Butler1984 Cavities

7/30/2019 Butler1984 Cavities

1/13

SICS. VOL. 49, NO. 7 (JULY 1984); P. 1084-1096, 23 FIGS.

and gravity gradient techniquesdetection of subsurface cavities

K. Butler*-_ __ -- _- __ .._

.ABSTRACTMicrogravim etric and gravity gradient surveying

techniques are applicable to the detection and delinea-tion of sha llow subsu rface cavities and tunnels. Twocase histories of the use of these techniques to site inves-tigations in karst regions are presented. In the first casehistory, the delineation of a shallow (_ 10 m deep), air-filled cavity system by a microgravim etric survey isdemonstrated. Also, application of familiar ring andcenter point techniques produces derivative maps whichdemo nstrate (1 ) the use of second derivative techniquesto produce a residual gravity m ap, and (2) the abilityof first derivative techniques to resolve closely spaced orcomplex subsurface features. In the second case history,a deeper (-30 m deep), water-filled cavity system isadequ ately detected by a micrograv ity survey. Results ofan interval (tower) vertical gradient survey along a pro-file line are presented in the second case history; thisvertical gradient survey successfully detected shallow(< 6 m) anomalous features such as limestone pinnaclesand clay pockets, but the data are too noisy to permitdetection of the vertical gradient a noma ly c aused by thecavity system. Interval horizontal gradients were deter-mined along the same profile line at the second site, anda vertical gradient profile is determined from the hori-zontal gradient profile by a Hilbert transform techn ique.The measured horizontal gradient profile and the com-puted vertical gradient profile compare quite well withcorresponding profiles calculated for a two-dimensionalmodel of the cavity system.

~~-- __ -- -----BACKGROUND

Detection and de lineation of subsurface cavities is one of theequently cited applications of microgravim etry. Cavities

lled, water-filled, or filled with s ome secondarymaterial. A poten tial field method, su ch as gravimetry

represent a magne tic polarization contrast), is well suited forthe detection and delineation of cavities; whe reas cavities pres-ent a very difficult objective for detection by other geophysicalmethods (Franklin et al., 1980; Butler, 1977 ). Solution cavitiesare just p art of the geologic complexity to be expected in karstregions, and microgravimetry is an invaluable complem ent toother geophysical, geologic, and direct methods for site investi-gations in such areas.

Butler (1980) reviewed case histories of subsurface cavitydetection investigations by Arzi (1975) Neumann (1977), andFajklewicz (1976). The work by Arzi and Neumann involvedmicrogravim etric surveys which delineated karstic cavities andabandoned mines, respectively; while the work by Fajklewiczinvolved the use of a tower structure to me asure interval verti-cal gradients for the detection of shallow (< 15 m) abandonedmines. Although Fajklewicz reported an impressive anom alyverification record, his paper generated considerable dis-cussion. Much of the negative reaction to the work of Fajkle-wicz came from accuracy and precision claims for his datawhich se emed to be inconsistent with the accepted accuracy(f 20 uGa1 ) of the Sharpe gravimeter which he used.

A research program was initiated in 1976 at the U. S. ArmyEngineer Waterways Experiment Station to investigate geo-physical methodologies for detection and delineation of subsur-face cavities. The work was conducted in three phases:

(1) assessment of geophysical methods for cavity de tec-tion at a man-ma de cavity test site (Butler andMurphy, 1980);

(2) assessment of geophysical methods for cavity de tec-tion at a shallow (5 10 m), air-filled, natura l cavitytest site, Medford Cave, Marion Coun ty, Florida(Butler, 1980, 1983; Ballard, 1 983; Curro, 1983;Cooper, 1983); and

(3) assessment of the most prom ising g eophysical meth-ods, identified in phase 2, at a deeper (- 30 m), w ater-filled cavity test site, Mana tee Springs, Levy Cou nty,Florida (Butler et al., 198 3).

