 m square side (grey), 37 m square side (blue), 8-square 37.5 m

side (pink)and 8-square 19 m side (red)

†††††††††

((((((((((

((((((((((

99999999999999999999999990 250 m0 250 m0 250 m0 250 m0 250 m0 250 m0 250 m0 250 m0 250 m

BH1BH1BH1BH1BH1BH1BH1BH1BH1

Figure 4 – Location of MRS investigations at East-Isley. Different loops were used as transmitter and/or receiver : 8-square 37.5 m side (red)and 8-square 19 m side (rose).



PL28 is also equipped with jacking tensiometers down to 70 meters but measurements are not available because sensors had been out of the operation range for this kind of tensiometer (700 Hpa) and this type of tensiometer can not be refilled on-site.

At the East Isley site, it was not possible to plan MRS testing at the exact borehole location because of interference from an electric power-line and we had to move 300 m east of BH1 (Figure 4). At this site the main BH1 borehole has been equipped, with jacking tensiometers at depths below surface between12 and 25 meters within the framework of FLOOD1 project.

Hydrogeological conditions at the English sites are compared to those at Warloy-Baillon in Figure 5: suction values correspond to winter 2006 measurements (higher saturation in late April at East-Isley and Warloy-Baillon), neutron measurement at Beche Park Wood in April 2006 and laboratory moisture content at Warloy-Baillon in April and November 2005. Water-levels are from measurements from late April 2006.

Conditions at Warloy-Baillon and East Isley are very similar with a water level around 25 m depth and matrix potential (suction) more and more negative from the water level towards the ground surface. Water content is also similar at both sites with 30 to 40 % almost constant along the whole vertical profile.

At the Beche Park Wood site, matrix potential conditions, where measurements are available (in the first ten meters) contrast strongly to those from the other sites while the water content is a little bit smaller but of the same order of magnitude (mean value of about 35% from 2 to 8 meters depth).



0 10 20 30 40 50Neutron water content (%)

100

90

80

70

60

50

40

30

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(m

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-4000 -3000 -2000 -1000 0

Matrix potential (hPa)

WL

Beache Park Wood

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ford

0 10 20 30 40 50Moisture content (%)

50

40

30

20

10

0

De

pth

(m

)

-600 -400 -200 0


WL

Warloy-Baillon

Lew

es

S

ea

ford

0 10 20 30 40 50Saturated moisture content (%)

40

30

20

10

0

De

pth

(m

)

-800 -400 0


WL

East Isley

Ne

aw

Pit

L

ew

es

Figure 5 –Hydrogeological conditions on the English and French Warloy-Baillon experimental sites.

The very high suction values at Beche Park Wood site are the consequence of the severe drought that has prevailed in southern England for more than a year and the greater depth to water table on this site. One can wonder if between 50 and 75 m (that is to say 25 m above the water table) conditions are similar to those prevailing on East-Isley and Warloy-Baillon at the same period.

We have thus within the first 30 m depth, two contrasting matrix potential conditions at the English experimental sites. This is very convenient for testing the capacity of MRS method to distinguish various conditions of saturation.

1.3. OBJECTIVES

The objectives of the MRS survey conducted at the English sites were:

- to characterize MRS parameters for the chalk unsaturated zone at sites with different hydric conditions,

- to test the ability of the MRS method to distinguish different conditions of saturation. - to define an appropriate MRS methodology for monitoring variations in the chalk vadose

zone



2. Work carried out

2.1. PROPOSED METHODOLOGY

The proposed methodology for studying the English sites combined:

- measurements using the MRS prototype system having a short dead time (NumisPROTO) developed for a better determination of the short relaxation time signal in chalk and the usual equipment (NumisPLUS) for a deeper investigation;

- measurements with varying pulses Q (with fixed delay ∆t as defined in Figure 6) for the determination of the MRS relaxation time, T2* and water content distribution with depth.

- measurements with varying delay D1 (delay D1 = ∆t in Figure 6) when the pulse Q remains constant for the determination of the MRS relaxation time T1 for a constant investigation volume.

The vertical distribution of decay time (= relaxation time) and water content would then be built on the basis of :

- Relaxation times T1 evaluated with the best accuracy with the NumisPROTO measurements but limited to shallow depth ;

- Relaxation times T1 evaluated with a lower accuracy but along the whole profile using the conventional measurement by the NumisPLUS equipment.

- Vertical distribution of water content and relaxation time T2* at either shallow or greater depth with the NumisPROTO and NumisPLUS equipment respectively.

The combination of several measurements should make it possible to estimate the vertical distribution of MRS parameters with the best accuracy.

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20 ϕω +⋅−⋅= tTtete

Pulse P1

Ambiant

noise

Signal FID2

Pulse P2

Amplitudee0FID1Amplitudee0FID2

Decay time T2*

Signal FID1

Delay ∆∆∆∆t

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10

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Decay time T2*

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Delay ∆∆∆∆t

)/exp(1 1

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t

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Figure 6 – Conventional MRS sequence of measurement.



Tests of new TX/RX (Transmitter/Receiver) array were carried out to improve the S/N (Signal to Noise ratio). The efficiency of a 75 m side square loop as TX and 56 or 37m side square loop as RX was tested at the Beche Park Wood site. The objective was also to reduce an artefact during recording which pollutes the MRS signal. This artefact acts as a transient electromagnetic (TEM) signal after the pulse generated by the TX loop. These tests add to those performed in France at Orléans and the Warloy-Baillon site. This artefact being linked to the electric current in the TX loop is characterized by a frequency equal to the TX frequency (and not to the natural Larmor frequency), its amplitude is linked with the TX pulse moment, and is not linked to presence of water. That’s why several measurements were carried out using a TX frequency shifted from the Larmor frequency (2070 to 2072 Hz). Frequencies of 2060 Hz and 2087 Hz were used and varying TX power allowed separation of the TEM artefact from the MRS signal (see results in appendix 8).

2.2. MEASUREMENTS CARRIED OUT

The MRS soundings performed at East Isley and Beche Park Wood are listed with their main characteristics in Table 2.

It was concluded that both sites are suitable for MRS soundings and for methodological purposes. This implies that MRS parameters can be evaluated with acceptable uncertainty. Nevertheless it is still the case that MRS parameter uncertainty increases as signal to noise ratio (S/N) diminishes. The noise level and S/N encountered at the different experimental sites of the FLOOD1 project are listed in Table 1 and Table 2.

Two attempts were made at the end of the survey to carry out a MRS sounding using an eight-shape 37.5 m square loop as a transmitter and an eight-shape 19 m square loop as a receiver at Beche Park Wood in order to provide a comparison with the same configuration measured on East Isley. Unfortunately, due to instrumental problems, soundings were stopped before this could be achieved and insufficient records have been obtained with this loop configuration for a significant comparison to be made.

In Table 2, significant measurements for site comparison are highlighted using light grey for East Isley and dark grey for Beche Park Wood. Other measurements were mainly devoted to test potential instrumental artifacts which take place at the beginning of the measurement windows and which may interfere with chalk short MRS signals.

East Isley Beche Park Wood Warloy-Baillon

600-1200 nV 500-700 nV 180-200 nV

Table 1 : Noise level observed with NUMISPlus and a square8-shape antenna of 37.5 m side.



PMR sounding

Site Loop, side length

(m) Date Comments

Type of meas. / Qmax

Equipment

Pulse (ms)

Full Delay (ms)

Stack nb.

