Geophysical Prospecting 34,845-855, 1986.

SHEAR-WAVE REFLECTION PROFILING FOR NEAR-SURFACE LIGNITE EXPLORATION*

B. MILKEREIT** , H. S T U M P E L * * * and W . RABBEL***

A B S T R A C T MILKEREIT, B., STUMPEL, H. and RABBEL, W. 1986. Shear-Wave Reflection Profiling for Near- Surface Lignite Exploration, Geophysical Prospecting 34, 845-855.

We present the results of a shear-wave reflection experiment and in situ measurements in opencast lignite exploration. Near-surface coal seams have lower shear-wave velocities ( z 200 m/s) and lower densities than sand and clay layers. Due to strong reflection coeffi- cients, a shear-wave reflection survey provides a powerful tool in lignite prospecting. Due to shorter seismic wavelengths shear waves will yield a higher resolution of shallow subsurface structure than compressional waves. Low shear-wave velocities and strong lateral velocity variations, however, require a dense data acquisition in the field. The variation of stacking velocities can exceed & 15% within a profile length of 300 m. The different steps in processing and interpretation of results are described with actual records. The final CMP-stack shows steep-angle fault zones with maximum dislocations of 20 m within a coal seam.

1. INTRODUCTION

Lignite is one of the most important energy resources. In central Europe Tertiary coal deposits are excavated in opencast sites reaching depths of several hundred meters. The planning and procedure of excavation requires an a priori knowledge of the fine structure and layering, including the existence and extension of possible fault zones. So far, the exploration technique used consists of drilling a dense grid of boreholes. As an additional exploration method, high-frequency compressional P-wave reflection profiling for coal has been suggested by Ziolkowski and Lerwill (1 979), and case histories have been reported based on high-resolution reflection (Greaves 1984) and refraction surveys (Goulty and Brabham 1984).

First successful applications of horizontally polarized (SH) shear waves for coal exploration were given by Gaertner, Hartmann and Rische (1982), Gerstenberger and Rische (paper read at the 27th International Geological Congress, Moscow,

* Received July 1985, revision accepted November 1985. ** Present address: Energy, Mines and Resources Canada, Earth Physics Branch, Ottawa.

*** Institute for Geophysics, Neue Universitaet, 2300 Kiel, FRG.

845

846 B. M I L K E R E I T , H. S T U M P E L A N D W . RABBEL

1984), and Hasbrouck (paper read at the SEG meeting, Atlanta, 1984). Besides improvement in sources during the last 15 years (Shima and Ohta 1967, Helbig and Mesdag 1982), there is an increase in knowledge of petrophysical properties of young Tertiary and Quarternary sediments, above or within the groundwater level. S-wave studies seem to provide solutions for many structural problems. The range of P- and SH-velocities, Poisson’s ratios and densities for sand, clay and coal is shown in table 1. Due to acoustic impedance contrasts at the top and the bottom of coal seams strong reflections can be expected. Low S-wave velocities of 180-280 m/s within the coal and a central signal frequency of only 20 Hz can lead to wavelengths as short as 10-15 m. For comparable resolution, P-waves require frequencies of about 50-170 Hz depending on the Poisson’s ratio (see table 1). In our experiments

Table 1. Range of physical parameters of sediments in shallow depth (0-200 m) compiled from Stiimpel et al. (1984), Hager, Kothen and Spann (1981), Schon (1983), Gerstenberger and Rische (paper read at the 27th International Geological Congress, Moscow, 1984).

Maximum v, (m/s) Minimum for water (independent

for dry saturated of water Poisson’s Density sediments sediments saturation) Vp/V, ratio

Sand 1 .&2.2 400 2800 200-500 2-5.5 Clay 1.8-2.3 300 2500 100-600 3-4.2 0.33-0.49 Lignite 1.0-1.5 400 1800 180-280 2.2-6.4

P- and SH-velocities were checked by in situ measurements. Refraction profiling was carried out on top of a coal seam in an open mine. Additionally, a three- component vertical seismic profile (VSP) demonstrates the vertical resolution that can be achieved with surface exploration methods.

