c&g dec.2006vlfpros (1)
TRANSCRIPT
7/23/2019 c&g Dec.2006vlfpros (1)
http://slidepdf.com/reader/full/cg-dec2006vlfpros-1 1/9
See discussions, stats, and author profiles for this publication at:http://www.researchgate.net/publication/262171905
Short Note: VLFPROS-A Matlab code for
processing of VLF-EM data
ARTICLE in COMPUTERS & GEOSCIENCES · DECEMBER 2006
Impact Factor: 2.05 · DOI: 10.1016/j.cageo.2006.02.021
CITATIONS
14
READS
356
4 AUTHORS, INCLUDING:
Narasimman Sundararajan
Sultan Qaboos University
81 PUBLICATIONS 353 CITATIONS
SEE PROFILE
Ramesh Babu Veldi
Atomic Minerals Directorate for Explor…
6 PUBLICATIONS 29 CITATIONS
SEE PROFILE
Srinivas Yasala
Manonmaniam Sundaranar University
38 PUBLICATIONS 104 CITATIONS
SEE PROFILE
All in-text references underlined in blue are linked to publications on ResearchGate,
letting you access and read them immediately.
Available f rom: Narasimman Sundararajan
Retrieved on: 08 December 2015
7/23/2019 c&g Dec.2006vlfpros (1)
http://slidepdf.com/reader/full/cg-dec2006vlfpros-1 2/9
Computers & Geosciences 32 (2006) 1806–1813
Short Note
VLFPROS—A Matlab code for processing of VLF-EM data$
N. Sundararajana,, V. Ramesh Babub, N. Shiva Prasadb, Y. Srinivasa
aCentre for Exploration Geophysics, Osmania University, Hyderabad 500 007, IndiabAtomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, Hyderabad 500 016, India
Received 25 February 2006; accepted 25 February 2006
1. Introduction
The very low frequency electromagnetic (VLF-
EM) technique is well established for rapid geolo-
gical mapping and detection of buried conductive
targets. The technique makes use of signal radiation
from military navigation radio transmitters. There
are about 42 global ground military communication
transmitters1 operating in VLF frequency in the
range of 15–30 kHz. These stations, located aroundthe world, generate signals which are effectively
used for a variety of applications including naviga-
tion and communication, ground water detection or
contamination, soil engineering, cultural detection,
ionospheric, meteorological, archeological, nuclear
waste detection and VLF band transmission studies,
besides mineral exploration, mapping of fault zones,
etc. (Wright, 1988; Philips and Richards, 1975;
Sundararajan et al., 2006).
The detection of subsurface conductors is made
feasible by means of a portable VLF receiver,which, in most commercial instruments, provides a
measure of the inphase and quadrature components
of the vertical secondary magnetic fields relative to
the horizontal and primary field. Although both
inphase and quadrature components contain valu-
able diagnostic information about the subsurface
targets, only a few schemes exists for extracting the
required information and thereby relating the
observed anomalies to their causative sources. One
such a scheme first proposed by Fraser (1969) is a
simple filtering technique known as Fraser filtering.The technique is analogous to passing the inphase
data through a band pass filter which (i) completely
removes DC bias and greatly attenuates long
wavelength signals; (ii) completely removes Nyquist
frequency related noise; (iii) phase shifts all fre-
quencies by 901; and (iv) has the band pass centered
at a wave length of five times the station spacing.
Fraser filtering converts somewhat noisy, non-
contourable inphase components to less noisy,
contourable data which ensures greatly the utility
of VLF-EM survey. VLF-EM contour maps form ameaningful complement to magnetic maps.
Yet another filtering proposed by Karous and
Hjelt (1977, 1983), enables the geophysicist to
generate an apparent current density pseudosection
by filtering the inphase data and which provides a
pictorial indication of the depth of the various
current concentrations and hence the spatial dis-
position of subsurface geological features such as
mineral veins, faults, shear zones and stratigraphic
ARTICLE IN PRESS
www.elsevier.com/locate/cageo
0098-3004/$- see front matterr 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.cageo.2006.02.021
$Code available from server at: http://www.iamg.org/
CGEditor/index.htm.Corresponding author. Tel.: +91 40 27174793;
fax: +91 40 27099020.