EN/IN S/N

EI0506A East Isley T=37.5 m 8-square R=19 m 8-square

03 May 06 Ok Q sounding (1207 A.ms)

proto 20/22 237 200 1.6 4.6

BW0506A Beche Park

Wood Square, 75 m 03 May 06 Ok

Q sounding (10176 A.ms)

PLUS 39/42 337 200 4.0 3.4

BW0506B Beche Park

Wood T=75 m square R=56 m square

04 May 06 Freq. test Artifact test; variable

offset proto 20/20 235 400

BW0506C Beche Park

Wood T=75 m square R=37.5 square


offset proto 20/20 235

EI0506B East Isley T=37.5 m 8-square R=19 m 8-square


offset proto 20/22 237

EI0506C East Isley T=37.5 m 8-square R=19 m 8-square

04 May 06 Freq. test Artifact test; +-offset;

var. pulse proto 20/22 237 3.7 3.3

BW0506D Beche Park

Wood T=75 m square

R=37.5 m square 04 May 06 ok


proto 19/21 236 200 4.2 3.7

BW0506E Beche Park

Wood T=75 m square

R=37.5 m square 05 May 06 Freq. test

Artifact test; +offset; var pulse

proto 19/21 236

BW0506F Beche Park

Wood T=75 m square

R=37.5 m square 05 May 06 Ok

Var ∆t Udc=88 V Q=1195-1138 A.ms

proto 20/21 36 200 2.0 9.2

BW0506G Beche Park

Wood Square, 75 m 05 May 06 Ok

Var ∆t Udc=200V; Q= 3200- A.ms

PLUS 19/21 76 200 2.7 4.6

EI0506D East Isley T=37.5 m 8-square R=19 m 8-square

06 May 06 Ok Var ∆t Udc=88 V;

Q=1186-1130 A.ms proto 19/21 36 300 1.9 5.5

EI0506E East Isley 8-square 37.5 m 08 May 06 Ok Q sounding (9241 A.ms)

PLUS 39/41 335 300 1.8 5.0

EI0506F East Isley T=37.5 m 8-square R=19 m 8-square

09 May 06 Pb Var ∆t Udc=88 V;

Q=1186-1130 A.ms proto 19/21 36 300 1.6 6.7

EI0506G East Isley 37.5 m 8-square 10 May 06 G & Gbis Var ∆t Udc=140V;

Q=2400-2390 A.ms PLUS 19/21 36

300 & 400

2.0 3.0

EI0506H East Isley T=37.5 m 8-square R=19 m 8-square

10 May 06 Ok Vari ∆t Udc=88 V;

Q=1200-1190 A.ms proto 19/21 36 400 1.4 5.4

EI0506I East Isley T=37.5 m 8-square R=19 m 8-square

11 May 06 Failed Var ∆t Udc=88 V;

Q=1200-1190 A.ms proto 19/21 36 400

BW0506H Beche Park

Wood 8-Square, 37.5 m 11 May 06 Ok


PLUS 20/22 317 400 1.7 3.5

BW0506I Beche Park

Wood T=37.5 m 8-square R=19 m 8-square

12 May 06 twice failed Q sounding

(-) proto 20/21 236 400

Table 2- Soundings are listed with their main characteristics in chronological order; pulse = pulse duration in ms for P1/P2, full delay ∆t = deadtime + FID1 length + delay D1 + pulse2 in ms, Equipment = NUMIS

plus or

NUMIS proto



2.3. WORKFLOW OF ACQUISITION AND PROCESSING

The MRS results are summarized in a sheet which includes signal processing, MRS parameters estimation and inversion results (vertical logs) of water content and decay time. The display is kept constant for this survey and is illustrated in Figure 7 with results of Q sounding EI0506E (eight square 37.5m, Numisplus).

Inversions of MRS signal in terms of vertical log are performed using the SAMOVAR software.

Figure 7 – Standard result sheet of MRS measurements.

Legend:

a) Initial amplitude of Free Induction Decay signal FID1 (nV)+ noise (nV), b) FID1 frequency (Hz), c) FID1 phase (deg), d) Ambiant noise (nV), e) Initial amplitudes of FID1 and FID2 (nV), f) Apparent T2*(Q) in ms, g) Apparent T1(Q) in ms, h) Mean amplitude of FID1 and FID2 (nV), i) Recorded signals after synchronous detection and filtering (envelops of signals), j) Vertical log of water content (%), k) Vertical log of T2*(z) in ms, l) Vertical log of T1(z) in ms.

Note that the MRS signal (called FID) is recorded after an exciting EM pulse has been generated injecting a high intensity electrical current at the site specific Larmor frequency. Usualy, 2 pulses are used separated by a delay D1 (ms), each of them followed by a signal namely FID1 after the first pulse and FID2 after the second pulse.

a)

b)

c)

d)

e)

f)

g)

h)

i)

j)

k)

l)



3. MRS Methodology in chalk

3.1. DISCUSSION ON SPECIFIC CHARACTERISTICS OF MRS MEASUREMENTS IN CHALK

Figure 8 – Schematic MRS sequence of measurement. After the pulse a dead-time is applied before recording the signal: the 12 ms (proto) and 40 ms (plus) dead-times do not have the same impact when the

MRS signal is long or short (as in chalk)

Effect of instrumental dead time

The possible effects of instrumental dead-time on MRS measurement are illustrated in Figure 8. In the case of low S/N, the end of the signal curve is flattened by the noise level. It results in a longer decay time estimation.

If the MRS early time response is missing (because of dead time), estimation of the decay time may be biased for a quickly decreasing signal (as in chalk): the decay time is generally over-estimated. As a first consequence, the initial amplitude is under-estimated and thus the water content is under-evaluated. This is why Numisproto with a shorter dead time (12 ms) than the standard Numisplus (40 ms) was developed.

Effect of Bandpass filter

Because MRS is a mono-frequency signal, a band-pass filtering is applied during acquisition and processing. In the case of signals with very short decay times, such filtering may adversely affect the records. For example, a band pass filter, centred on a 2kHz frequency with +/- 80Hz band pass has a length around 40 ms in the time domain (Figure 9). In the case of MRS signals having a very short decay time, the time duration of the filter may be of the same length as the signal. (10 ms for instance) and the filtered signal may be corrupted by the filter impulse response: the apparent decay time of the filtered signal is made longer and the initial amplitude is reduced (Figure 10).

Pulse Dead times Recording time

Numisproto

, dead time « 12ms »

Numisplus

, dead time « 40ms »

Noise threshold

MRS signal with short decay time

MRS signal with long decay time



T1 measurements

The measurement of T1, which is based on the ratio of the MRS signal amplitude called Free Induction Decay FID1 and FID2, is theoretically less affected by instrumental limitations and processing artefacts than T2* estimation. Nevertheless, a good signal / noise ratio is needed, particularly for the measurements with the shortest delays ∆t when a low FID2 signal is expected because the ratio FID2/FID1 is increasing with a T1 exponential behaviour when the delay, ∆t, between pulses is increased.

T1 is physically less sensitive to magnetic heterogeneities and its estimation, based on an amplitude ratio, is more robust than T2* exponential fit on the FID which is always affected by the dead time effects and more so for short decay times. Nevertheless, a T2* estimation can be performed for each MRS measurement whatever the sequence (Q sounding or ∆t-scan) and then a huge number of estimations can be performed. We expect that a statistical approach based on numerous evaluations may provide a reliable estimation of T2*. In addition, we should keep in mind that lab measurements on samples have shown that T1 and T2* were nearly identical in chalk. We expect this to be the same for field use of MRS.



Figure 9 – Frequency response of a bandpass filter centered on 2kHz with a +/-80 Hz bandpass (above). Impulse response of the same bandpass filter in the time domain (axis in second, below).

≈40ms



Figure 10 – Effect of bandpass filter on MRS data : 100ms (blue) and 10 ms (red) decay time signals; Above : raw synthetic data; Below : filtered data. The longest signal is less affected.



3.2. REPROCESSING RAW FID1 AND FID2: T2* ESTIMATION ON FID2 AND T1 FIT.

FID1 and FID2 curves were reprocessed using specific software to estimate initial amplitude and T2* using raw data. We obtained a similar fit for FID1 to those performed with the standard software SAMOVAR whereas the FID2 fit is not currently displayed with SAMOVAR.