In hydrocarbon exploration the comparison of S- and P-wave data is used to obtain additional information about petrophysical parameters (e.g., porosity). Often, the S-wave data are collected either by separate measurements (Ensley 1985) or together with a P-wave survey using converted SV-phases (Dohr and Janle 1980, Fertig 1984). We demonstrate that for shallow-target horizons an SH-reflection profile alone provides sufficient structural information.

In this study the shear waves were generated with a light transportable source which was developed at the Institute of Geophysics at Kiel University together with THOR Geophysical Prospecting GmbH (see Meissner, Stumpel and Theilen 1985). A piston is accelerated with high pressure to run alternatively against the right or left side of a cylinder generating SH-waves. This is followed by vertical stacking of single recordings with change in polarity.

SH REFLECTION PROFILING 847

2. I N SITU ESTIMATION OF SEISMIC V E L O C I T I E S I N LIGNITE

Since the knowledge of interval velocities is essential in reflection seismic interpreta- tion, two experiments with refraction profiles and a VSP were conducted to gather in situ velocity information in sand layers and coal seams.

2.1 Refraction survey

On an 80 m thick coal seam we carried out an SH- and P-wave refraction survey. The total profile length was 190 m with 4 m geophone spacing. The P-wave record section is shown in fig. l a (vertical-source and geophones). In spite of vertical

( a ) (rn) I20 100 80 60 40 20 0

0

T (rns)

250

50C

Fig. 1. (a) P-wave refraction profile along a coal seam; (b) SH-wave refraction profile along a coal seam.

848 B. M I L K E R E I T , H. S T U M P E L A N D w. R A B B E L

stacking P-wave energy dies out after 120 m. This indicates a material partially saturated with water with a high absorption of P-waves (Meissner et al. 1985, Muckelmann 1985). The time-distance curve indicates a strong velocity gradient from 400 to 800 m/s. Figure l b shows the SH-record section (SH-source and SH- geophones). Each trace is normalized to its maximum amplitude. The overall hori- zontal velocity is 180 m/s. This represents the lower limit of known velocities in lignite (table 1). In general the S-wave recording shows a less complicated wavefield. This experiment yields VJV, ratios in the range of 2-6 for dry to partially water- saturated lignite.

2.2 Vertical seismic projile ( V S P ) In situ velocity information from within the strata can be obtained from a VSP. A three-component borehole geophone (one vertical component and two perpendicu- lar horizontal components) gathered the information from a P-wave and an SH-wave source, respectively, at various depths. The P-wave seismogram is a display of the vertical component of the borehole geophone (fig. 2a). The SH- recording (fig. 2b) is represented by the absolute amplitude values of both horizontal components. No attempt was made to determine the exact orientation of the hori- zontal components. The P-wave records show a slight increase in traveltime with depth, i.e., high velocities within the lignite, whereas the S-waves show larger trav- eltimes, indicating low S-wave velocities. From traveltimes picked at adjacent geo- phones interval velocities can be estimated. Figure 3 summarizes borehole geological information and the calculated velocity-depth distribution. Beneath the overburden (sand and gravel), lignite occurs from 32 to 68 m depth followed by a sand layer.

0 250 T (ms)

20

30

40

50

60

I-- *U ' - I

70

80

Fig. 2. Vertical seismic profile: (a) P-wave recording; (b) SH-wave recording. As the azimuth of the horizontal components could not be observed, H = ,/m is plotted.

SH R E F L E C T I O N P R O F I L I N G 849

Fig. 3. Geological log and P- and SH-wave velocities corresponding to the record sections of fig. 3.

Figure 2 clearly demonstrates that S-waves provide higher resolution than P-waves. The traveltime within the coal seam is 25 ms for P-waves, but it reaches 160 ms for SH-waves. Sand and gravel have S-velocities of 30&500 m/s, S-velocities in lignite are from 180 to 250 m/s. The analysis of the P-wave data gives a com- pletely different result. In dry or partially saturated sands P-wave velocities are 60&900 m/s. There is no pronounced velocity change at the sand/lignite boundary, but there is a strong increase in velocity to values larger than 1500 m/s beneath 40 m depth due to water saturation. The degree of water saturation does not influ- ence S-wave velocities. Therefore, in contrast to the P-wave data, the velocity-depth distribution of S-waves represents the lithology parameters and not the saturation state.

