ENWRONMENTAL UPDATE

REVIEW: GEOPHYSICAL METHODS FOR KARST DETECTION AND MAPPING

By T. L DOBECKI lNTRODUcmN

b~meationofkarst and karst-related features ieatDsk which figures in numerous economic and safety related enterp&. For example, recent collapse of homes and businosaesinto sinkholes inOrlando. Fbrida have provided wecatchins headlines and maior financial 1- to owners a d insu&e companies. ~ u ~ c e s f u l p r ~ n of~ndplent collapse zones would ease a lot of human sdferina. In related circumstances, investigation of karst development may be mandated bv Federal redation t e a foundation stubies for nuclear power stations). ~ro&&ly the most ubiquitous requirement for karst mapping isin relation to its conaiderabie inflwnceongrormdwater - both froma supply standpoint and its potential tor disiributing hazardous wasie3.

Sucecssjul application of geophysics to the pmblems of detecting and mapping karst features depends upon our abilitv to define the tamet (Mi cbaracterktb) and

these to geophysid meth& rhat are h i i d i to those spedfic characteristics. The thrust of this article will be to review, by mefhod, ways in which karst features may be ddineated and to prwide some examples and reference materials for ilIushation.

For anv geophysical target, the greater the contrast, the better the chance of detection. A void often ~mducesa maximum contrast; shear zones somewhat I& In one sense, detecting karst features should be a relatively easy task because they differ Lrom the high seismic velociiy and pwr elecMcd conductivity of the SLwO~dlng, unaltered

If we consider the N o r types of karst faaturea we can determine which characteriaics they have that make them

attractive wph& targetg. Table 1 prouides a general htimof targets, partkukr physical characteristics of each type, and most sensitive -physical method for those characteri&cs.

SEISMIC METHODS The presence of voids and fractures in a rock mass

significantly affects the way in which scSsmic waves have1 through the rock. In texms of transmission, waves are dowed, attenuated, and sometimes e l i i t e d by passing through a void or densely fractured rack A void/rock interface creates a significant acoustic impedance contrast whieh resulta in large reflection coefficients (nearly 100% of the seismic energy is reflected off the void). The m t u d e d the seismic effect, indeed withany geophysical measure. ment,isverymuchafunctionof thesizeofthekaratfeature. Therefore, there are practical Gmitsas to havsmalla targel can beand sfill be dofined geophysically. As wave propa* tion methods usually offer the best h w of auchresolutim, ssismic teehniquesare among the betterfordc6nlnsemdlor targefs.

lb fh t i i . Seismic reflection is the standard gee physical technique in dl exploration and is onky recently becaning popular in shallow, engineeringandgroundwater invarr@ations. Voids/caves r e k t eeimnic e w back toward the surface where it is detected. in such circum skances, the seismic event fhdicative of a karst feature is recognized as either a) a large arnplihule, relatively wide reflection ( f ~ r a large cave), b) a di&adon hyperbola (for a small cave or feature), c) a disruption of an otheMZse continuous retledion event (bedrock surface or sedi- mmtary layer), ord) an area whew &per reflections are hkhly attenuated or disrupted. 'Ihe last event occurs because voids reflect incident energy with such m t efficiency that the energy available to k s m i t through the

TABLE 1. Li.Lina of karst targ.t. and geophysld void to interrogate seater depths is greatly minimhed dur r t c rb t i c s . Few case histories are avaibble d e a h awdicaUv with

seismic reflection for llmestone d i ~ s o l ~ n ieatures: One study considers bedrock diswlutia, beneath an earth-fio dike suffering from seepage problems (Dobecki, et el., 1989 Fiiure 1 is a wrth of a shallow seismic ref*etion section showing bedrock dissolution features linked to fractu~faultimg of the s d d e limestone. Other studies deal with sinkholes related to salt dis.soIurion [Steeples, et al., 19861, abandoned coal mine wo- Wbecki, 1989; Steeples and Mikr. 1987). and tunnel detection (Dobeeki and E)aW, 198%.