E-mail address: [email protected]
(N. Sundararajan).1ABEM Printed Matter No. 93062 (http://www.abem.se/
products/wadi/vlf-freq.pdf ).
7/23/2019 c&g Dec.2006vlfpros (1)
http://slidepdf.com/reader/full/cg-dec2006vlfpros-1 3/9
conductors (Ogilvy and Lee, 1991). The finite Hjelt
filter method is a more generalized and rigorous
form of the widely known Fraser filter (Fraser,
1969), however is derived directly from the concept
of magnetic fields associated with the current flow in
the subsurface. Application of either Fraser or Hjeltfilter or both to VLF inphase component enables
one to obtain the equivalent current densities at a
constant depth which would cause a magnetic field.
In the absence of numerical modeling, Fraser and
Hjelt filtering techniques are proved to be effective
as they provide a simple scheme for semi-quantita-
tive interpretation.
The aim of this work is to present a simple and
elegent code with GUI-based MATLAB functions
for processing and interpretation of VLF-EM data.
The utility of this program includes: (i) plotting of
VLF-EM anomalies as stacked profiles using a userdefined filter; (ii) contouring and imaging of the
filtered data for understanding the spatial distribu-
tion of EM conductors; and (iii) preparation of
pseudodepth section of a chosen traverse using
Fraser and Hjelt filters. Stacking of profiles,
contouring and imaging of magnetic data can be
carried out by this code. These features are
illustrated graphically over a set of inphase compo-
nent of VLF-EM besides contours of magnetic
signal, over a mineralized fracture zone from
Chhattisgarh, India (Ramesh Babu et al., 2004).
2. Filtering of VLF-EM data
Modern commercial VLF-EM instruments read
directly the inphase and quadrature components in
digital form of the vertical secondary magnetic field
expressed as percentage of the primary horizontal
magnetic field. VLF inphase data often yield
complex patterns which require a considerable
study for proper interpretation of the profiles. The
most popular form of presenting 2-D VLF-EM data
over a given area is in the form of stacked profiles.
These are profiles along each survey line plotted on
a 2-D plane in the same relative position as the lines.
VLFPROS facilitates plotting the data in the form
of stacked profiles, which in turn demarcates spatial
location of conductors.
Linear filtering technique developed by Fraser
(1969) transforms non-contourable inphase data to
contourable form. Inphase component (VIP) ex-
hibits crossovers in the presence of a conductor/
conducting zones (McNeill and Labson, 1991) as
shown in Fig. 1(a). The filtering process simply
involves running a four-point weighted average
using the weights of 1, 1, +1, +1. This simple
digital filter operator passes over the inphase
component as shown in Fig. 1(b).
Fraser filtered inphase component when plotted
generally peaks over the top of the conductor asshown in dotted line (Fig. 1(a)). This alteration of
the raw data is termed as Fraser filtering and can be
summarized in two statements: (i) the filter phase
shifts all spatial frequencies by 901, i.e., it turns
crossovers into peaks or troughs; and (ii) it exhibits
a band pass response, in other words, it greatly
diminishes either sharp irregular responses (noise)
or long rolling responses. At the same time, it
accentuates responses which are the nearest to the
filter’s shape. Subsequently, the filtered output is
plotted again at the center of the filter shown (Fig.
1(b)) as Fraser filter coefficients. The filter is thenmoved ahead of one data spacing and the procedure
is repeated. Once all the profiles are filtered, the
filtered output is contoured and imaged. The
interpretation of maps/images generated from the
Fraser filtered output is qualitative in nature. A
large amplitude can be considered as a large
conducting zone. Very sharp and low amplitudes
indicate shallow sources, and, conversely, broader
amplitudes of large wavelengths indicate progres-
sively deeper sources.