Our in situ measurements clearly show low S-wave velocities in coal (V, < 250 m/s). Coupled with reduced density (table 1) high acoustic impedance contrasts can

be expected.

3. SH-REFLECTION SEISMIC MAPPING OF LIGNITE DEPOSITS An S-wave reflection survey was carried out in a sedimentary basin where detailed geological information is available from boreholes. Tertiary-lignite deposits of varying thickness are embedded in sand and clay layers. Superimposed extensional tectonics has resulted in steep-angle fault zones.

3.1 Gathering of shallow SH-reflection data Acquisition and processing of shallow S-wave data differs significantly from stan- dard procedures :

850 B. M I L K E R E I T , H. STUMPEL A N D w. R A B B E L

- frequencies ranging from 12 to 50 Hz and low SH-velocities require fine horizontal-data sampling with geophone-group spacing of a few meters, to avoid aliasing ;

~ small two-way traveltimes and low stacking velocities of shallow reflectors pro- hibit stacking large offsets to avoid wavelet distortion due to stretch effects resulting from normal moveout (NMO) correction;

-lateral variation of S-wave velocities can be determined from first breaks of refracted arrivals. The computation of static corrections demands either the knowledge of a deep refractor (Wiest and Edelmann 1984), or the analysis of surface wave dispersion (Mari 1984). However, for shallow targets the refraction method is not promising and in regular SH-reflection surveys dispersive Love waves are suppressed by effective shot- and/or receiver patterns.

In our experiment a geophone group spacing of 4 m and a weighted 9-geophone pattern was used in order to suppress the Love waves. The maximum source- geophone offset was 120 m. We obtained a 6-fold coverage of common midpoints along a profile of 1150 m length. Figures 4 and 5 display two (24-channel) SH- record sections at shotpoints 250 and 180 separated by 150 m. Trace amplitudes are both normalized (figs 4a and 5a) and displayed with a short-window automatic gain (AGC; figs 4b and 5b). Reflections can be recognized at 0.3 and 1.1 s. Application of AGC to this data set shows coherent low-frequency noise which has been induced by mining activity. Since the signal frequencies are centered at 20 Hz, this noise could be reduced by bandpass filtering and extensive vertical stacking in the field.

0 20 40 60 ( m ) 0 20 40 60 (rn) I I I I

SP 250

Fig. 4. Example of SH-wave record without muting: (a) normalized amplitudes; (b) ampli- tudes displayed with short-window AGC.

S H R E F L E C T I O N P R O F I L I N G 851

0

I

T ( 5 )

2

0 20 40 60 (rn) 0 20 40 60 (m)

SP 180 P

Fig. 5. Example of SH-wave record without muting. (a) normalized amplitudes; (b) ampli- tudes displayed with short window AGC.

3.2 Laterally varying stacking velocities

The record sections of figs 5 and 6 are typical examples for the strong variation of near-surface SH-velocities. Even for closely spaced shotpoints the first arrivals yield different velocities (380 m/s and 330 m/s). For P-waves (V, > 1500 m/s) a velocity variation of 50 m/s corresponds to a variation of only a few percent and has almost no influence on the stacked seismogram. However, for S-waves (V, z 300 m/s), the

0 30 (m) 60 30 (rn) 60 30 (rn) 60

Fig. 6. CMP-stack: (a) Stacking velocities 20% too low (-65 m/s); (b) Optimum stacking velocities; (c) Stacking velocities 20% too high ( f 6 5 m/s).

852 B . M I L K E R E I T , H. STUMPEL A N D w. R A B B E L

500 300 I00 CDP

500 450-n ~ ' 0 . 5 ~ 400

350 8

300

LlVL

T - I s 350

300

V(rn/s) (rn)

500 250 0 I +

Fig. 7. Display of optimum stacking velocities at 0.5 and 1 s.

same velocity variation can easily result in errors of more than 20%. To demon- strate the sensitivity of S-wave stacking to small velocity changes in early part of the section, we changed the optimum stacking velocities by _+20%. The influence of such a velocity error on the stacked section is shown in fig. 6.