In general, seismic reflection is Wter at detedim discrote karst targets likecaves or side and lessapplicable to detecting fracture zones. This is changing, however, owing to oilfield rsaearch which is utilizing shear wave

Fmre 1 Shallow seismic reflection section showing bedrock surface anddissolution features associated with mapped faults (from Dobeck et al.. 1989).

bireftinsence (splittins into fast and slow components) which ate 1) shallow, 2) large and3) air-fined The first two when such a wave passes through a fractured rock mass points are valid for any geophysical method; the latter is (Martin and Davis. 198n. Whensuchanalusea are oerfected wrticukrlv true for aravitv as it msximiues the densitu and mxkd d& w i t ~ k d y be &y app6priate to. s i~s tu~Uesinka3:s t delbrpatton.

R*Seismk~&actiQR,.while:beinsa mainmainspay of engineerinsand groundwater geophysfcs, 'is not a m d method lordiict dete&n of k-t features. M& karst featureshave-lowseismii: velocity, whkhrefradion ten& to ekip.wer initsmost classicd ap$fice@n. Howevei, ~efrac- Hon methods can bo- employed with eat^ succ08s in mapping bedrock~surface; Vkarst.featurs~are developedori this htwface; then rehctfon can .man them. instance where. relractioti $.an be. u d when the ref rdse ismic wavemustpass-throu& the karat feature. ,htb.- delamin t h s d a f z h e r e f r a c t e d ~ e a n indicate the karst fedture. When cdkpeo of a cavern accurs, sisnifieant modffication of' the ovefburden is induced. Sub*tanW fiaeturing. results in a.bd&ed lower- ing of the ~ a w r b d e n veLocify or localieod horizontal

wlocity. h, too, can bedetacted by refraction methods as an indirect moans of cavitv detection (Dobecki 1989).

Anovelseisrnicmethod ~ o r h h g & d w a t d & in a karst re$bn is neither dection nor &action, but most aim& to m i c r o o a t t h q ~ k e ~ l o s g r . Armdjelovie(l969) placed small timebombs into flowins water in a cavern swtem. Thebombs ware set todetonate at varied intervals. A surface a r w of earthquake seismometers recorded the blasts and. using earthauake location alsorithma located the bbsts wfrkt-enabled tracing out thehoRow p a b n and caw system.

MlCROGRAMn AND MAGNEnCS MicrogmVitV. The ma6.s deficiency caused by b y e -

ins dense limestone with a wid or void/fill alters the local gravity Cild. Using mkmgmtty to map karst feature8 has been fai& popular in Europe (Colley, 1963; Neumann. l%7l but less frequently applied in the United States (Arzi. 1975; Butler, 1984). The method is mort effective for voids

eonhsst k t h the Ii&st&e. Fairly sephiitioatd dak processing and filterins are required to extract the fairly small void anon-& due to a cave - especially if the region is one of substantial topographic relief. This precludes gnrvity from being a real-time method which enables making Iceation judgementsin the field F i r e 2 isa pracessedand

a CAVE

Fire 2 Residual microgravity map over a knawn cave system (after Butlw, 1983).

filtered gravity map over a series of caves in Florida. The gravity low over the cave is apparent.

Magnetics. Use o f magnetics for karst exploration is not intuitively obvious. The hypothesis suggesting mag- netics might be useful is that clay f i l l in fractures, sinks, or cavities typically has a higher magnetic susceptibility than limestone. This would not be an overwhelming effect and, in the case of a dry, air-filled cavern, might have no effect at all. This concept has been tested (Butler, 1983) and has not proven to be very successful. However, special circum- stances (e.g. unusually high concentration of paramagnetic minerals in fi l l ) might make a specific site very appropriate for magnetic investigation.

ELECTRICAL/EM (ELECTROMAGNETIC) METHODS

These methods which rely on contrasts in electrical resistivity, are probably the most frequently used geophysics for karst delineation. Fill, such a s water and/or clay in karst features is electrically conductive while intact limestone is poorly conductive. O n e of the other attractive features of electrical/em methods is the variety of ways in which they can be applied. While quantitative interpretation of almost all electrical,!em data is fairly complicated, the qualitative interpretalion is more straightforward and, with particular methods, rather simple.

Standard Galvanic Resistivity. These methods employ physical contact (via stakes, for example) with surface materials while inducing electrical currents to flow in subsurface layers. These generally work well (e.g. Dutta, 1970; Vincenz, 1968), albeit at a relatively slow pace, inareas of fairly conductive surficial soils.