The Hjelt filter technique is a more generalizedand rigorous form of the Fraser filter but is directly
derived from the concept of magnetic fields asso-
ciated with current flow in the earth. From the
filtered VLF inphase component, one obtains the
equivalent current densities at a constant depth
which would cause a magnetic field. That is, the
filter attempts to determine the current distribution
responsible for producing the measured magnetic
field. Determination of the filter shape (coefficients)
is fairly a simple mathematical process (Karous and
Hjelt, 1977). A number of filters with various
lengths and shapes can be developed, however the
optimized Hjelt filter (Karous and Hjelt, 1983) can
be expressed as
DZ =2PI að0Þ ¼ 0:205 H 3 0:323H 2 þ 1:446H 1
1:446H 1 þ 0:323H 2 0:205H 3,
where I a (0) ¼ 0.5 [I (DX /2)+I (DX /2)] and where
H 3, H
2, etc., are the measured data at six
consecutive stations, DX is the measurement inter-
val and I a is the apparent current density. Proce-
dures for applying the Hjelt filter is exactly the same
ARTICLE IN PRESS
N. Sundararajan et al. / Computers & Geosciences 32 (2006) 1806–1813 1807
7/23/2019 c&g Dec.2006vlfpros (1)
http://slidepdf.com/reader/full/cg-dec2006vlfpros-1 4/9
as that of Fraser filter, including proper orientation
so as to match expected anomaly shapes. While the
filter passes along the data profile, the inphase
components are multiplied by the filter coefficients
and then all the six products are added to produce
one output value which is represented at the center
ARTICLE IN PRESS
-6
-4
-2
0
2
4
6
8
10
12
14
0 9060 70 805040302010 100
inphase componentFraser filteredHjelt filtered
X-axis
(operator facingtransmitter i.e., Y-axis)
(conductor)
F i l t e r c o e f f i c i e n t s
F i l t e r c o e f f i c
i e n t s
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
0 2 4 5 6
-2
-1
0
1
2
(centre)
(profile)
(-1) (-1)
(1)(1)
Hjelt filter coefficients
Fraser filter coefficients
Conductor response
1 3
(a)
(b)
(c)
Fig. 1. (a) Schematic sketch of conductor response. (b) Schematic sketch of Fraser filter coefficients. (c) Schematic sketch of Hjelt filter
coefficients.
N. Sundararajan et al. / Computers & Geosciences 32 (2006) 1806–18131808
7/23/2019 c&g Dec.2006vlfpros (1)
http://slidepdf.com/reader/full/cg-dec2006vlfpros-1 5/9
of the filter as shown in Fig. 1(c). The process
continues till the end for all the profiles. Then the
output is contoured and interpreted in a similar way
to that of the Fraser filter.
3. Pseudosection
An additional interpretative tool is based on
pseudosection of the filtered outputs. This is
obtained by processing a chosen single data profile
either by Fraser or Hjelt or both at various lengths
or spans. As the length of the filter increases,
responses from increasing depths are successively
emphasized. Therefore if the outputs are arranged
on a section such that greater depths correspond to
longer filters than the section should approximately
resemble the current pattern in the ground. How-
ever, it must be emphasized that this is only anapproximation to the section (Wright, 1988; Fraser,
1981). Thus, construction of the pseudosection
consists of a number of steps like processing the
profiles with as many as number of levels (approxi-
mately 5 or 6), at each level, in terms of integer
multiples of the station spacing (n Dx) where n is the
number of levels and Dx is the station spacing.
Finally, plotting the results separated by nD
x ateach level one below the other so as to form a
section. Interpretation of the pseudosection is also
fairly general and consists of the following se-
quences: (i) areas with high current-densities corre-
spond to good conductors; (ii) negative contours are
due to high resistivities; and (iii) the trend of the
contour pattern indicates the dip of the conductor.
4. Matlab implementation
VLFPROS is a Matlab Version 6.0 R12-based
program for processing of VLF-EM data and whichcan also be implemented on Version 7.0 R14. This
program can be activated either by an ‘m’ file
ARTICLE IN PRESS
Fig. 2. Flow chart of VLFPROS.