One of the conclusions of this work is that, for investigationsrequiring detection and delineation of shallow cavities ( 6 4 to 6effective cavity diameters in depth), microgravimetry is themost promising surface method in most cases.

received by the Editor January 11, 1983; revised manuscript received January 16, 1984.Army Engineer Waterways Experiment Station, P.O. Box 631, Vicksburg, MS 39180.was prepared by an agency of the U.S. government.

1084


2/13

1085ravity Detection, Subsurface Cavities

FIG. 1. Cavity map, survey grid, and borehole locations at the Medford Cave site.

In this paper, results of microgravimetric surveys at theMedford Cave an d Man atee Springs test sites are presented.Details of site characteristics, topographic survey procedures,microgravim etric field procedures, data collection procedures,etc., are presented in the references given und er the phase 2 and3 descriptions of the research program . T he p resentation herewill concen trate on the aspects of work at the two sites relatedto gravity-gradient measurem ents and/or de terminations. Me d-ford Cave is a complex, three-dimensional (3-D) system; thusgravity gradient methods were restricted to analytical determi-nation by the familiar ring and center point techniques, albeiton a very dense grid of stations. In the vicinity of the microgra-vimetric survey, the Man atee Springs cave system can be con-sidered an approxim ately two-dimensional (2-D) feature. Thusat Manatee Springs, interval vertical and horizontal grad ientswere determined along a profile line approximately perpendicu-lar to the axis of the main cavity system , and p rocedures for

calculating vertical gradient profiles from horizontal gradientprofiles by application of a discrete Hilbert transform, valid for2-D cases,are investigated.

MEDFORD CAVE SITE INVESTIGATIONS

Scope of microgravimetric survey

The microg ravity survey at Medford Cave site consisted of420 stations over a 260 by 260 ft (approximately 80 by 80 m)area. A b asic grid dimen sion of 20 ft (6.1 m) was used, with aIO-ft (3-m) grid used in the central portion of the area over theknown cavity system. A LaCoste and Romberg m odel D-4

Grid and profile dimensions for the two test sites are in feet. Gradi-ents are converted to mGal/m.


3/13

Butler

FIG. 2. Cross-section cavity maps of the Medford Cave site.

eter was used for the survey. Figures 1 and 2 presentnd cross-section views of the known cavity system , and3 is the site topographic map.

Grid point (0, 0) was selected as the base station an d wa s

the field base station drift curve.base station drift data . The long-term, cum ulative

, although the re are nontidal meter drifts larger than

selective drilling of small negative anomalies in a reas awayfrom the known cavity system intercepted a ir- or clay-filledcavities or clay pockets in the top of the limestone. Elevenboreholes were located in positive anom aly areas, and onlythree of these boreholes intercepted cavities (Z2 ft in verticaldimension). Figure 7, for example, compares a gravity profile

The data w ere processed and corrected using the proceduresned in Butler (19 80) and Bu tler et al. (1983). A density ofg/cm w as used for the Boug uer and terrain corrections

es required terrain corrections > 10 uGa1. Figures 5 andset and for all the data (includ ing 10 ft grid data),

nspection. Correlation of gravity anom alies with features ofcavity system as shown in Figure 3 is excellent, and

model-D gravimeter has a sensitivity to gravity change or vari-approximately 1 PGal and an accuracy of k4 pGal in theof a single relative gravity value using exacting field FIG. 3. Topographic map of the Medford Cave site.


4/13

Gravity Detectfon, Subaurffme Cavltles

FIG. 4. Drift curve and measuredearth tide curve for the Medford Cave site microgravimetricsurvey.

along a north-south line (the 8O W line) with a geologiccross-sectionalong the line determin edby closelyspaced xploratorydrilling. Th e co rrelation of gravity lows and highs with claypocketsand limestonepinnacles, espectively,n the 110 to 260ft profile range s quite good.Gravity gradient maps

Two typesof gravity-gradient maps were generated rom theMedford Cave site microgravity survey data. The familiar ring

and ce nter point (spatial filtering) techniqueswere utilized tocompute first (vertical gradient) and second derivative map sfrom the gravity data. These techniqueswere used or this sitefor two reasons: 1) to investigate he application of the tech-niques o small-scalesurveys o r improved resolution and thedetermm atron of residual gravity m aps; and (2) beca use heknown cavity sys tem is clearly threedimensiona l. Since thetechniques re familiar and standard,detailsabout their formu-lation and usewill not be given.The secondderivative map in Figure 8 was produced using

FIG. 5. Residual gravity anomaly map, 20 ft data spacing, FIG. 6. Residual gravity anomaly ma p, 10 ft data spacing,Medford Cave site microgravimetricsurvey. Medford C ave sitemicrogravimetricsurvey.