We separately processed the FID early times (0-40ms) and late times (40-200ms). The objectives were:

- to suppress the effect of the artefact in the early time of FID (as expected for transient EM noise);

- to characterise the long decay time signal when observed on FID (the signal is sometimes above the noise level after 150ms of recording).

The estimation of T2* is biased and unreliable when the amplitude of FID is low (for low pulse or short delay ∆t) and close to the noise level. In this case, late time signals are not reliable but early time signals can be correctly processed and used.

3.3. ARE WE ABLE TO DISCRIMINATE T1 RELAXATION TIMES WITHIN THE RANGE 40-100 MS ?

Laboratory NMR measurements on chalk samples from the Warloy-Baillon experimental site have shown that T2 relaxation times are within the 40-100 ms range. The free induction decay curve decreases exponentially, and this decay is usually modelled by a single decay time constant (also called relaxation constant) but it may also be modelled by a distribution of decay times: decay time distribution is physically interpreted as the existence of various pore diameters in the material. The range of observed T2* relaxation time constants from surface MRS measurement at the same site are in the same range. Based on this observation, an attempt was made to model mean FID2 amplitude curves for variable delay ∆t and considering single exponential relaxation T1 at 40, 70 and 100 ms and bi-exponential relaxation T1 at 40 and 100 ms. The results of the modelling are shown in Figure 11.

If we consider that the better accuracy (under high S/N condition) of MRS amplitude measurement is about 10% (Girard et al. 2005), it follows that:

- bi-exponential chalk responses with 40 (50%) and 100 ms (50%) T1 is equivalent to 70 ms mono-exponential signal

- bi-exponential chalk responses within the range 40-100 ms will be impossible to discriminate from each other

- 40 ms and 100 ms mono-exponential responses can be separated

- 40 ms and 100 ms mono-exponential can hardly be discriminated from 70 ms mono-exponential

These modelling results indicate that under the conditions of our experiments we are at the limit of the current instrument sensitivity.



50 100 150 20040

50

60

70

80

90

100

Delay (ms)

Am

plit

ude n

V

T1:40 ms

T1:100 ms

bi-exp T1 40 and 100 ms

T1:70ms

Figure 11 – Modelling FID2 amplitudes when varying delays and relaxation times corresponding to those observed in chalk FID2(∆t)=100 *(1-exp(∆t/T1)



4. Results and interpretation of field survey

Remarks on interpretation:

- There is a known trade-off between water content and decay time T2* (Girard et al., 2005). The energy of the full length signal is linked to the initial amplitude (water content, W) and the decay time (T2*): the same signal can be fitted within a certain range of couple (W,T2*). In order to reduce uncertainty, we chose to interpret MRS soundings while keeping the water content W nearly constant as it is based on hydrogeological measurements.

- Apparent T2* on FID1 and FID2 for the low pulses may be not significant because it corresponds to a signal with weak amplitude (low Signal/Noise ratio).

- Inversion results are obtained using the SAMOVAR software (Legchenko & Shushakov, 1998).

4.1. COMPARISON OF MRS RESPONSE AT THE EAST ISLEY (EI) AND BECHE PARK WOOD (BW) SITES

Two soundings, EI0506E and BW0506H, performed with the same loop (square 8-shape, 37.5 m side) and the NUMISPlus are compared in Figure 12, Figure 13 and Figure 14. Both have very similar FID1 (initial amplitude): max. 110 nV followed by a slow decrease down to 90 nV.

Two differences can be underlined:

- the mean amplitude is lower at the BW site than at the EI site. In parallel, the apparent T2* estimation is shorter at the BW (T2* ≈ 55-80 ms) than at the EI site (T2* ≈ 100-120 ms). These observations are consistent with similar water content (same initial amplitude) but with shorter decay time at BW than at EI (energy of signal is lower when signal decreases quickly).

- FID2 at EI is significantly lower than FID1 (it is observed in both the initial amplitude and the mean amplitude) whereas FID1 and FID2 are stuck in BW. As a consequence, T1 estimation is twice as long as at EI than as at BW.

The apparent T2* of FID1 and FID2 are similar (and nearly constant over the set range of Q moments) for a particular site, but the apparent T2* is lower at BW (50-70ms) than at EI (100-140ms).

A third sounding EI0506A, measured with the NumisProto at EI is presented in Figure 15 and Figure 16. The corresponding one at BW is unfortunately not available since several attempts made to measure it failed because of instrument failure.

This sounding at EI with the NumisProto has characteristics similar to BW. One should note that most of the signal on EI0506E corresponds to pulses stronger than the maximum pulse recorded with the NumisProto which is limited to shallow investigation (Q<1200 A.ms) if compared to sounding



EI0506E with the NumisPlus (Q<9000 A.ms). Hence, sensitivity to water from 20 to 50 m is better with the NumisPlus sounding. This may explain why the apparent T2* values as measured using the NumisPlus (100-140ms) are longer than those using the NumisProto (60-80ms) at the same place in the EI site.

Processing of Q sounding EI0506E (Numisplus) TX/RX : eight shape 37.5m.

103

104

60

80

100

120

140

160

Init

. A

mp

. (n

V)

Q (A.ms)

FID1 Initial Ampl. (nV)


103

104

40

60

80

100

120

140

160

180

200

T2

* (m

s)

Q (A.ms)

FID1 app. T2* (ms)

FID2 app. T2* (ms)

Processing of Q sounding BW0506H (Numisplus) TX/RX : eight shape 37.5m.

103

104

60

80

100

120

140

160

Init

. A

mp

. (n

V)

Q (A.ms)



103

104

40

60

80

100

120

140

160

180

200

T2

* (m

s)

Q (A.ms)

FID1 app. T2* (ms)

FID2 app. T2* (ms)

Figure 12 – Initial amplitude (left) and apparent T2* (right) fit on FID1 (blue) and FID2 (green) curves (full record 0-200 ms). Same configuration of Q sounding at East Isley (above) and Beche Park Wood (below).

T2* ≈ 50-70 ms

T2* ≈ 90-140 ms



Figure 13 – East Isley site : EI0506E Q sounding data and inversion results (eight square 37.5m, Numisplus

).

Figure 14 – Beche Park Wood site : BW0506H Q sounding data and inversion results (eight square 37.5m, Numis

plus).

Mean T1app ≈ 190 ms

T2*app ≈ 100-120 ms

T1 ≈ 200 ms

T2* ≈ 100 ms

No reliable T1app

T2*app ≈ 55-80 ms

T1 ≈ 80 ms

T2* ≈ 80 ms



Figure 15 – East Isley site : EI0506A Q sounding data and inversion results (TX eight shape 37.5m RX sq19mx2, Numis

proto).

102

103

20

40

60

80

100

120

140

160

180

200

220

Init

. A

mp

. (n

V)

Q (A.ms)



102

103

40

60

80

100

120

140

160

180

200

T2

* (m

s)

Q (A.ms)

FID1 app. T2* (ms)

FID2 app. T2* (ms)

Figure 16 – East Isley site, EI0506A Q sounding: Initial amplitude (left) and apparent T2* (right) fit on FID1 (blue) and FID2 (green) curves (calculated on full record 0-200 ms). T2* for low pulses are not significant

because of low S/N.

Mean T1app ≈ 80 ms

T2*app ≈ 70-90 ms

T2*app ≈ 60-80 ms

T1 ≈ 80 ms

T2* ≈ 100 ms



At Beche Park Wood, because the water table is deep (75 m), a deep sounding was performed using a 75m square loop (Figures 16 and 17) which confirmed the results of sounding BW0506H.

