Geophones on a boardD. W. Steeples*, G. S. Baker, C. Schmeissner, and B. K. Macy, The University of Kansas, Lawrence

Summary

We examined the feasibility of using seismic reflectionsto image the upper 10 m of the earth’s subsurface quicklyand effectively by attaching geophones to a wooden boardat 5-cm intervals. The shallow-seismic-reflection infor-mation obtained was equivalent to control-test data gath-ered using classic, single-geophone plants with identical5-cm intervals. The results were surprising: We foundlittle intergeophone interference in response to our use ofa high-resolution, .22-caliber-rifle seismic source at off-sets of a few meters. Furthermore, we noted very little dif-ference between the 60-ms intra-alluvial reflection ob-tained from the standard geophone plants and the reflec-tion obtained from the board-mounted geophones. Ampli-tude spectra were nearly identical for geophone data col-lected with and without the board. The results suggestthat deploying large numbers of closely spaced geo-phones simultaneously—perhaps even automatically—may be possible. Should this method of planting geo-phones prove practical after further testing, the cost effec-tiveness of very shallow seismic-reflection methodsmight be improved. The “board” technique also may beuseful at greater depths and in broader applications, e .   g . ,petroleum exploration, in which geophones are some-times bunched together tightly.

Introduction

Seismic-reflection methods can be useful when analyzingvery-near-surface geology at some locations. However,the expense of shallow subsurface seismic imaging maybe prohibitive when shotpoint and geophone intervals ofonly a few centimeters are required to maintain the coher-ency and distinctness of recorded shallow reflections.Hence, in an effort to develop a fast and cost-effectivemethod of deploying large numbers of closely spacedgeophones for use in seismic-reflection imaging, we con-ducted experiments in which 12 geophones were attachedfirmly to a wooden board at 5-cm intervals (Figure 1a).The geophones did not interfere with each other exten-sively, and the presence of the board did not substantiallydistort useful seismic signals. As a result, we were able toobtain shallow-seismic reflections that were comparableto control-test data gathered using classic, single geo-phones planted at identical 5-cm intervals.

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Figure 1. Photos of (a) 12 geophones mounted ona 66.7-cm-long board, and (b) the board-mountedgeophones, pictured near the center of the left-handgeophone line, when deployed at the test site.

Our experiments were designed to determine whethernumerous geophones could be deployed rapidly, at thesame time, while maintaining good coupling to theground and ensuring negligible interference between geo-phones. We collected experimental data at two sites nearLawrence, Kansas. Because the experiments undertaken atthe second location were more comprehensive than thoseat the first, we include here data gathered from the secondsite only; however, we found the results from the two sitesto be comparable. A .22-caliber rifle and a 30.06 riflewere used as energy sources. Because results from the twosources were similar, we present only the .22-caliber-rifledata here.

Geophones on a Board

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Figure 2. Pseudo-walkaway field file for the .22-rifle source, without board-mounted geophones, displayed with a 25-msAGC window and a band-pass filter from 300-400 Hz with 12 dB/octave slopes. The receiver interval is 5 cm. The promi-nent reflection at 60 ms is from a clay-sand interface at a depth of ~ 9 m.

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Figure 3 . Pseudo-walkaway field file for the .22-rifle source, with board-mounted geophones, displayed with a 25-msAGC window and a band-pass filter from 300-400 Hz with 12 dB/octave slopes. The traces produced by the board-mountedgeophones are denoted by brackets at the bottom of the figure. Note that the static shifts at these locations are a function ofthe shorter (8 cm) spikes used with the board-mounted geophones.

Geophones on a Board

Field Methods

We collected data in the alluvial valley of the KansasRiver near Lawrence, Kansas. The valley floor is com-posed of 20 m of clay and gravel alluvium overlying al-ternating beds of Pennsylvanian limestones and shales,typically only a few meters thick. Previous boreholecheckshot experiments conducted at this site showed thetwo-way traveltime for a reflection from bedrock to be82 ms; intra-alluvial reflections have also been detectedat this location (Steeples and Knapp, 1982).

We placed two parallel lines 20 cm apart, eachconsisting of 48 geophones (Figure 1b). To ascertainwhat effects, if any, the board might have on thegeophone plants and on the recorded data, we used theline of geophones without the board as an experimentalcontrol. On both lines, Mark Products L-40A, 100-Hzgeophones were positioned at intervals of 5 cm andequipped with spikes 12.5 cm long, except on the boarditself, where 8 cm spikes were used. The method ofattaching the 12 geophones to the board follows.

A board of solid birch 5.5 cm wide, 2.0 cm thick, and66.7 cm long was used, with 12 geophones attached toit at intervals of 5 cm. The line of geophones wasaligned parallel to the grain of the wood. First, thegeophones were screwed into 9.5-mm (3/8-in)

NF-threaded nuts welded to the heads of 9.5-mm NF-threaded bolts 4 cm long. Next, the bolts were inserteddownward into the board through 10 mm drillholes andfastened with 9.5-mm NF threaded nuts. Geophonespikes 8 cm long were then screwed onto the ends ofthe bolts.