5/13

Comparison of the 80W north-south residual gravityprofile with the known geologic cross-section.

due to Elkins (1951). This technique is sometimesas the Elkins residual method, since it is designed to

ap closely resembling a residual gravity map. Usea ring at rr = a = 20 ft (6.1 m) introduces a second derivative

ng with coefficients chosen to smo oth high spatial fre-

Second derivative map (Elkins residual) produced fromBouguer anomaly data from the Medford Cave site survey. FIG. 9. First derivative map produced from the Bouguer anom-ur interval = 1 (arbitrary un its). aly data from the Medford Cave site survey. Contou r inter-val = 1 (arbitrary units).

quencies; while a second ring at r4 = & a = 44.7 ft (13.6 m) isused to approximate a local regional field for the center point.The contour values in Figure 8 should be considered in arelative sense with a rbitrary units.3 Com paring the secondderivative m ap in Figure 8 with the residual gravity m ap inFigure 5, the similarity is evident. All of the primary features ofthe residual gravity map can be found in the second derivativemap. The second derivative technique is a more objective pro-cedure than the inspection or graphical techniques, and it canbe advantag eously app lied to microgravity survey results whenit is difficult to recognize the proper scale regional field.

Figure 9, the vertical gradient or first derivative map, wasproduced using an equation due to Baranov (1975). The equa-tion does not have coefficients chosen to produce smoothing asin the second derivative equation. Thus , in principle, the firstderivative map should have greater resolution than the secondderivative an d residual gravity map. The contour values inFigure 9 should be considered in a relative sense with arbitraryunits.4 All of the anom aly features identified on the residua lgravity map can be seen on the first derivative m ap; however,the spatial ex tent of given an omalies is generally less on the firstderivative map than on the residual gravity map. Also, someanomalies observed as single features on the residual gravitymap seem to be resolved into two or more features on the firstderivative m ap, such as the negative anomaly between 80N and180N in Figure 5 along the eastern boundary of the surveyarea.

3As emphasized by one reviewer, this procedure produces only a verypoor approximation to the true second vertical derivative due to thestrong smoothing involved in the filter operator. Thus the secondderivative map should be used only for the location of anomalies inplan and not for any type of quantitative interpretation.Strictly speaking,, first derivative units, the Eotvos (E), can be ob-tained by multiplymg contour values by 18.31365.


6/13

Gravity Detection, Subsurface Cavities 1089

FIG. 1 0. Com parison of residual gravity (g,), first derivative (g:),and second derivative (9:) profiles along the 0 north-south line.

In order to compare and evaluate the features of the deriva-tive and residual gravity ma ps, two north-south profile lineswere selected for study. The 0 north-south profile line waschosen due to the interesting negative anomaly centered at(110 , 0) and because it is representative of areas at the siteabou t which nothing was known prior to verification d rilling.The residual gravity, first derivative, and sec ond derivativeprofiles along the C north-south line are shown in F igure 10. Allthree profiles show.the negative anomaly feature between pro-file locations 80 and 1 80. The gra vity p rofile suggests hat theremight be two closely spaced subsurface features causing theanomaly (or at least a significant change in shape, size, ordensity contrast of the feature). The second derivative profileshows essentially the sam e information as the residua l gravityprofile. The first derivative profile, however, clearly resolves theanom aly into tw o negative anomalies centered at the 1 o- an d160-ft profile locations. Verification drilling was not extensive

FIG. 1 1. Com parison of residual gravity (g,), first derivative (gi),and second derivative (gi) profiles along the 8OW north-southline.

enough to confirm in detail the predictions of multiple subsur-face features causing the negative anomaly, but two boreholesplaced at (110 , 0) and (1 17, - 5) confirmed the presence of asignificant cavity feature at this location which varied in dimen-sion and depth laterally.