N. Sundararajan et al. / Computers & Geosciences 32 (2006) 1806–1813 1809
7/23/2019 c&g Dec.2006vlfpros (1)
http://slidepdf.com/reader/full/cg-dec2006vlfpros-1 6/9
‘‘VLFPROS ’’ or by a figure file of ‘‘VLFPROS ’’ as
found in flowchart (Fig. 2). From Matlab command
window, once the VLFPROS is loaded on activa-
tion, a window consisting of push buttons with
various options such as (i) contouring, (ii) stacked
profiles and (iii) pseudosection display on the screen.The VLF inphase component which is the input to
VLFPROS has to be created in three different
columns separated by a comma or space and free
from headers. The first column should contain X
location, second column Y location and the third
column should contain the inphase component of
VLF-EM data. Example ‘vlfip.dat’ is a VLF inphase
component data which is available for demo
(Fig. 3).
To begin with stacked profiles option is selected
and which displays filtering and stacked profiles
window. This window enables plotting of raw data,
filtering (with various options like Fraser, Hjelt_1,
Hjelt_2) and plotting of filtered outputs in the form
of stacked profiles. The input data file has to bespecified in the appropriate diallog box enter file
name option and then click plot profile for raw data
stacked profiles. Further, one of the three available
filters can be made use of to filter the input data by
entering the appropriate filter name (Fraser.txt/
Hjelt_ 1.txt/Hjelt_ 2.txt). Then specify an output file
name against enter output file name dialog box and
then enter to obtain the stacked profiles of the
filtered data. To contour/image of the filtered
output, go to VLFPROS and click contouring.
Contouring and imaging window appears on the
screen. Contouring and imaging can be realized byspecifying a data file name in the appropriate diallog
box, the maximum and minimum value of the input
data file appear. An appropriate cell size has to be
specified. Select a gridding method out of available
options linear, cubic, nearest, V4 (Matlab 4 griddata
method) from the popup menu, which results the
interpolated minimum and maximum contour
values. The cubic and V4 methods produce smooth
surfaces while ‘linear’ and ‘nearest’ have disconti-
nuities in the first and zeroth derivatives respectively
(Sandwell, 1987). Finally specify the appropriatecontour interval for contouring.
To prepare pseudosection, activate VLFPROS
and click pseudosection which displays the pseudo-
section window. Enter the input file name of raw
data and select a specific profile line number which
is to be pseudosectioned. Once the line number is
selected from the popup menu, enter the appro-
priate filter and output files in their respective dialog
boxes. Then choose the number of levels from popup
menu and then click to write the filtered output to a
file which can further be used for obtaining the
pseudosection using contouring option.
The entire operations are illustrated with VLF
inphase component (vlfip.dat) pertaining to miner-
alized fracture zone from Chhattisgarh, India. The
stacked profiles of raw data and its image are shown
in Fig. 4(a) and (b), respectively. While the Fraser
and Hjelt filtered outputs in the form of stacked
profiles are given in Fig. 5(a) and (b), and the
corresponding images are given in Fig. 6(a) and (b),
respectively. The pseudosections of the chosen line
(traverse 50) based on Fraser and Hjelt filters are
shown in Fig. 7(a) and (b). The magnetic data from
ARTICLE IN PRESS
Fig. 3. Location and geological map of area.
N. Sundararajan et al. / Computers & Geosciences 32 (2006) 1806–18131810
7/23/2019 c&g Dec.2006vlfpros (1)
http://slidepdf.com/reader/full/cg-dec2006vlfpros-1 7/9
ARTICLE IN PRESS
100 200 300
0
100
200
300
400
500
0 %
50 %
100 200 300
0
100
200
300
400
500
600
distance in metres distance in metres
d i s t a n c e
i n m e t r e s
d i s t a n c e
i n m e t r e s
0 %
50 %
Fraser filtered Hjelt filtered
(a) (b)
Fig. 5. (a) Stacked profiles of Fraser filtered VLF-EM inphase component. (b) Stacked profiles of Hjelt filtered VLF-EM inphase
component.
Fig. 4. (a) Stacked profiles of VLF-EM inphase component. (b) Image with contours of VLF-EM inphase component.
N. Sundararajan et al. / Computers & Geosciences 32 (2006) 1806–1813 1811
7/23/2019 c&g Dec.2006vlfpros (1)
http://slidepdf.com/reader/full/cg-dec2006vlfpros-1 8/9
ARTICLE IN PRESS
Fig. 7. (a) Pseudodepth section of Fraser filtered VLF-EM inphase component. (b) Pseudodepth section of Hjelt filtered VLF-EM inphase
component.