Figure 17 – Beche Park Wood site : BW0506A Q sounding data and results (TX/RX : square 75m; Numis

plus)

103

104

50

100

150

200

250

300

350

Init

. A

mp

. (n

V)

Q (A.ms)



103

104

40

60

80

100

120

140

160

180

200

T2

* (m

s)

Q (A.ms)

FID1 app. T2* (ms)

FID2 app. T2* (ms)

Figure 18 – Beche Park Wood site, BW0506A Q sounding : Initial amplitude (left) and apparent T2* (right) fit on FID1 (blue) and FID2 (green) curves (full record 0-200 ms)

T2* ≈ 50-70 ms

No reliable T1app

T2*app ≈ 50- 70 ms

T1 ≈ 75 ms

T2* ≈ 70 ms



To investigate deeper than with the EI0506A array with the Numisproto system, we used a larger TX loop (75 m) and a square 37 m RX loop. Results of all MRS soundings are in good agreement at BW.

Figure 19 – Beche Park Wood site : BW0506D Q sounding data and inversion results (TX: square 75m, RX square 37m, Numis

proto).

102

103

0

100

200

300

400

500

600

Init

. A

mp

. (n

V)

Q (A.ms)



102

103

40

60

80

100

120

140

160

180

200

T2

* (m

s)

Q (A.ms)

FID1 app. T2* (ms)

FID2 app. T2* (ms)

Figure 20 – Beche Park Wood site, BW0506D : Initial amplitude (left) and apparent T2* (right) fit on FID1 (blue) and FID2 (green) curves (full record 0-200 ms).

Not reliable T1app

T2*app ≈ 50-60 ms

T1 ≈ 75 ms

T2* ≈ 50 - 75 ms

T2* ≈ 50-70 ms



Comparing the Numisplus and Numisproto results shows that the water content obtained with the Numisplus is always lower than with the Numisproto (Table 3, Figure 13, Figure 14, Figure 17 and Figure 19). The difference is explained by the fact that more water is visible with the improved apparatus.

Conventional evaluation

Numisplus measurement

Numisproto

measurement

East Isley 30-40 % (lab water saturated content)

8.5 % 11 %

Beche Park Wood 30-40 % (in situ calibrated neutron)

9 - 10.5 % 11.5 %

Table 3 : Comparison of water content estimations.

The water content as measured with the neutron probe is much higher. This is consistent with the fact that some part of the water remains invisible by MRS, even with the prototype.

The MRS water content increases from the surface to about 20 m at East Isley and 30m depth at Beche Park Wood while the conventional methods don’t show such an increase (neutron log only shows an increase from 2 to 10 m on Beche Park wood). One should note that this can be associated with a change in decay time but it is not clear here even on the reprocessed data.

4.2. METHODOLOGICAL PROBLEMS ENCOUNTERED

The Numisproto is a prototype and some tests were performed to verify its performance. All the tests which were performed have shown that some artifacts are recorded, at least in the early times (first 40-60 ms of records). These artifacts are revealed by :

- a fast decreasing shape in the early times (for examples, see BW0506D and EI0506A in Appendix 6 and Appendix 7)

- a frequency peak close to the Larmor frequency (Appendix 8).

It is not clear to what extent this artefact affects the MRS signal but records seem suitable for processing and inversion. In addition, the gain in recording the early time response (because of a shorter dead time) also improves the mean amplitude processing.

Another problem with the NumisProto is due to the amplitude shifts observed when performing ∆t-scan measurements. Significant amplitude shifts are observed within the same set of delay measurement or between repeated set measurements (see EI0506D, H, F in Appendix 4). When the reliability of the T1 measurement by ∆t-scan with the Numisproto was uncertain it was not used in the sites comparison.



4.3. SYNTHESIS OF OBSERVATIONS ON THE ENGLISH SITES

Both the Beche Park Wood (BW) and East Isley (EI) sites were found to be suitable for carrying out MRS soundings in order to characterize them in terms of MRS parameters and to compare them with each other and with the French test site of Warloy-Baillon (WB).

There was a significant difference in the static water level at these sites: 77 m in BW and 25 m in EI. This latter site has similar properties to the French site WB in terms of depth to water table (around 30m) and suction values. BW presents extreme conditions since the unsaturated zone is 77 m thick and extremely high suction values are observed within the first 10 m and are expected throughout most of the profile between 10 and 50 meters depth.

For testing the variations in the MRS parameters (decay time particularly) and their sensitivity to negative pressure, BW is a perfect candidate for comparison with the more common conditions encountered in EI and WB. In addition, electromagnetic conditions are particularly good there.

The different characteristics obtained with the different configurations of measurements are summarized in

Table 4.

Both sites appear similar in terms of amplitude and water content. When the same sounding is performed at both sites with a square eight (37.5m), sounding curves FID1 and the water content estimation are similar (8.5 % for EI0506E and 10.5 % for BW0506H). This is also confirmed by the results obtained with other configurations.

The water content calculated from the Numisproto data is higher than when obtained from the Numisplus. This is consistent with the fact that the early time response is better recorded, and the initial amplitude (that is directly linked with water content) is then more reliably evaluated.

From statistical estimation of the T2* fit on raw data, the estimation of T2* is globally higher in EI (60 to 100ms) than in BW (50 to 70ms).

Using the ∆t-scan sequence of measurement, T1 was successfully measured at Beche Park Wood and is about 50ms, but data quality was not sufficient from East Isley. At this latter site T1 (if significant) and T2* appears lower and close to 50 ms in contrast to the evaluation of T2* from Q-sounding but this can be explained by the different investigation depths between the soundings.

The main result of this field survey is that at Beche Park Wood all soundings have FID1 and FID2 curves stuck together, whereas in East Isley they are clearly separated. The separation between FID1 an FID2 curves, which is a measure of T1, reveals a longer decay time response when the water table is shallow (25 m), which disappear in BW where the water table is 77m deep.



4.4. COMPARISON WITH THE FRENCH EXPERIMENTAL SITE OF WARLOY-BAILLON (WB)

The comparisons are summarized in Table 5.

Globally, two main results stand out :

- The Beche Park Wood relaxation times are lower than the Warloy-Baillon and East-Isley ones.

- Water content is higher and relaxation times shorter when evaluated with the NUMISProto than with the NUMISPlus system.

4.5. INTERPRETATIONS

Static level is around 25m bgl at the East Isley site and 77 m bgl at Beche Park Wood. Negative pressure is then 10 times higher in BW than in EI. The weak but systematic 20-30ms difference in decay times observed between both sites is thought to be linked with the pressure conditions.

Unsaturated zone conditions are similar at Warloy-Baillon (France) and East Isley. At both sites the MRS results reveal a clear difference in FID1 and FID2, whereas it is not the case when the water table is deep as at Beche Park Wood. Are these observations linked with the pressure only or are there any other phenomena that control the MRS responses? Lithological variations with depth and between the different sites should not be neglected.

It seems that despite the artefacts, the NUMISProto provides information consistent with the other measurements. The NUMISProto which allows recording of the very early times response, provides a higher evaluation of water content. Nevertheless, the data quality of the Numisproto remains unstable compared to the Numisplus which provided stable and consistent results. Thanks to an adaptation of the measuring sequence (shorter pulse, shorter FID) it remains the more reliable MRS equipment for use in chalk.