The 80W north-south profile line was discussed previously inconnection with the residual gravity profile; the gravity profileis comp ared w ith the gravity-gradient profiles for this line inFigure 11. Qualitatively, all three profiles in Figure 11 aresimilar. The smoothing inherent in the second derivative pro-cedure is evident in the subdued nature of the highs and lowscorresponding to the limestone pinnacles and clay pockets. Thefirst derivative profile in this case, however, is nearly iden ticalto residual gravity profile in delineating the top of limestonetopograph y and detecting the known cavity (see Figure 7 ).

MANATEE SPRINGS SITE INVESTIGATIONS

Scope of microgravimetricand gravity-gradient surveys

The microg ravity survey at the Mana tee Springs site consist-ed of 1 86 stations over a 100 by 400 ft (- 30 by 122 m) area witha basic grid interval of 20 ft (6.1 m). A LaCoste and Rombergmodel D-25 gravity m eter was used for the survey. The surveygrid was oriented approxim ately perpend icular to the knowntrend of the cavity system as shown in Fig ure 12. Grid point(0, 200) was used as a base station and was reoccupied on anaverage of once every 30 minutes. Details of the microgravitysurvey procedure can be found in Butler et al. (1983). In addi-tion to the micrograv ity survey, a tower ve rtical gradient surveywas conducted along the southwest-northeast line extendingfrom (40, 0) to (40 , 400) ; this survey consisted of 21 verticalgradient stations. The purposes of the tower vertical gradientsurvey were (1) to refine tower field procedures, (2) to investi-gate the utility of the results, and (3 ) to com pare the intervalvertical gradient profile with the vertical gradient profile com-

FIG. 12. Microgravimetric survey area and plan map of themain cavity, Manatee Springs site.


7/13

FIG. 13. Bouguer gravity anomaly map, Manatee Springs site.A

. . . .*ooJ. . .. ./i100.400)I,L 20p0.1the discrete Hilbert transform of an interval h orizontal

Part of the research effort at the Man atee Springs site was

as polynom ial surface fitting. Figure 13 is the Bou guermap (1.8 g/cm3 used for Bouguer and terrain correc-

or the survey area; the maximu m gravity differenced is only - 80 uGa1. A careful

to the selection of a planar regional field dipping from south-east (SE) to northwest (NW ) with a gradient of 0.22 uGal/ft(0.72 @al/m ). Subtracting this inspection regional gives theresidual map show n in Figure 14; the plan view of the cavitysystem, determined by cave divers during the course of the fieldwork, is also shown (the plan map shown in Figure 12 is thedetail known prior to the field work).The broad negative anomaly over the known cavity systemin Figure 14 is consistent in magnitude and width with theknow n size and depth of the cavity system. Howe ver, there arecomplexities or smaller anomalous features in the residual mapwhich canno t be attributed to the main cavity; som e of thesesmaller anomalies may be due to smaller and shallower solu-tion features or other density ano malies. The basic concept of

FIG. 14. Residual gravity anomaly map, M anatee Springs site.


8/13

Gravity Detection, Subsurface Cavities 1091. . . .

. .

2.0

/./

P

/ &+ // . s?(O*OSECONDRDER

.. . . . x(L\L

FIG. 1 5. First- throu gh fourth-order polynom ial surface fits to the Boug uer gravity data (see Figure 13); contour interval= 10 PGal.

the polynomial surface-fitting technique for determining re-gional fields is that successively higher order surface fits to theBoug uer anoma ly d ata accou nt for the gravity effects of suc-cessively smaller a nd shallower subsurface features (Coons e tal., 1967; Nettletoli, 1971 ). Figure 15 contains contoured poly-nomial surface fits.to the Bougu er data through fourth o rder. Itis noteworthy that, although the first-order (planar) su rface dipthrough the grid is on a different azimuth than the planedetermined by inspection, the southeast-northwest gradient isthe same, i.e., -0.22 uGal/ft. The residual anom aly map , ob-tained by subtracting the first-order surface fit, is shown inFigure 16. Further details of the surface-fitting procedure an d

features of higher order residual maps are given in Bu tler et al.(1983 ). The map of second-order residual, for example, displaysa small closed negative anomaly feature at location (100, 220)which was verified when a wheel of a drill rig collapsed a soilbridge revealing a vertical solution pipe about 80 ft deep.

