The term sinkhole receives a lot of use, and equal amountsof abuse, in the popular media. Generally, anytime a hole ordepression forms in the land surface, sometimes in a short periodof time, it is called a sinkhole. Sinkholes are geologic features formedby movement of rock or sediment into voids created by thedissolution of water-soluble rock. Some sinkholes, such as theinfamous Winter Park, Florida, sinkhole of 1981 (Figure 1),capture the attention of society as we view expensive homesand automobiles teeteringon a precipice about to dis-appear into the underworld.Subsidence features causedby other processes, such asmine collapse and washoutsresulting from broken waterand sewer mains, are not truesinkholes, but may be equallyas damaging. These featuresalso result from rock or soilmoving into a void, but thevoid was a result of humanactivities.

This paper compares theeffectiveness of various geo-physical techniques in lo-cating these two types ofsubsidence features. For thepurposes of this paper, bothtrue sinkholes and anthro-pogenic subsidence featureswill be referred to as sink-holes in keeping with the ver-nacular usage.

Anthropogenic subsidencefeatures. Some of the morecostly sinkholes, in terms ofresource and financial loss, golargely unnoticed. For exam-ple, the Retsof salt mine nearCuylerville, New York (USA)collapsed in March 1994 dueto weakened pillar design ata depth of over 335 m in twominor portions of the mine.Two sections of the mine col-lapsed providing a pathwayfor the massive aquifer of theoverlying Genessee River val-ley sediments to access theopen mined section and theremaining unmined sections.The subsequent flooding intothe mine approached 76 m3

per minute. This event low-ered groundwater levels from15 to 30 m in local wells andover 125 m in the area of thecollapse. The collapse and

flooding resulted in abandonment of this, the second largestsalt mine in the world, plus destruction of farmland, highways,and bridges as the collapse worked its way to the surface(Figure 2). This event had an impact on the national supplyof road salt and created substantial losses of jobs, but it is notwell known outside of the local community or the miningindustry. Geophysical surveys, including self potential andseismic reflection, were employed in the early stages after the

initial collapse to define thearea of significant downwardwater flow, collapsed featuresat depth, and affected regionsin the overburden.

Similar sinkholes involv-ing salt have happened inLouisiana, when an oil explo-ration well drilled into anopen subsurface salt mine(Jefferson Island salt dome)and emptied a surface lakeinto the mine, and in sectionsof Kansas where leaky brinedisposal wells have dissolvedsalt beds and generated sur-face collapse.

Loss of property and in-frastructure is not uncommonin towns such as RockSprings, Wyoming (USA)where collapse of coal-mineworkings has resulted in sink-holes. In such areas, geo-physical surveys, mainlyseismic reflection, have beenemployed to map existingunderground workings,thereby aiding hazard delin-eation studies.

Compare these to hun-dreds of times each yearwhen the nightly commutehome is blocked off becausea backhoe operator broke awater main, and the roadwayis diverted around the “sink-hole.”

Sinkholes and Florida. Forthe remainder of this paper,we will focus on examplesdemonstrating the state ofgeophysical practice in foren-sic evaluation of sinkholepotential within a well-known karst (limestone) envi-ronment of the UnitedStates—namely Florida. InFlorida, concerns for publicsafety and minimizing the

Geophysical applications to detect sinkholes and ground subsidenceTHOMAS L. DOBECKI, Shaw Environmental & Infrastructure, Tampa, Florida, USASAM B. UPCHURCH, SDII Global, Tampa, Florida, USA

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Figure 1. Aerial photograph of the 1981 (limestone) sinkhole in WinterPark, Florida. This collapse feature was approximately 100 m in diameter,30 m deep, and formed in a matter of hours.

Figure 2. Photograph of edge of surface collapse/sinkhole feature formednear Cuylerville, New York over a collapsed salt mine section approxi-mately 335 m below land surface.

potential threat of sinkhole damage to commercial and resi-dential properties has fueled the search for noninvasive toolsthat could be used to warn the public of an imminent sub-terranean threat.

Florida is famous for its limestone bedrock platform andmany impressive sinkholes. Many of the large, round sparklinglakes found scattered throughout Florida owe their origins tocarbonate dissolution activity. The combination of geologicallyyoung and shallow limestone, mobile sandy soil overburden,and abundant groundwater yields an almost perfect recipefor limestone dissolution, subsidence, and sinkhole formation.Referring again to Figure 1, it is obvious that sinkholes canform rapidly and without concern for current land use.