Sounding Type Loop / Instrument East Isley (EI) Beche Park Wood

(BW)

TX 8sq37 RX 8sq19 Numisproto

A W ≈ 11%

T2* ≈ 60-90 ms T1 < 80 ms

failed

TXRX 8sq 37m Numisplus

E W ≈ 8.5%

Similar to H (BW) T2* ≈ 90-140 ms

T1 =180 ms

H W ≈ 10.5%

Similar to E (EI) T2* ≈ 50-80ms

TX sq75m RX sq37m Numisproto

D

W ≈ 11.5 %, T2* ≈ 50-70ms on

Q sounding

TX RX sq75m Numisplus

A W ≈ 9 %, z>30m T2* ≈ 50-60ms

TX RX sq75m Numisplus

G T1 ≈ 56ms

45 ms (early times) 67 ms (late times)

T2* ≈ 50-70ms (FID2) 50-70 ms (early times)

100 ms (late times)

TX sq75m

RX sq37m Numisproto

F T1 ≈ 50ms


T2* ≈ 50ms (FID2) 50 ms (early times) 80 ms (late times)

TX 8sq 37m RX 8sq19 m Numisproto

D,F,H T1 ≈ 50 ms ? (poor quality

estimation)

T2* ≈ 40-60 ms (FID2) 50 ms (early times) 80 ms (late times)

D1 scan

TXRX 8sq 37m Numisplus

G,Gbis T1 failed

T2* ≈ 50-90 ms (FID2)


Table 4 – Summary of observations on English exprimental sites



Instrumen-

tation NUMIS PLUS NUMIS PROTO

Loop Sq 8-shape TX-RX 37.5 m 1t

Sq TX-RX 37.5 m 2t

Sq 8-shape Tx 37.5 m 1t Rx 19 m 3t

Sq Tx 75 1t Rx 37.5 m 1t

Warloy-Baillon Oct.06 T2*=60-130 ms T1=110-140 ms W=18 %

Oct.05 T2*=70-85 ms T1= 100-150 ms W=20%

Mar 06 T2*=45-75 ms T1= 75-170 ms W=20%

East Isley May 06 T2*=100-120 ms T1=200 ms W=8.5 %

May 06 T2*=70-90 ms T1=80 ms W=11%

Beche Park Wood

May 06 T2*=55-80 ms T1< 80 ms W=10.5 %

May 06 T2*=50-60 ms T1= 75 ms W=11.5%

Table 5 – Comparison with the french experimental site of Warloy-Baillon.



5. Conclusion

Within the framework of the FLOOD1 project two sites were investigated using Magnetic Resonance Sounding in the Pang Catchment, West Berkshire, England. These experimental sites, East Isley and Beche Park Wood, had been instrumented within the framework of the FLOOD1 and the LOCAR projects respectively and benefit from in situ suction and water content measurements which are useful for the comparison and calibration of MRS measurements.

The EM noise conditions are relatively good at these sites and make them suitable for methodological investigations and for determining MRS parameters (water content, W and relaxation time constants T1 and T2*) with reasonable uncertainty.

An artefact of measurement is observed when measuring early times with the prototype. Evaluation of T1 relaxation times with the NUMISProto remains unusable because of some unresolved instrumental problem. Hardware improvements are envisaged that could enhance the sensitivity to short relaxation times variations. Finally the the Numisplus system with an adaptation of the measuring sequence (shorter pulse, shorter FID, fine frequency tuning) is nowadays an efficient tool for MRS chalk investigation.

The main result concerning non-destructive monitoring of chalk vadoze zone variation is that relaxation times, T1 and T2* measured at Beche Park Wood are globally lower than those measured at East Isley. A parallel is made between these contrasted MRS observations and the higher suction values measured at Beche Park Wood than at East Isley.

MRS water content measurements at both sites are similar. Water content calculated from the Numisproto data is higher than when obtained from the Numisplus. This is consistent with the fact that the early time response is better recorded, and the initial amplitude (that is directly linked with water content) is then more reliably evaluated.

None of the experimental results disqualify the possible and expected relationship between MRS relaxation times and suction. Despite very contrasted pressure conditions between EI and BW, the relationship is not clearly demonstrated as some observations remain inconsistent and some modelling shows that in the conditions of unsaturated chalk the distinction of varying relaxation time are at the sensitivity limit of the method.



6. References

Baltassat J.M, Boucher M., Girard J-F, Mathieu F., Dupont F. (2005) – Projet INTERREG III A FLOOD1 : Rôle des eaux souterraines dans le déclenchement des crues – Caractérisation géophysique du site expérimental de Warloy-Baillon. BRGM/RP-55258-FR, x ill., x ann.

Girard, J-F., Legchenko A., and Boucher M., 2005, Stability of MRS signal and estimating data quality, Near Surface Geophysics, 2005, 3, 187-194.

Legchenko A. and Shushakov O, Inversion of surface NMR data, 1998, Geophysics, Vol. 63, No. 1 (January-February 1998); pp. 75–84.



Appendix 1

Plan of instrumentation at the Beche Park Wood site

PL28 BECHE PARK WOOD

Profile

Probe

1

NP d-1

9 m

Profile

Probe 4

NP c-3

4 m

NP a-2

4 m

NP b-4

4 m

1.0 m 0.5 m 0.5 m

2.0 m

Multiplexer

Logger

2

Suction

Samplers

Purgeable

Tensiometers

Puncture

Tensiometers

18 m

43 m

NOT TO SCALE

1.0 m 0.5 m

2.0 m 0.5 m

Suction

Samplers

0.5 m

Equitensiometers

1.0 m 0.5 m

2.0 m 0.5 m

Teflon

Suction

Samplers 0.5 m

Borehole with

tensiometers



Appendix 2

Variable Q sounding at East Isley



EAST ISLEY May 06 EI0506E NUMIS+ P1/P2=39/40 ms

Site: East Isley Numis plus sounding Q var

Loop: 4 - 37.5 Date: 07.05.2006 Time: 13:01

NUMIS data set:

D:\Etudes2\flood1\RMP_ENGLAND\jf\East_Isley\EI0506E_interpbis\EI0506E.inp

matrix: D:\Etudes2\flood1\RMP_WB_06\matrix\East_Isley\EI_8sq37_0_0.mrm

loop: eight square, side = 37.5 m

geomagnetic field:

inclination= 66 degr, magnitude= 48622.07 nT

filtering window = 198.9 ms

bandpass = 10.00 Hz

average S/N = 4.52; EN/IN = 2.08

fitting error: FID1 = 5.74%; FID2 = 11.39 %

param. of regular.: E,T2* = 5000.0; T1* = 300.000

permeability constant Cp = 7.00e-09



EAST ISLEY May 2006 EA0506A NUMISPROTO P1/P2=20/20 ms

Site: East Isley sounding Q var

Loop: 4 - 37.5 x 1 Date: 03.05.2006 Time: 19:13

NUMIS data set:

D:\Etudes2\flood1\RMP_ENGLAND\jf\East_Isley\EI0506A_interpbis\EI0506A.inp

matrix: D:\Etudes2\flood1\RMP_WB_06\matrix\East_Isley\EI_T8sq37_R8sq19_p20.mrm


geomagnetic field:



bandpass = 10.00 Hz







Appendix 3

Variable Q sounding at Beche Park Wood



BECHE PARK WOOD May 06 BW0506A NUMIS+ P1/P2=39/42 ms

Site: Beche Park Wood

Loop: 2 - 75.0 Date: 02.05.2006 Time: 20:20

NUMIS data set:

D:\Travail\Etudes2\flood1\RMP_ENGLAND\jf\Beche_park_wood\BW0506A_interpbis\BW050

6A.inp

matrix: D:\Travail\Etudes2\flood1\RMP_ENGLAND\matrix\Beche-Park-

wood\sq75_40ms.mrm

loop: square, side = 75.0 m

geomagnetic field:



time constant = 15.00 ms



param. of regular.: modeling




BECHE PARK WOOD May 06 BW0506d NUMIS Proto P1/P2=19/21 ms

Site: Beche wood park sound Q var rx sq37 x 2

Loop: 2 - 75.0 x 1 Date: 05.05.2006 Time: 00:01

NUMIS data set:

D:\Travail\Etudes2\flood1\RMP_ENGLAND\jf\Beche_park_wood\BW0506D_interpbis\BW050

6D.inp

matrix: D:\Travail\Etudes2\flood1\RMP_ENGLAND\matrix\Beche-Park-

wood\BW_Tsq75_Rsq37_p20.mrm

loop: square, side = 75.0 m

geomagnetic field:



time constant = 15.00 ms







BECHE PARK WOOD May 06 BW0506H NUMIS+ P1/P2=20/22 ms

Site: Beche Park Wood, sq8-37 Q sounding 20ms

Loop: 4 - 37.5 Date: 10.05.2006 Time: 17:57

NUMIS data set:

D:\Etudes2\flood1\RMP_ENGLAND\jf\Beche_park_wood\BW0506H_interpbis\BW0506H.inp

matrix: D:\Etudes2\flood1\RMP_WB_06\matrix\East_Isley\EI_8sq37_0_0.mrm


geomagnetic field:



bandpass = 10.00 Hz







Appendix 4

Variable delay at East Isley



1 2 3 4 5 6 7 8 940

60

80

100

120

140

160

file number

Initial Ampl. nV

T2 fit (ms)

50 100 150 200 25026

28

30

32

34

36

38

40

42

44

46

Delay Time msec

Am

plit

ude n

V

EI0506H, mean ampl. from 0.00 to 196.98 ms

T1= 48.33 ms

Processing of D1 scan EI0506H (Numisproto) TX : square 37.5m, RX square 19m .