Vertical gradient survey results

Using a specially adapted tripod, the five measurement eleva-tions illustrated in Figure 17 were utilized during the verticalgradient survey along the (40, 0) to (40, 400) survey line. Only

FIG. 16. First-order residual gravity anom aly map , Mana tee Springs site.


9/13

Butler

--h,

Illustration of the tower or tripod m easurem ent con-for vertical gradient determination; for the Manateeh,, h, , h, , and h, are 0,1.38, and 1.63 m, respectively.

0.27

five gravity values at the upper elevation h, were obtainedalong the profile line. The measu rement seq uence at each pro-file location required 1.5 o 25 minutes; thus the ground stationh, was reoccupied at the end of each sequence and the datawere drift-corrected in the usua l mann er.

Considering elevations h,, h,, h, , an d h, , six interval gradi-ents can be determined as well as differential gradients at anypoint within the interval h, to h, using a parabolic fittingprocedure. Results of three of the determinations of verticalgradients along the (40, 0) to (40, 400) survey line are shown inFigure 18; Agb,/Az,,, and AgbJAze3, where AgbI = go - gr,A,,, = h, - h,, etc., and (Cg/iiz),, which is the differentialgradient at h, determined from a parabolic fit to the data at h,,h,, and h,. The five values of Agb,/Az,,, are also shown. Allthree profiles exhibit considerable variation, with se veral gradi-ent anomalies as large as 10 percent of the normal verticalgradient. The Agb3/Azo3 prefile is smoother than the otherprofiles, since it is less affected by very shallow density an oma-lies (Butler, 198 4). All three profiles behave qualitatively thesame except at profile positions 0,40 to 60,200, and 3 60 wherethe Agb3/Azo3 profile behavior is clearly at variance with theother two profiles. In many locations the three values are nearlyidentical; an d at the 1 00 and 3 00 ft profile positions all fourvalues are nearly equal and also nearly equal to the normalgravity gradient, which implies a linear variation of gravitywith elevation at these locations. There are, however, no obvi-ous indications of an anomaly which could be caused by themain subsurface cavity system.Horizontal gradient determinations

Using the gravity d ata along the selected profile line, hori-zontal gradient profiles can be determined using various values

0 50 100 150 200 250 300 350 400X, FT

(DISTANCE ALONG SURVEY GRID PROFILE LINE FROM 40.0 TO 40.4001

FIG. 18. Profile of interval v ertical gradient determinations, Mana tee Springs site.

0.02

0.01

E22

0 2>Bz

-0.01

-0.02


10/13


EXTENT OF CAVITY(FIG. 14)

50 100 150 200 250 3U 3!xl 40 0X. FT

FIG.19. Profiles of interval horizontal gradient determinations, Mana tee Springs site.

of AX. Horizontal gradient profiles for Ax equal to 20, 40, and80 ft (6.1, 12.2, and 2 4.4 m) are shown in Figure 19, where theresidual gravity values from Figu re 15 (planar least-squaresregional) were used. The profiles in Figure 19 clearly becom esmoother with increasing Ax, and all three profiles showaverage behavior consistent with the known cavity systemwith center at profile position 200 ft. The Ax = 20-ft profile,however, is so erratic that the cavity gradient signature iseffectiveiy maske d. The g radient signature of rhe cavity isenhanced by Ax values which a re larger than the effectivedepths of the shallow anomalous features causing the erraticbehavior of the AX = 20-ft profile (Butler, 198 4). Accordingly,the AX = SO-ft profile data will be used for the considerationswhich follow.Comparison of results with 2-D model calculations