Because residential construction rarely includes thor-ough subsurface testing, many homes and small commer-cial structures are constructed over ancient sinkholes(paleosinkholes) that have been buried and obscured bysubsequent depositional cycles of marine terrace deposits.In addition, the limestone is soluble and often acidic shal-low groundwater continues the process of dissolution andformation of new voids in the limestone. Because of theseprocesses, the Floridalegislature has includeda statutory requirementthat homeowner’s in-surance include a provi-sion that coverssinkhole-related dam-age. The statute cur-rently defines sinkholeactivity as the move-ment of soil into voidscaused by dissolution inthe underlying lime-stone. It goes on to de-fine sinkhole loss asdamage to the insuredstructure as a result ofsinkhole activity. There-fore, losses due to sub-sidence associated withmines, washouts, andother human causes aregenerally not covered.

There are manyother ways in which adepression can form ora house may experiencedamage from move-ment of earth materials.For example, holes,depressions, and/ordamage can be relatedto heaving by tree roots,decaying or compress-ing organic matter (peatsoils, buried wood, de-caying tree roots andstumps), raveling ofsoils into broken sewerlines or septic tanks,leaking or poorly con-structed water wellsand swimming pools,buried construction de-bris, shrinking/ swel-ling clay soils, leaking

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Figure 3. Photograph of a residential-scale sinkhole. The portable drillingrig seen within the depression was in the process of drilling/testing soilswithin a GPR anomaly area near a home when the drilling activitycaused sudden ground subsidence.

Figure 4. Example illustrat-ing the sequence of develop-ment of a cover-collapsesinkhole in Florida.

water lines or irrigation pipes, very loose natural soils, andanimal burrowing.

Estimates of monetary damages caused by sinkholeactivity are difficult to quantify because much informationconcerning property damage and payouts is proprietary. A2002 investigation by Florida State University reported onan analysis of 877 Florida sinkhole claims filed between1996 and 2001. Average payout per claim was US$62 628. In2004, one of the largest of the insurers in Florida reported pay-ing out about $6 million in sinkhole claims, and it estimatedthat overall owner compensation for sinkhole claims could beas high as $25 million in 2004.

In 2005, the Florida Legislature rewrote the law requiringsinkhole insurance. Previously, geophysical investigationswere not required, but they had become “industry accepted”investigation techniques. In 2005, the statutes specified cer-tain investigation protocols, including geophysical investiga-tions, to identify anomalous conditions that might reflectsinkhole activity. Occasionally, the type, location, and degreeof damage makes defensible identification of sinkhole activ-ity simple (Figure 3), but these cases are the exception and notthe rule. Ground truth confirmation of the components of asinkhole (as defined) is required, but blind drilling across aproperty is fruitless because of the low probability of drillinginto such a potential sinkhole feature as well as the possibil-ity of breaching confining layers and exacerbating sinkhole

development. Drill testing should be minimized and per-formed within anomalies interpreted from geophysical find-ings. In Figure 3, we see a portable drilling rig that has falleninto a ground depression adjacent to a home. The depressionformed during the drilling of an anomalous feature detectedduring a geophysical survey. Geophysical surveys provide animportant link in the chain of tests that will determine sink-hole activity or relate damage to some other cause.

First, geologic evidence must support the assumption thatwe are in a geologic locale that is conducive to sinkhole gen-eration (e.g., shallow carbonates overlain by mobile sedi-ments). Given that, there are three characteristics under Floridastatutes that must be present before a site can be associatedwith sinkhole activity (Figure 4):

• There must be a void volume in the limestone that is capa-ble of accepting sediments.

• There must be evidence of material raveling downwardinto such a void.

• There must be damage consistent with sinkhole activity,or a surface depression, and/or evidence that a depres-sion is forming.

The first and third criteria are of secondary importance ascompared to evidence of raveling. First, it is almost impossi-ble to drill a borehole in Florida and not eventually encounter

338 THE LEADING EDGE MARCH 2006

Figure 5. Two-dimensional GPR transect showing significant depression and discontinuity of a deep clay horizon and, to a much lesser extent, distur-bance of overlying clastic sediments. Limestone bedrock (not observed) underlies this section approximately 30 m below land surface.

a void — voids are ubiquitous so theydo not define a sinkhole.

Certain types of damage to a struc-ture can be construed as an indicationof ground movement. Therefore, afiled damage claim identifying spe-cific types of structural damage rep-resents a testable argument that thesurface is moving or has moved.