Left, Initial amplitude and T2* fit on FID2 curves (full record 0-200 ms), right T1 estimation from mean FID2 amplitude.

0 5 10 15 20 25 30 35 400

50

100

150

200

250

300

file number

Initial Ampl. nV

T2 fit (ms)

50 100 150 200 250 300 35020

25

30

35

40

45

50

55

60

65

70

Delay Time msec

Am

plit

ude n

V

EIO5O6F, mean ampl. from 0.00 to 196.98 ms

T1= 43.95 ms

Processing of D1 scan EI0506F (Numisproto) TX : square 37.5m, RX square 19m .


T2* ≈ 50-70 ms

T1 ≈ 50 ms

T2* ≈ 50-70 ms T1 ≈ 48 ms



0 2 4 6 8 10 1250

60

70

80

90

100

110

120

130

140

file number

Initial Ampl. nV

T2 fit (ms)

70 80 90 100 110 120 130 140 15020

22

24

26

28

30

32

34

36

Delay Time msec

Am

plit

ude n

V

EIO5O6G, mean ampl. from 0.00 to 195.05 ms

T1= 2.95 ms

Processing of D1 scan EI0506G (Numisplus) TX/RX : eight square 37.5m.


T2* ≈ 50-90 ms

Cannot be used for T1

estimation



Appendix 5

Variable delay at Beche Park Wood



0 5 10 15 20 250

50

100

150

200

250

300

350

400

file number

Initial Ampl. nV

T2 fit (ms)

50 100 150 200 250 300 350 40055

60

65

70

75

80

85

90

95

100

105

Delay Time msec

Am

plitu

de n

V

BWO5O6F, mean ampl. from 0.00 to 197.10 ms

T1= 49.33 ms

Processing of D1 scan BW0506F (Numisproto) TX : square 75m, RX square 37 .


0 2 4 6 8 10 12 14 16 18 200

50

100

150

200

250

300

file number

Initial Ampl. nV

T2 fit (ms)

80 90 100 110 120 130 140 150 160 170 18045

50

55

60

65

70

75

80

Delay Time msec

Am

plitu

de n

V

BW0506G, mean ampl. from 0.00 to 239.61 ms

T1= 56.12 ms

Processing of D1 scan BW0506G (Numisplus) TX/RX : square 75m.


T2* ≈ 50-70 ms

T1 ≈ 50 ms

T2* ≈ 50-70 ms

T1 ≈ 56 ms



Appendix 6

Re-processing of Q sounding at East Isley : Mean FID1 and FID2 amplitude integrated over the full record and

for late times



0 20 40 60 80 100 120 140 160 180 2000

50

100

150

200

250

Time msec

Am

plit

ude n

V

FID1

EI0506A.05

EI0506A.06

EI0506A.07

EI0506A.08

EI0506A.09

EI0506A.010

EI0506A.011

EI0506A.012

EI0506A.013

EI0506A.014

EI0506A.015

EI0506A.016

0 20 40 60 80 100 120 140 160 180 2000

50

100

150

200

250

Time msec

Am

plitu

de n

V

FID2

EI0506A.05

EI0506A.06

EI0506A.07

EI0506A.08

EI0506A.09

EI0506A.010

EI0506A.011

EI0506A.012

EI0506A.013

EI0506A.014

EI0506A.015

EI0506A.016

0 2 4 6 8 10 1210

20

30

40

50

60

70

File number

Am

plit

ude n

V

EI0506A, mean ampl. from 0.00 to 196.98 ms

0 2 4 6 8 10 125

10

15

20

25

30

35

40

45

File number

Am

plitu

de n

V

EI0506A, mean ampl. from 40.55 to 196.98 ms

Processing of Q sounding EI0506A (Numisproto) TX eight shape 37.5m RX sq19mx2

Top, FID1 and FID2, bottom, mean amplitude of FID1 and FID2 calculated over the full record (0-200 ms, bottom left) and for late times (40-200 ms, bottom right).



0 20 40 60 80 100 120 140 160 180 2000

50

100

150

Time msec

Am

plit

ude n

V

FID1

EI0506E.03

EI0506E.04

EI0506E.05

EI0506E.06

EI0506E.07

EI0506E.08

EI0506E.09

EI0506E.010

EI0506E.011

EI0506E.012

EI0506E.013

EI0506E.014

EI0506E.015

EI0506E.016

0 20 40 60 80 100 120 140 160 180 2000

50

100

150

Time msec

Am

plit

ude n

V

FID2

EI0506E.03

EI0506E.04

EI0506E.05

EI0506E.06

EI0506E.07

EI0506E.08

EI0506E.09

EI0506E.010

EI0506E.011

EI0506E.012

EI0506E.013

EI0506E.014

EI0506E.015

EI0506E.016

0 2 4 6 8 10 12 1425

30

35

40

45

50

55

60

65

File number

Am

plit

ude n

V

EI0506E, mean ampl. from 0.00 to 195.05 ms

0 2 4 6 8 10 12 1415

20

25

30

35

40

45

50

55

File number

Am

plit

ude n

V


Processing of Q sounding EI0506E (Numisplus) TX/RX : eight shape 37.5m




0 20 40 60 80 100 120 140 160 180 2000

50

100

150

Time msec

Am

plit

ude n

V

FID1

EI0506E.03

EI0506E.04

EI0506E.05

EI0506E.06

EI0506E.07

EI0506E.08

EI0506E.09

EI0506E.010

EI0506E.011

EI0506E.012

EI0506E.013

EI0506E.014

EI0506E.015

EI0506E.016

0 20 40 60 80 100 120 140 160 180 2000

50

100

150

Time msec

Am

plit

ude n

V

FID2

EI0506E.03

EI0506E.04

EI0506E.05

EI0506E.06

EI0506E.07

EI0506E.08

EI0506E.09

EI0506E.010

EI0506E.011

EI0506E.012

EI0506E.013

EI0506E.014

EI0506E.015

EI0506E.016

0 2 4 6 8 10 12 1425

30

35

40

45

50

55

60

65

File number

Am

plit

ude n

V


0 2 4 6 8 10 12 1415

20

25

30

35

40

45

50

55

File number

Am

plit

ude n

V


Processing of Q sounding EI0506E (Numisplus) TX/RX : eight shape 37.5m




Appendix 7

Re-processing of Q sounding at Beche Park Wood : Mean FID1 and FID2 amplitude integrated over the full

record and for late times



0 20 40 60 80 100 120 140 160 180 2000

50

100

150

200

250

300

350

Time msec

Am

plit

ude n

V

FID1

BW0506A.03

BW0506A.04

BW0506A.05

BW0506A.06

BW0506A.07

BW0506A.08

BW0506A.09

BW0506A.010

BW0506A.011

BW0506A.012

BW0506A.013

BW0506A.014

BW0506A.015

BW0506A.016

BW0506A.017

BW0506A.018

BW0506A.019

BW0506A.020

0 20 40 60 80 100 120 140 160 180 2000

50

100

150

200

250

300

350

Time msec

Am

plit

ude n

V

FID2

BW0506A.03

BW0506A.04

BW0506A.05

BW0506A.06

BW0506A.07

BW0506A.08

BW0506A.09

BW0506A.010

BW0506A.011

BW0506A.012

BW0506A.013

BW0506A.014

BW0506A.015

BW0506A.016

BW0506A.017

BW0506A.018

BW0506A.019

BW0506A.020

0 2 4 6 8 10 12 14 16 1830

40

50

60

70

80

90

100

File number

Am

plit

ude n

V

BW0506A, mean ampl. from 0.00 to 195.05 ms

0 2 4 6 8 10 12 14 16 1820

25

30

35

40

45

50

55

60

65

70

File number

Am

plit

ude n

V

BW0506A, mean ampl. from 40.55 to 195.05 ms

Processing of Q sounding BW0506A (Numisplus) TX/RX : square 75m.