The cavity system was modeled as a 2-D prism with rec-tangular cross-section as shown in Figure 20 (based on cavitydetails known prior to the field work), and interval horizontaland vertical gravity gradients were computed. In Figure 21, thecomp uted horizontal gradient profile is compa red with themeas ured horizontal gradient profile for Ax = 80 ft. Theaverage behavior of the measured profile approximates thecalculated profile quite well in amplitude and spatial wave-length, with the amplitude of the measured profile slightlylarger on the right-hand side. The vertical gravity gradientg_(x, z) on the surface z = 0, due to a 2-D subsurface struc-ture, is related to the horizontal gravity gradient g_(x, z) onthe surface by a H ilbert transform (Sneddon, 1 972, Bracewell,1965),

where x is the profile point at which CJ,, is to be determined. Analgorithm for computing the vertical gradient of a discretehorizontal gradient profile data set is presented in Butler et al(1982) using a procedure suggested by Shuey (1972). A vertica

FIG. 20. Two-dime nsional model of the main ca vity at theMana tee Springs site and the calculated gravity an omaly.


11/13

Butler0.002 -

- MEASURED HORIZONTAL$ 0.001 - GRADIENT, Ag,/AX. AX=80 FTm:

---- CALCULATED HORIZONTALGRADIENT. 2-D MODEL,

K6

AX=80 FT

E2 0u -0.001 -

EXTENT OF MODEL CAVITYEXTENT OF MAPPED CAVITY

-0 002 I0 100 200 300 400

X. FT

22. Com parison of a vertical gravity gradient profile com puted as the discrete Hilbert transform HD of the measuredhorizontal gradient profile (Figure 21) with a vertical gradient profile computed for the 2-D model (Figure 20).


12/13


mGal/m0 002T - 2-D MODEL- * - FROM FIELD

DATA

1 AgJAz-0.002 0.002 mGal/m

-0.002A-FIG. 23. Com parison of gradient sp ace plots from field data an dfrom 2-D model calculations, Mana tee Springs site.

system very well. Indeed, both reports of the cave diving teamand a very limited verification drilling effort confirm that thecavity system is extremely complex. The cavity varies errati-cally in cross-sectional shape and size; a vaulted ceiling iscommon and num erous smaller branching cavities are present.Also, drilling and detailed mapping indicates more extensivesolutioning to the northea st of the (0, 200 ) to (100, 200 ) linethan southwest of it, which is consistent with both the residualgravity map and the gravity-gradient results.

Verification of drilling resultsOnly a limited n umb er of verification borings were possible,

and the borings were located to investigate various gravityanom alies as well as anom alies indicated by o ther geop hysicalsurveys and not specifically to investigate anom alies along thegravity-gradient profile line. Likewise, borings placed to ac-c~omm odate crosshoie geophysicai su rveys of various typeswere placed to the northeast of the gradient profile line for themost part. Two of the borings, however, allow direct confir-mation of vertical gradient ano malies shown in Figure 18. Theborings indicate that, typically, limestone is encountered atdepths of 13 to 17 ft, although limestone pinnacles are within 5ft of the surface in places and clay-filled pockets in the top ofthe limestone extend to depths of 27 ft in places. A boring neargradient profile position 120 ft encountered a clay pocket w hichextended to the 27-ft depth (limestone is typically encounteredat the 17-ft depth in this area); the vertical gradient profilesshow a prominent negative anomaly a t this location. Anotherboring near gradient profile position 280 ft encountered a claypocket extending to the 16-ft depth (limestone is typically en-countered at the 13-ft depth in this area); the vertical gradientprofiles show a~negativeanomaly at~tbis ocation.