Subsurface evidence of raveling,therefore, is generally the most impor-tant criterion to be resolved. The finalword on raveling is determined bygeotechnical testing (i.e., standard pen-etration tests or cone penetrometersoundings), but the key role in apply-ing geophysics is, as usual, determin-ing the proper or optimal locations forsuch tests.

The role of geophysical investiga-tions. “Raveling” is the process bywhich water transports soil down-ward into voids/cavities in underly-ing strata. The ravel zone is, therefore,a volume of soil that is looser (higherporosity) and has less subsequent strength than nonraveledsoils. So the targets for subsurface testing are areas wheresoil layers are depressed, discontinuous, or truncated andwhere there is a volume of loose or low-strength soil. So,methods that are sensitive to changes in density, porosity,and strength are ideal for identification of areas for sub-surface testing.

The most commonly used and recommended geophysi-cal surveys for the investigation of potential sinkhole activityinclude ground-penetrating radar (GPR), two-dimensionalelectrical resistivity tomography (ERT), and seismic methods(refraction tomography, reflection, surface wave inversion).Microgravity surveying has been used for sinkhole detectionbut, because of the ease and success of the other mentionedtechniques, little has been done in Florida. The following willdescribe the application and advantages/disadvantages ofeach method using real data examples.

GPR. GPR is the overwhelming geophysical method ofchoice for sinkhole investigations. This stems from Florida’sfamous dry, high-resistivity surficial sands, which translateinto relatively deep GPR penetration (typically 5-10 m). GPRprovides the opportunity for dense data coverage (includ-ing 3D coverage), high resolution, and very good penetra-tion with quick turnaround and relatively low cost. Anotherplus is that GPR profiles can be acquired within some struc-tures by shooting through the floor slab. Reinforcing steel(rebar) limits the range of antenna frequencies (typically >400MHz) that may be used to shoot through the slab. Difficultiesarise when a site has shallow clay or shallow, salty ground-water. However, when the target of the investigation is toestablish evidence of subsidence, the presence of clay is lessof an issue than might be expected. If all we can image isthe top of the clay and it can be shown that the clay is mov-ing or is locally disrupted due to raveling, the GPR sectionstill provides useful information.

A ravel zone can also be viewed directly on GPR sec-tions as a near-vertical zone of discrete scatterers and localincreases in reflection amplitudes in the shallow sectionbecause of localized increases in soil porosity. Also, if fines(clay) are removed by the raveling process, such featuresare sometimes identified by a localized interval of increased

GPR signal penetration due to the reduced electrical con-ductivity/attenuation.

The GPR section of Figure 5 exhibits a depression in astrong reflection. This GPR reflection correlates with a thinclay layer identified in local drill holes. The clay horizonshows a substantial depression, but we also note that over-lying soil horizons exhibit only minor disturbance. Multiple,orthogonal GPR transects over the same general area of thesingle section enabled mapping of the top of this clay hori-zon (Figure 6). The surface map of this single layer exhibitsa significant depression that has a quasi-circular footprint,which are key indicators that this anomalous area is a poten-tial buried sinkhole structure even though the ground sur-face is quite level. The apparent, undisturbed nature of theshallower soil horizons suggest that this may be an inactive(paleo) sinkhole, but follow up geotechnical drilling wouldbe required within the feature to determine its potentialactivity. However, geophysical survey results ensure thatsuch invasive testing is performed in optimum locations.

ERT. 2D electrical resistivity tomography profiling (sur-face electrode arrays) is also used commonly for sinkholeinvestigations as a means of identifying the ravel zone and,under ideal circumstances (shallow limestone and large dis-solution features), the underlying void or cavity. The ravelzone is characterized by increased porosity and reduced per-centage of fines. Depending on depth to the local ground-water table, the ravel zone can either be a high-resistivityanomaly (if dry) or a low-resistivity anomaly (if saturated).Deeper void space is typically characterized by a low-resis-tivity feature indicative of carbonate materials being replacedby looser clastic sediments or by water. Of course, an air-filled void would generate a high-resistivity anomaly fea-ture, but this would not constitute a sinkhole (by legaldefinition) because it has not collapsed or accepted overly-ing sediments.