Fig. 6. (a) Image with contours of Fraser filtered VLF-EM inphase component. (b) Image with contours of Hjelt filtered VLF-EM inphase
component.
N. Sundararajan et al. / Computers & Geosciences 32 (2006) 1806–18131812
7/23/2019 c&g Dec.2006vlfpros (1)
http://slidepdf.com/reader/full/cg-dec2006vlfpros-1 9/9
the same location is imaged and shown in Fig. 8 and
which may be compared with the Fraser and Hjelt
images (Fig. 6(a) and (b)) of the VLF-EM inphase
component.
5. Conclusion
The VLFPROS, thus a simple Matlab code with
GUI which produces stacked profiles of both raw
and filtered data, in addition to their images with
contours. This code can also be used for processing
of magnetic data to obtain the contours/images/
stacked profiles. This code can be modified for
additional features. Further, pseudosections provide
first hand information regarding the number, size,
depth and relative disposition of the conductors.
Acknowledgements
The authors wish to record their sincere and
profound thanks to Dr. Roger Guerin for many
useful suggestions to prepare this manuscript.
Further, they extend their heartful thanks to Prof.
A.V.R.S. Sarma, Head, Department of Electrical
Engineering, Osmania University for provi-
ding computational facilities. The second author
expresses his gratitude to Dr. R.N. Sinha, Director,
Atomic Mineral Directorate for Exploration and
Research (AMD), Department of Atomic Energy,
Hyderabad, India for his kind permission to publish
this paper and also his encouragement to do Ph.D.
One of the authors Y. Srinivas would like to thankthe Council of Scientific and Industrial Research
(CSIR), New Delhi, India for financial support.
Appendix A. Supplementary Materials
Supplementary data associated with this article
can be found in the online version at doi:10.1016/
j.cageo.2006.02.021.
References
Fraser, D.C., 1969. Contouring of VLF-EM data. Geophysics 34,
958–967.
Fraser, D.C., 1981. A review of some useful algorithms in
geophysics. Canadian Institute of Mining Transactions
74(828), 76–83.
Karous, M., Hjelt, S.E., 1977. Determination of apparent current
density from VLF measurements: Report. Department of
Geophysics, Univeristy of Oulu, Finland, Contribution No.
89, 19pp.
Karous, M., Hjelt, S.E., 1983. Linear filtering of VLF dip-angle
measurements. Geophysical Prospecting 31, 782–794.McNeill, J.D., Labson, V., 1991. Geological mapping using VLF
radio fields. In: Nabighian, M.N. (Ed.), Electromagnetic
Methods in Applied Geophysics, vol. 2. Society of Explora-
tion Geophysicists, Tulsa, OK, pp. 521–640.
Ogilvy, R.D., Lee, A.C., 1991. Interpretation of VLF-EM in-
phase data using current density pseudosections. Geophysical
Prospecting 39, 567–580.
Philips, W.J., Richards, W.E., 1975. A study of the effectiveness
of the VLF method for the location of narrow, mineralized
fault zones. Geoexploration 13, 215–226.
Ramesh Babu, V., Subash Ram., Srinivas, R., VeeraBhaskar, D.,
Bhattacharya, A.K., 2004. VLF-EM surveys for uranium
exploration in Dulapali area, Raigarh district, Madhyapra-
desh, India. XXV (2–3), 27–33.Sandwell, D.T., 1987. Bihramonic spline interpolation of GEOS-
3 and SEASAT altimeter data. Geophysical Research Letters
2, 139–142.
Sundararajan, N., Narasimah Chary, M., Nandakumar, G.,
Srinivas, Y., 2006. VES and VLF—an application to ground-
water exploration, Khammam, India. The Leading Edge, in
press.
Wright, J.L., 1988. VLF Interpretation Manual. EDA Instru-
ments (now Scintrex. Ltd.), Concord, Ont.
ARTICLE IN PRESS
Fig. 8. Image of raw magnetic data.
N. Sundararajan et al. / Computers & Geosciences 32 (2006) 1806–1813 1813