An interesting variation on standard galvanic surveying is the misc-a-la-masse method. Here, the current is forced to flow in the conductive target by emplacing one current electrode into the target mass (water-filled cave, for example) at a point where it can be reached (an opening or an intersecting borehole). By causingmuchof the current to flow within the target, a n investigator can trace out the target by mapping induced voltages at the surface along a grid of lines (e.g. Chandra et al., 1987; Sheriff, 1989).

Self Potential (SP). Water flowing in a confined conduit, like a clayey fracture set , generates an electro- kinetic voltage which can be measured at ground surface. This has been an effective way of mapping seepage from reservoirs and has been used in India (Chandra et al., 1987) for mapping solution channels through pumping of a karst water supply well and inducing flow in the recharge system.

Passive EM. These methods employ electromagnetic waves (radio frequency, generally) generated by huge transmitters located around the world for submarine (naval) commun~cations (the "VLF" method). These waves travel out radially from the transmitters and are modified (tilt, phase shift, e tc . ) when they pass over conductors in the subsurface. The field gear is light, single operator portable, rapid, and cheap. There is a preferred orientation of the strike of the target with respect to direction from site to the transmitter which means the VLF may be good for certain strikes and poor for others, and this is not something which can be controlled by the operator.

Controlled Source EM. These are the more typical (and more expensive) em systems in which the em field is controlled (frequency, type, orientation) by the operator making them more flexible than the VLF. They are still quite portable and rapid; many offer direct, hard copy output from the device such that near real-time interpretations (qualitative) can be made while still in the field. As an example of such an em survey over karst features, Figure 3 shows the distinctive pattern in em response over a clay- filled chimney in Germany.

t 5 0 lnphase 3555 Hz -

9'.

,- - - - J -- .--- - _ _

-40 Outphase 3555 H z - '

ELECTROMAGNETIC SECTION

6 3 s o l 1 GEOLOGIC SECTION

r l C l a v

Shale

L l m ~ s l o n e o 1 7 1 l o o m

Figure 3 Electromagnetic profile showing resonses over a clay-filled chimney and normal fault zone (after Vogelsang, 1987).

Radar. This method is popularly viewed a s an electro- magnetic analog of seismic reflection; both show similar types of reflection time sections and features "look" the same o n each. Both, however, are quite different in terms of resolution (radar can see smaller targets) and penetration depth (seismic wins hands down!). The method has been applied in both cross-borehole (Kaspar and Pccen, 1975) as well a s surface (Ballard, 1983) modes. These have been successful, from the surface, at detecting voids at several tensof feet deeponly if the surface soilsare poor conductors (dry and sandy). When they d o work, radar methods probably have the best resolution of any geophysical method (with, correspondingly, the least amount of penetration).

SUMMARY

Geophys~cs should never be viewed as a substitute for exploratory boreholes. It should always be viewed as a cost effective means of cutting down on the total number of boreholes required to a d e q ~ ~ a t e l y complete an explorat~on

Houston Geoloqra l Society Bu lcl ln Janudry 1990

program. Geophysics can be fooled by "geologic noise" (e.g. a clay filled voidmay have the sameapparent effect as a clay lens in the overburden) and requires companion boreholes toa) calibrate the geophysics so it can betrusted over a site and b) verify the geophysical interpretation. For karst delineation, electrical/em is probably the best overall method simply because of itscost and itseffectiveness in a wide variety of of applications. Close behind are gravity and seismic methods. Magnetics may have occasional, special- ized application.

Much information can be gained if part of the karst system is accessible (through a chimney or intersecting borehole). Even if boreholes fail to hit a given target, additional geophysics can determine where the next bore- hole should be placed to intersect the anomaly.

A number of fairly expensive and uncommongeophysi- cal surveys have been described for void detection (e.g. cosmic ray tomography and induced void resonance) but I have purposely limited thisdiscussion to familiar and readily available technologies.

REFERENCES Arandielovic, D., 1969, A possible way of tracing ground

water flows in karst: Geophysical Prospecting, 17, 404-418.

Arzi, A.A., 1975 Microgravity for engineering applications: Geophysical Prospecting, 23,408.425.