The microg ravity survey of the Manatee Springs site sucessfully detected the main water-filled cavity system. Results drilling at the Manate e Springs site confirm tha t the largmagnitude, short spatial wavelength anomalies which appear ithe measured interval vertical gradient profiles are due prmarily to relatively shallow ( < 20 ft) density anomalies such aclay pockets and limestone pinnacles. The lower amplitudelonger spatial wavelength anomalies which appear in the measured horizontal gradient and Hilbert transform vertical gradent profiles are due to the deeper (> 80 ft) main cavity systemThe large amplitudes of the vertical gradients due to shallofeatures at the Manatee Springs site completely m ask anpossible expression in the measured interval vertical gradienprofile of the low amplitude anomaly due to the deeper cavitsystem. A mu ch taller tower (>20 ft in height) with lowemeasurement stations several feet above the ground would brequired to have any chance of detecting the small verticagradient anom aly caused by the cavity system.

The considerable flexibility in the selection of horizontaintervals from the Mana tee Springs survey for determinininterval horizontal gradient profiles allowed a profile to bselected which (1) appears to be free from significant perturbation due to shallow anomaious features, (2) is consistent witthe known location and general features of the main cavitsystem, and (3) compares quite well with an interval horizontagradient profile computed from an approximate 2-D model othe cavity. Using the horizontal gradient profile with an interval selected to attenu ate gra dient an omalies c aused by shallowdensity variation, a vertical gradient profile was compu ted by discrete Hilbert transform which compares satisfactorily witthe vertical gradient profile of the approximate 2-D modeWhile these results are demo nstrated for a specific case studthe procedures are general and can be applied to any featurwhich is approxim ately two-dimensional. The gradient profileproduced by this procedure can then be utilized in combinegradient interpretive procedures such as discussed by Nab ighian (1972), Stanley and Green (19 76), Hammer and Anzoleaga (I 975), and &tier et al. (198j.

The boring near gradient profile position 280 ft encountered The usefulness of interval vertical grad ient surveys, usina significant clay-filled cavity in the 90- to 105-ft depth range; towers of mana geable height (l-4 m), is primarily limited tthis is the same depth range as the known water-filled cavity to exploration for shallow targets (< 10-15 m), such as solutio

the southwest. The discovery of this clay-filled cavity featursuggests hat solution features extend considerably northeast othe known cavity system under the gradient profile line, whicis completely consistent with the gradient profile da ta in Figures 21-23.

CONCLUSIONSThe m icrogravity survey at the Medford Cave site demon

strates the capability of microgravime try to detect and delineate shallow, complex cavity systems. Fam iliar spatial filterintechniques were applied to the dense grid of gravity stations tproduce first and second vertical derivative maps. Suitabselection of ring radii and coefficients in a second derivativequation successfully produced a map which compares q uiwell with residu al gravity maps produced by the usua l regionaresidual separation procedu re. Examination of a selected profile line from the first derivative (vertical gradient) map demonstrates the greater resolving power of the first derivative profilcompared to the gravity profile.


13/13

Butleries and abandoned mines (Fajklewicz, 1976; Butler, 1980).

is no flexibility to select large vertical intervals in orderlarge gradient anom alies caused by shallow den-

variations. Also, since terrain variations produce largeon short tripod measurem ents (Fajkle-

6; Ager and Liard, 198 2), interval v ertical gradientwill b e most successful n area s with flat terrain. Thus ,vertical grad ient su rveys are not use ful, in general, for

REFERENCESA., and Liard, J. O., 1982, Vertical gravity gradient surveys:Field results and interpretations in British Columbia. Canada: Geo-physics, v. 47, p. 919-925.A., 1975, Microgravimetry for engineering applications: Geo-phys. Prosp., v. 23, p. 408425.R. F., 1983, Cavity detection and delineation research, Report5, Electromagnetic (radar) techniques applied to cavity detection:Tech. Rept. CL-83-1, U. S. Army Engineer Waterways ExperimentStation, CE, Vicksburg, MS.1975. Potential fields and their transformations in au-plied geophysics: Geoexpl. Monographs, Series 1, no. 6, Berlin,Geopublication Associates.1965, The Fourier transform and its applications:

New York, McGraw-Hill Book Co. Inc., 352 p.K., Ed., 1977, Proc. of the symposium on detection ofsubsurface cavities: U. S. Army Engineer Waterways ExperimentStation, CE, Vicksburg, MS.1980, Microgravimetric techniques for geotechnical appli-cations: Miscellaneous Paper CL-80-13, U. S. Army EngineerWaterways Experiment Station, CE, Vicksburg, MS.1983, Cavity detection research, Report 1, Microgravimetricand magnetic surveys, Medford Cave Site, Florida: Tech. Rep. GL-83-1, U. S. Army Engineer Waterways Experiment Station, CE,Vicksburg, MS

1984, Gravity gradient determination concepts: Geophysics, v.49, p. 8288832., D. K., and Murphy, W. L., 1980, Evaluation of geophysicalmethods for cavity detection at the WES cavity test facility: Tech.Rep. CL-80-4, U. S. Army Engineer Waterways Experiment Station,CE, Vicksburg, MS.

Butler, D. K., Gangi, A. F., Wahl, R. E., Yule, D. E., and Barnes, D. E.,1982, Analytical and data processing techniques for interpretation ofgeophysical survey data with special application to cavity detection:Misc. paper CL-82-16, U. S. Army Engineer Waterways ExperimentStation, CE, Vicksburg, MS.

Elkins, T. A., 1951, The second derivative method of gravity interpreta-tion: Geophysics, v. 16, p. 29950.

Butler, D. K., Whitten, C. B., and Smith, F. L., 1983, Cavity detectionresearch, Report 4, Microgravimetric survey, Manatee Springs Site,Florida: Tech. Rep. CL-83-1, U. S. Army Engineer Waterways Ex-periment Station, CE, Vicksburg, MS.Cooper, S. S., 1983, Cavity detection and delineation research, Report3, Acoustic resonance and self-potential applications_MedfordCave and Manatee Springs Sites, Florida: Tech. Rep. CL-83-1, U. S.Army Engineer Waterways Experiment Station, CE, Vicksburg, MS.Coons, R. L., Woollard, G. P., and Hershey, G., 1967, Structuralsignificance and analysis of Mid-Continent gravity high: Bull., Am.Assoc. Petr. Geol., v. 51, p. 2381-2409.Curro, J. R., 1983, Cavity detection and delineation research, Report 2,Seismic Methodology-Medford Cave Site, Florida: Tech. Rep. GL-83-1, U. S. Army Engineer Waterways Experiment Station, CE,Vicksburg, MS.

Fajklewicz, Z. J., 1976, Gravity vertical gradient measurements for thedetection of small geologic and anthropomorphic forms: Geophys-ics, v. 41, p. 10161030.Franklin, A. G., Patrick, D. M., Butler, D. K.? Strohm, W. E., andHvnes-Griffin. M. E.. 1980. Foundation constderations in siting ofnuclear facilities in karst terrains and other areas susceptibl; toground collapse: NUREGCR-2062, U. S. Nucl. Reg. Commission,Washington, D. C.Hammer, S.: and Anzoleaga, R., 1975, Exploring for stratigraphic trapswith gravtty gradients: Geophysics, v. 40, p. 256268.Nabighian, M. N.. 1972. The analytic sianal of two-dimensional mag-netic bodies with polygonal cross-secti&-Its properties and use forautomated anomaly interpretation: Geophysics, v. 37, p. 507-517.Neumann, R., 1977, Microgravity method applied to the detection ofcavities: Symposium on Detection of Subsurface Cavities, D. K.Butler, Ed: U. S. Army Engineer Waterways Experiment Station,CE, Vicksburg, MS.Nettleton, L. L., 1971, Elementary gravity and magnetics for geologistsand seismologists: Monograph No. 1, Tulsa, Sot. of Expl. Geophys.Shuey, R. T., 1972, Applications of Hilbert transforms to magneticprofiles: Geophysics, v. 37, p. 1043-1045.

Sneddon, I. N., 1972, The use of integral transforms: New York,McGraw-Hill Book Co. Inc., 539 p.Stanley, J. M., and Green, R., 1976, Gravity gradients and the interpre-tation of the truncated plate: Geophysics, v. 41, p. 137&1376.

Documents

Butler1984 Cavities