0 20 40 60 80 100 120 140 160 180 2000

50

100

150

200

250

300

350

400

450

500

Time msec

Am

plit

ude n

V

FID1

BW0506D.04

BW0506D.05

BW0506D.06

BW0506D.07

BW0506D.08

BW0506D.09

BW0506D.010

BW0506D.011

BW0506D.012

BW0506D.013

BW0506D.014

BW0506D.015

BW0506D.016

BW0506D.017

BW0506D.018

BW0506D.019

BW0506D.020

0 20 40 60 80 100 120 140 160 180 2000

50

100

150

200

250

300

350

400

450

500

Time msec

Am

plitu

de n

V

FID2

BW0506D.04

BW0506D.05

BW0506D.06

BW0506D.07

BW0506D.08

BW0506D.09

BW0506D.010

BW0506D.011

BW0506D.012

BW0506D.013

BW0506D.014

BW0506D.015

BW0506D.016

BW0506D.017

BW0506D.018

BW0506D.019

BW0506D.020

0 2 4 6 8 10 12 14 16 180

20

40

60

80

100

120

140

File number

Am

plit

ude n

V

BW0506D, mean ampl. from 0.00 to 197.10 ms

0 2 4 6 8 10 12 14 16 1810

20

30

40

50

60

70

80

File number

Am

plitu

de n

V

BW0506D, mean ampl. from 40.58 to 197.10 ms

Processing of Q sounding BW0506D (Numisproto) TX: square 75m, RX square 37m.




0 20 40 60 80 100 120 140 160 180 2000

20

40

60

80

100

120

140

Time msec

Am

plit

ude n

V

FID1

BW0506H.04

BW0506H.05

BW0506H.06

BW0506H.07

BW0506H.08

BW0506H.09

BW0506H.010

BW0506H.011

BW0506H.012

BW0506H.013

BW0506H.014

BW0506H.015

BW0506H.016

BW0506H.017

BW0506H.018

BW0506H.019

BW0506H.020

0 20 40 60 80 100 120 140 160 180 2000

20

40

60

80

100

120

140

Time msec

Am

plitu

de n

V

FID2

BW0506H.04

BW0506H.05

BW0506H.06

BW0506H.07

BW0506H.08

BW0506H.09

BW0506H.010

BW0506H.011

BW0506H.012

BW0506H.013

BW0506H.014

BW0506H.015

BW0506H.016

BW0506H.017

BW0506H.018

BW0506H.019

BW0506H.020

0 2 4 6 8 10 12 14 16 1815

20

25

30

35

40

File number

Am

plit

ude n

V

BW0506H, mean ampl. from 0.00 to 195.17 ms

0 2 4 6 8 10 12 14 16 1810

12

14

16

18

20

22

24

26

28

File number

Am

plitu

de n

V

BW0506H, mean ampl. from 40.58 to 195.17 ms

Processing of Q sounding BW0506H (Numisplus) TX/RX : eight shape 37.5m




Appendix 8

Artefact tests – frequency spectra



EI0506B : Artifact test, variable frequency offset ; (Larmor frequency =2071 Hz) Pulse

Number ( Q1 and Q2) Transmitting

frequency (Hz) Number of stack

Comments

14 (858 -913 A.ms) 2071 117 15 (1035-1109 A.ms) 2060 (-10Hz) 105 16 (1261-1357 A.ms) 2087 (+17Hz) 103

frequency of the highest peak is unchanged and corresponds to

Larmor, clearly above noise level

frequency (Hz)

2 1202 1002 0802 0602 040

am

plitu

de

(n

V)

60

55

50

45

40

35

30

25

20

15

10

5

0

frequency (Hz)

2 1202 1002 0802 0602 040

am

plitu

de

(n

V)

60

55

50

45

40

35

30

25

20

15

10

5

Q14 Q15

Q16

FID1 & FID2 spectra for varying transmitting pulse frequency (-10 HZ and +18Hz) but the dominant frequency remains the same around 2071 Hz. We conclude that main component of signal is controlled by MRS signal and not by any artifact.

frequency (Hz)

2 1202 1002 0802 0602 040

am

plitu

de

(n

V)

60

55

50

45

40

35

30

25

20

15

10

5

∆∆∆∆F=+17Hz

∆∆∆∆F= 0 Hz ∆∆∆∆F= -10 Hz



EI0506C : Artifact test; with constant +17 Hz frequency offset but increasing pulse (Larmor frequency f0=2071 Hz). FID1 is not recorded to reduce the delay between the two pulses, to have the minimum amplitude for FID2 and help to detect the artefact.

Pulse voltage (Udc V, Q1/ Q2

A.ms)

Transmitting frequency (Hz)

Number of stack

Filter

Comments

20 – 242/250 2087 100 Notch 40 – 532/532 2087 100 SNotch 60 – 825/825 2087 100 Notch 80 – 1099/1150 2087 100 SNotch 100 – 1431/1515 2087 100 SNotch

Transient EM effect is known to be at TX freq. and increasing with TX power.

0 20 40 60 80 100 120 140 160 180 2000

50

100

150

200

250

Time msec

Am

plit

ude n

V

EI0506C.01

EI0506C.02

EI0506C.03

EI0506C.04

EI0506C.05

Time domain 5 FID2 curves recorded.



frequency (Hz)2 1202 1002 0802 0602 040

am

plitu

de

(n

V)

38

36

34

32

30

28

26

24

22

20

18

16

14

12

10

8

6

4

2

0

frequency (Hz)

2 1202 1002 0802 0602 040

am

plitu

de

(n

V)

55

50

45

40

35

30

25

20

15

10

5

0

FID2 spectra for Udc 20, 40 V.

frequency (Hz)

2 1202 1002 0802 0602 040

am

plitu

de

(n

V)

70

65

60

55

50

45

40

35

30

25

20

15

10

5

0

frequency (Hz)

2 1202 1002 0802 0602 040

am

plitu

de

(n

V)

55

50

45

40

35

30

25

20

15

10

5

0

FID2 spectra for Udc 60, 80 V.

frequency (Hz)

2 1202 1002 0802 0602 040

am

plitu

de

(n

V)

45

40

35

30

25

20

15

10

5

0

FID2 spectra for Udc 100 V.

Spectra appear to be relatively noisy, and we are not able to detect clearly a transient EM artefact in the frequency domain (with TX frequency and magnitude increasing with pulse power).

In addition, the maximum peak remains at Larmor frequency in any cases.