The first experiment with the board-mounted geophonesunderscored the difficulty of pushing 12, 12.5-cm-longgeophone spikes simultaneously into firm ground.Therefore, in the second experiment, we used spikes only8 cm long. Overall deployment became much easierduring the second test, and the effect of the board itselfremained negligible. However, we noticed that theshorter spikes produced receiver statics of about+0.5 ms.

Experimental Data

Walk-away noise tests were performed on each geophoneline. The geophone configuration remained fixed as thesource was moved away, in 2.4-m increments, from oneend of each line. To remove source variation from thedata comparisons, data were recorded simultaneously onboth lines. The source used in Figs. 2 and 3 was a com-mercially available, single-shot, .22-caliber rifle withthe tip of the barrel placed about 10 cm below the sur-face of the ground. On the closest offsets, subsonic

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Frequency-Component Comparison for .22-rifle Source

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Figure 4. Response comparison of standard-plant versus board-mounted geophones, at different frequency ranges,for the .22-rifle source. Traces from the 12 unmounted and 12 board-mounted geophones are from 6.65-m to 7.25-msource-to-receiver offsets. The doublet event arriving at 60 ms is the intra-alluvial reflection from the clay-sandinterface at a depth of ~ 9-m.

Geophones on a Board

.22-caliber short ammunition was used to avoid clippingdata. At offsets of 3 m or more, supersonic .22-caliberlong-rifle ammunition was used to increase the S/N ratio.

A prominent intra-alluvial reflection is visible at~ 60 ms (Figure 2) for the band-pass-filtered data re-corded on the line without the board. This reflection i sfrom a clay-sand interface at a depth of ~ 9 m; i. e. ,~ 1 m below the water table. Data recorded using theboard-mounted geophones can be seen in Figure 3, inwhich we used the same band-pass filter as in Figure 2.A slight phase variation can be seen in the arrivals atthe geophones attached to the board, which is consistentwith the static delay associated with the shorter spikesused on these geophones.

To provide a more detailed 1:1 comparison of the data,we plotted only the 12 traces obtained from the board-mounted geophones and the comparable 12 traces ob-tained from standard geophone plants. Figure 4 showsthe data comparison for the .22-rifle source with shot-to-geophone offsets of 6.65 m to 7.25 m, with four differ-ent band-pass filters. The fundamental question i swhether the reflection data are comparable. Specifically,the reflection noted at ~ 60 ms is essentially identicalfor the two data sets, regardless of the filter passbandused. Some minor differences in ground roll are evident,particularly with the higher frequency passbands.However, we did not examine the differences in groundroll closely, as they were not a primary concern at thispoint in our research.

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Figure 5. Frequency spectra comparison for theboard-mounted geophones versus the standard-plant geophones. The spectra are averaged overthe same 12 traces used in Figure 4.

Variations in geophone deployment can affect frequencyresponses as well as data amplitude. Figure 5 presentsamplitude spectra, with and without the board, for the 12comparable traces of data used in Figure 4. Note thatfrequency variations in the data are negligible for theboard-mounted geophones when compared to thestandard-plant geophones.

C o n c l u s i o n s

Conventional wisdom—or myth—might predict that theboard-mounted geophones used in these experimentswould interfere with each other as a result of their firmconnection to the board. However, this was not gener-ally the case; in fact, the wave modes identified withinthe board were expected to be much more pronouncedthan they proved to be.

The ramifications of these results may be significant tothose interested in performing shallow-reflection sur-veys in which very small geophone intervals are needed.For example, an apparatus might be devised that coulddeploy large numbers of geophones very quickly, and ifgeophones were affixed permanently to a board or otherrigid medium, then electrical wiring also could beattached permanently. Instead of connecting each geo-phone to a master cable equipped with standard takeouts,geophones outfitted with individual pairs of wires couldbe attached several at a time to a master cable. A smallall-terrain vehicle equipped with hydraulically controlledgeophone “planters” might even be envisioned. On sucha vehicle, geophones would be wired permanently to acable connected to an onboard seismograph. This wouldresult in a significant decrease in the time required todeploy receivers, thus greatly increasing the efficiencyof shallow-seismic surveying.

Note that these results may not be applicable to all sitesor situations, as larger energy sources might induceinterference between geophones and could produceundesirable modes of motion within the rigid medium.

Acknowledgments

This work was supported in part by the U.S. Departmentof Energy under Contract DE-FG07-97-ER14826 and theNational Science Foundation under Grant EAR97061218.

References

Steeples, D. W., and Knapp, R. W., 1982, Reflectionsfrom 25 feet or less: 1982 Ann. Internat. Mtg., Soc.Expl. Geophys., Expanded Abstracts, 469-471.