Ballard, R.F. Jr.. 1983, Electromagnetic (radar) techniques applied to cavity detection: Tech. Report GL-83-1 (Report 5 of a series), Geotech. Lab. US Army Engineer Waterways Exper. Station, Vicksburg, MS.

Butler, D.K., 1983, Microgravimetric and magnetic surveys - Medford Cave Site, Florida: Tech. Report GL-83-1 (Report 1 of a series), Geotech. Lab. US Army Engineer Waterways Exper. Station, Vicksburg.

Butler, D.K., 1984, Microgravimetric and gravity gradient techniques for detection of subsurface cavities: Geo- physics, 49, 1084.10%.

Chandra, P.C., Tata, S., Raju, K.C.B., 1987, Geoelectrical response of cavities in limestones - an experimental field study from Kurnool district, Andhra Pradesh, India: Geoexploration, 24,483-502.

Colley, G.C., 1963, The detection of caves by gravity methods: Geophysical Prospecting, 11, 1-9.

Dobecki, T.L. and Balch, A.H., 1987, Seismic reflection applications to the dormant tunnel detection problem: unpublished report prepared for US Army Engineer Waterways Experiment Station by Department of Geophysics, Colorado School of Mines, Golden.

Dobecki,T.L.,Mueller,T.L.,andSavage,M.B., 1989, High- resolution seismic reflection investigations at Beaver Dam, Arkansas: Tech. Report REMR-GT-10, USArmy Engineer Waterways Exper. Station, Vicksburg.

Dobecki, T.L., 1989, Seismic velocity anomalies due to stress concentrations above shallow voids: The First Break, 7, (in press).

Dutta, N.P., 1970, Detection of solution channels in lime- stone by electrical resistivity method: Geophysical Prospecting. 18, 405.414.

Kaspar. M. and Pecen, J., 1975, Detection of caves in karst formation by means of electromagnetic waves: Geo. physical Prospecting, 23, 611-621.

Martin, M.A. and Davis, T.L., 1987, Shear wave birefrin- gence - a new tool for evaluating fractured reservoirs: Geophysics The Leading Edge, 6, 22-28.

Neumann, R., 1967. La gravimetrie de haute precision a ~ ~ l i c a t i o n aux recherches de cavites: Geo~hvsical . . . . Prospecting. 15, 116134.

Sheriff, R.E., 1989, Geophysical Methods: Prentice Hall, En~lewood Cliffs. N.J.. 605 D.

steeples, D.M., ~ n a p p , R.w., and McElwee, C.D., 1986, Seismic reflection investigations of sinkholes beneath Interstate Highway 70 in Kansas: Geophysics. 51, 295~301.

Steeples, D.M. and Miller, R.D., 1987, Direct defection of shallow subsurface voids using high-resolution seismic reflection techniques: Second multidisciplinary conf. on sinkholes and the environmental impacts of karst, A.A. Balkema, Boston.

Vincenz, S.A., 1968, Resistivity investigations of limestone aquifers in Jamaica: Geophysics, 33,980-994.

Vogelsang, D., 1987, Examples of electromagnetic pros- pecting for karst and fault systems: Geophysical Pros- pecting, 35.604-617.

T. L. DOBECKI-Biographical Sketch

Dr. Thomas L. Dobecki is President of LCT, Inc. of Houston and Denver. He received a BS (Physics). MA (Geology), and PhD (Geophysics) all from Indiana Uni- versity. Sincecompletion of his schooling, Dr. Dobecki has worked, primarily, as a geotechnical and groundwater geophysicist, having been Chief Geophysicist for D'Appolonia Consultants, Staff Research Geophysicist at Sandia National Labs, and Associate Professor of Geo- physics at Sandia National Labs, and Associate Professor of Geophysics at the Colorado School of Mines prior to joining LCT. Professionally, he has served as Chairman of the Engineering and Groundwater Geophysics Committee of the SEG (Society of Exploration Geophysicists) and was recently invited by SEG to author a state-of-the-art assess- ment of engineering and groundwater geophysics for the SEG 50th Anniversary volume. Dr. Dobecki serves as a technical reviewer for Geophysics and Groundwater. His primary areas of research interest are shallow, high reso. lution seismic and potential field applications.