For static corrections the near-surface velocities are calculated from first breaks and the traveltimes are reduced to a datum plane where lateral velocity variations are minimized. The observed strong variations of S-velocities would, therefore, demand a datum plane at great depth, which is in conflict with the objectives of shallow exploration. Laterally varying thicknesses and the low S-wave velocities of the shallow coal deposits provide additional problems,

In this experiment stacking velocities decrease with increasing two-way time due to the presence of thick coal seams. Variable velocities in sand, clay and gravel and variations of the thickness of coal seams have to be taken into account. The optimum stacking velocity distribution could only be obtained by trial and error. Figure 7 shows the variation of the optimum stacking velocity at 0.5 and 1.0 s, respectively, along the profile. On the left side the higher stacking velocities are caused due to a change in composition of the overburden. Lower velocities on the right side indicate thick coal seams. The strongest decrease in stacking velocities is observed next to CDP 250.

3.3 Final stack and interpretation

Figure 8a shows the final stack using the velocity distribution of fig. 7. Figure 8b shows our interpretation with coal seam reflectors, reflected refractions (dashed) and

S H R E F L E C T I O N P R O F I L I N G 853

1 -

T ( ' )

2-

500 400 300 200 100 0 CDP

I000 800 600 400 200 0 (rn) 0 1 '

A - -/ .,,." , ,' I ..

,,, , '-:,< I

( b )

--

Fig. 8. (a) CMP-stack: lignite structure with normal faults; (b) interpretation of (a).

- 0

150

-- 300

diffractions (dotted). Steep-angle dislocations within the layers are indications of major normal fault zones. The top of the lignite (A) and the locations of faults can be mapped despite the low 6-fold CDP-coverage. Also, the bottom reflection (B) related to the base of the coal seam can be identified particularly along those parts of the profile where the top of the coal seam is not affected by faulting (CDP 75-350). Figure 9 shows a detailed section of fig. 8a. This part of the profile (CDP

0 50 100 150 (rn)

Fig. 9. CMP-stack. Detail of fig. 8a with interpretation.

854 B. M I L K E R E I T , H. S T U M P E L A N D w. R A B B E L

190-310) shows the lowest stacking velocities (fig. 7) as well as continuous strong reflections. The depth scale is computed, using an average velocity of V, x 300 m/s. Diffracted energy is shown by dots. Our interpretation of the events are also given. The strong reflectors A and B represent the top and bottom of the coal seam. The dip of the fault is approximately 80" with a vertical displacement of nearly 20 m. Reflections between A and B may indicate a change in the sand content of the coal. The quality of this record section could be further improved byf-k-filtering. On the other hand, the filtering would cancel the reflected refractions. A higher CDP- coverage would reduce the noise level significantly. Processing this dataset using ray-tracing procedures for the partially disturbed top of the coal might be helpful to reconstruct the bottom contact along the whole profile.

4. CONCLUSIONS Shear-wave velocities as low as 180 m/s were recorded for shallow Tertiary lignite deposits. In water-saturated sediments, S-waves provide a higher resolution of the subsurface structure than P-waves. Since S-waves are not affected by the degree of water saturation, large impedance contrasts will lead to strong reflection coefticients. In view of the low S-wave velocities, stacking of reflection data should take lateral inhomogeneities into consideration.

For exploration at shallow depths these inhomogeneities have to be corrected for by applying laterally varying stacking velocities. High resolution S-wave reflec- tion surveys in the upper 50-200 m depth can provide valuable additional informa- tion, e.g., for - exploration of lignite, peat, limestone etc., - stratigraphic extrapolation of existing borehole information, - detection of shallow fault zones and their amount of dislocation, - calculation of average shear moduli for engineering purposes.

ACKNOWLEDGMENTS We are grateful to R. Meissner and A. Green for critical comments on the manu- script. This study was partially sponsored by the Bundesministerium fur Forschung und Technologie of the FRG, grant: 03-R-246, and Prakla-Seismos GmbH, Han- nover. The authors wish to thank the management of Rheinbraun AG for support- ing the field work and permission to publish the data.

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