∆∆∆∆F=+17H ∆∆∆∆F=+17Hz

∆∆∆∆F=+17H ∆∆∆∆F=+17Hz

∆∆∆∆F=+17Hz



BW0506B : Artifact test; variable frequency offset; f0 =2070 Hz (Numisproto, TX sq75, RX sq56)

Pulse nb Transmitting

frequency (Hz) Number of

stacks Filter

Comments

14 2070 100 Notch 15 2060 (-10 Hz) 50 Notch 16 2087 (+17Hz) 177 Notch

0 20 40 60 80 100 120 140 160 180 2000

100

200

300

400

500

600

700

800

900

Time msec

Am

plit

ude n

V

BW0506B.014

BW0506B.015

BW0506B.016

0 20 40 60 80 100 120 140 160 180 2000

100

200

300

400

500

600

700

800

Time msec

Am

plit

ude n

V

BW0506B.014

BW0506B.015

BW0506B.016

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 30

100

200

300

400

500

600

700

800

file number

Initial Ampl. nV

T2 fit (ms)

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 30

100

200

300

400

500

600

700

800

file number

Initial Ampl. nV

T2 fit (ms)

Left FID1 curves (top) and initial amplitude and T2* (bottom), right FID2 curves (top) and initial amplitude and T2* estimation (FID2) on full length records (0-200 ms).



frequency (Hz)

2 1202 1002 0802 0602 040

am

plitu

de

(n

V)

210

200

190

180

170

160

150

140

130

120

110

100

90

80

70

60

50

40

30

20

10

0

frequency (Hz)

2 1202 1002 0802 0602 040

am

plitu

de

(n

V)

220

200

180

160

140

120

100

80

60

40

20

Q14 Q15

frequency (Hz)

2 1202 1002 0802 0602 040

am

plitu

de

(n

V)

210

200

190

180

170

160

150

140

130

120

110

100

90

80

70

60

50

40

30

20

10

Q16

Some peaks appear surrounding the dominant Larmor frequency, and follow the frequency shift between the injected pulse frequency and the Larmor frequency : on Q15 (-10Hz = 2060 Hz) and Q16 (+17Hz = 2077 Hz).

Because we suspect this observation to be the signature of a transient EM artefact, we decided to reduce the size of the receiving loop: the maximum coupling between TX and RX loops (i.e. the same size) should be the cause of this artefact.

∆∆∆∆F= 0 Hz

∆∆∆∆F=+17Hz

∆∆∆∆F=-10Hz



BW0506C : Artifact test; variable offset; f0 =2070 Hz (Numisproto, TX sq75, RX sq37.5)

Pulse nb / Udc V

Transmitting frequency (Hz)

Number of stack

Filter

Comments

13 / 59 V 2070 150 Notch 14 / 72 V 2070 67 Notch 15 / 89 V 2060 (-10 Hz) 54 Notch 16 / 109 V 2087 (+17 Hz) 120 Notch

0 20 40 60 80 100 120 140 160 180 2000

50

100

150

200

250

300

350

400

Time msec

Am

plit

ude n

V

bw0506c.013

bw0506c.014

bw0506c.015

bw0506c.016

0 20 40 60 80 100 120 140 160 180 2000

50

100

150

200

250

300

350

400

450

Time msec

Am

plit

ude n

V

bw0506c.013

bw0506c.014

bw0506c.015

bw0506c.016

1 1.5 2 2.5 3 3.5 450

100

150

200

250

300

350

400

file number

Initial Ampl. nV

T2 fit (ms)

1 1.5 2 2.5 3 3.5 4

0

50

100

150

200

250

300

350

400

file number

Initial Ampl. nV

T2 fit (ms)

Left FID1 curves (top) and inital amplitude and T2* (bottom), right FID2 curves (top) and initial amplitude and T2* estimation (FID2) on full length records (0-200 ms).



frequency (Hz)

2 1202 1002 0802 0602 040

am

plitu

de

(n

V)

90

85

80

75

70

65

60

55

50

45

40

35

30

25

20

15

10

5

frequency (Hz)

2 1202 1002 0802 0602 040

am

plitu

de

(n

V)

110

100

90

80

70

60

50

40

30

20

10

Q 13 Q 14

frequency (Hz)

2 1202 1002 0802 0602 040

am

plitu

de

(n

V)

110

100

90

80

70

60

50

40

30

20

10

frequency (Hz)

2 1202 1002 0802 0602 040

am

plitu

de

(n

V)

110

105

100

95

90

85

80

75

70

65

60

55

50

45

40

35

30

25

20

15

10

5

Q15 Q16

When there is no frequency shift between transmitting pulse and Larmor frequency, spectra have a symmetrical shape. When transmitting frequency is -10 Hz (Q15, FTX=2060 Hz) or +17Hz (Q16, FTX=2087 Hz) we observe a distortion along the border of the spectrum, following this frequency shift.

So, even with this loop, we observe a signature which we attribute to the existence of a transient EM artefact. Because it is always far below the level of PMR signal, we kept this loop to perform the field study.

∆∆∆∆F= 0 Hz ∆∆∆∆F= 0 Hz

∆∆∆∆F=+17Hz ∆∆∆∆F=-10Hz



BW0506E : Artifact test; with +17 Hz frequency offset; increasing pulse (Larmor frequency f0=2071 Hz). FID1 is not recorded to reduce the delay between the two pulses, to have the minimum amplitude for FID2 and help to detect the artefact.

Pulse nb

Udc voltage (V) Transmitting

frequency (Hz) Number of stack

Filter

Comments

1 / 20 V 2087 (+17Hz) 200 Notch 2 / 40 V 2087 (+17Hz) 200 Notch 3 / 60 V 2087 (+17Hz) 200 Notch 4 / 80 V 2087 (+17Hz) 200 Notch 5 / 100 V 2087 (+17Hz) 200 Notch 6 / 110 V 2087 (+17Hz) 200 Notch

Transient EM effect is known to be at TX freq. and increasing with TX power.

0 20 40 60 80 100 120 140 160 180 2000

50

100

150

200

250

300

350

400

Time msec

Am

plit

ude n

V

BW0506E.01

BW0506E.02

BW0506E.03

BW0506E.04

BW0506E.05

BW0506E.06

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 650

100

150

200

250

300

350

file number

Initial Ampl. nV

T2 fit (ms)

Left FID2 curves and initial amplitude and T2* (right) estimation on full length records (0-200 ms).

frequency (Hz)

2 1202 1002 0802 0602 040

am

plitu

de

(n

V)

42

40

38

36

34

32

30

28

26

24

22

20

18

16

14

12

10

8

6

4

2

0

frequency (Hz)

2 1202 1002 0802 0602 040

am

plitu

de

(n

V)

65

60

55

50

45

40

35

30

25

20

15

10

5

0

∆∆∆∆F=+17Hz ∆∆∆∆F=+17Hz



Udc = 20 V Udc = 40V

frequency (Hz)

2 1202 1002 0802 0602 040

am

plitu

de

(n

V)

90

85

80

75

70

65

60

55

50

45

40

35

30

25

20

15

10

5

frequency (Hz)

2 1202 1002 0802 0602 040a

mp

litu

de

(n

V)

95

90

85

80

75

70

65

60

55

50

45

40

35

30

25

20

15

10

5

Udc = 60V Udc = 80V

frequency (Hz)

2 1202 1002 0802 0602 040

am

plitu

de

(n

V)

95

90

85

80

75

70

65

60

55

50

45

40

35

30

25

20

15

10

5

0

frequency (Hz)

2 1202 1002 0802 0602 040

am

plitu

de

(n

V)

95

90

85

80

75

70

65

60

55

50

45

40

35

30

25

20

15

10

5

0

Udc = 100V Udc = 110V

The main peak of the spectrum remains at the same frequency (centered on the 2071 Hz Larmor frequency). But a distortion is systematically observed around 2080Hz (less than the frequency shift, but in the same trend) which should be related with a artefact, but in any case, its magnitude is far below the Larmor peak.

∆∆∆∆F=+17Hz ∆∆∆∆F=+17Hz

∆∆∆∆F=+17Hz ∆∆∆∆F=+17Hz

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