ERT, in the context of sinkhole exploration within resi-dential or commercially developed properties, is generallymore affected and, therefore, influenced by cultural noisesources (utilities, piping) and the limited areal space avail-able for surveying (considering the maximum dimension ofa typical residential lot may be 20 � 30 m or less and the

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Figure 6. Surface representation of the top of the samedeep clay horizon in Figure 5 as constructed from anorthogonal grid of GPR transects. The depression seenin the section view exhibits a nearly circular pattern inplan view, also suggesting sinkhole conditions.

adjacent house may be only 3 m away). Also, space limita-tions mandate ERT lines be acquired so close to houses orstructures that offline (3D) effects of foundations and swim-ming pool structures influence the readings and can pro-duce false anomalies. In spite of these limitations, at leastone major insurer in Florida suggests a combination of GPRand ERT be used for all its sinkhole claim investigations.

ERT sections acquired in a karst area but not in an urban-

ized environment clearly display thetypes of anomalies routinely encoun-tered and confirmed as sinkholes(Figure 7). In the area where thesesample data sets were acquired,dolomitic carbonates are known toexist in the upper 10 m, and expo-sures (quarries) in the area showsignificant karst development (dis-solution features, pinnacles). TheERT sections show a mantle of lower-resistivity silts and clayey sandsoverlying higher-resistivity dolo-mite. Anomalous areas are observedwhere we see penetration of lower-resistivity materials into the dolomiteindicating potential dissolution fea-tures that have been filled with surfi-cial sediments or areas withextensive weathering of the dolomite(e.g., along fractured volumes).

ERT sections acquired in anurban setting (within 4 m of a housefoundation) clearly possess lowersignal-to-noise in comparison withsections acquired in undevelopedareas (Figure 8). We note the veryhigh-resistivity surficial sands (acommon Florida feature) and ananomalous area near the 11-m markon the left side of the section wherewe see a thickening/deepening ofthe high-resistivity layer, possiblyindicative of raveling. However, thesection between 9.1 and 27.4 m isconsistent with where the ERT tran-sect abuts against the footprint of thehouse (also indicated on Figure 8),and anomalous areas coincide withthat interval. This, too, is a commonobservation when conducting ERTadjacent to a house foundation.Anomalies generally occur at the cor-ner of a foundation. This suggeststhat the house itself, in the exampleof Figure 8, could be influencing theinversion results (3D effects on a 2Dinversion process). However, com-plementary GPR profiles in the samearea plus a visible depression thatwas forming beneath the slab of thehouse (both shown on Figure 8) sug-gest that this ERT anomaly is actu-ally a raveling anomaly. Subsequentdrilling confirmed this interpreta-tion. This example also indicates whymultiple data sets and complemen-tary views of the subsurface are nec-essary for proper interpretation in

a developed urban setting.Seismic methods. The key feature of a ravel zone that

makes it an attractive target for seismic surveying is the loos-ened, weak nature of the sediments relative to adjacent,undisturbed soils. This makes a ravel zone an anomalousarea where reduced seismic velocities are commonlyexpected and observed. In areas of shallow water tabledepths, this would mean that shear-wave-based seismic

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Figure 7. Two examples of electrical resistivity tomography (ERT) images acquired in a rural settingwithin a karst geologic environment. The dolomitic bedrock is identified with the higher (>1000 ohm-m)green-to-red resistivities and the clayey soil mantle by the bluer shades (<50 ohm-m). Note in bothexamples the apparent breaches in the bedrock indicated by finger-like projections of lower-resistivitymaterials into the bedrock.

Figure 8. Example data sets (ERT and GPR) acquired in close proximity to a house. The ERT shows apossible ravel zone anomaly (circled) near the 11-m mark; however, that location also corresponds to thecorner of the house foundation (indicated above the ERT section). The GPR section shows a broaddepression plus a discontinuity in a deep reflector that correlates with the position of the ERT anomaly.A void area beneath the house slab (photograph) found in the same general location confirms the inter-pretation of sinkhole conditions.

methods would be particularly sensitive to the detection andmapping of such loose soil volumes. Further, because of thetypically large velocity contrast between the soils and car-bonate bedrock, seismic methods are also very good at map-ping depressions on the bedrock surface.

The most common seismic method utilized for sinkholedefinition has been seismic refraction tomography (SRT)using S-waves, when possible. Tomographic processing ofthe refraction data set is required because the ravel zone willrepresent a local, lateral variation/anomaly in the shear-wave velocity field (and, to a lesser extent, the P-wave veloc-ity field if fully saturated) that is poorly handled by moreclassical ray-trace interpretation methods. Figure 9 is a P-wave refraction tomography section acquired not in Floridabut in Texas where a road failure occurred associated withdrilling near a salt dome structure. The section clearly showsdevelopment of a pair of lobe volumes of reduced strength/lower-velocity materials that are extending downward intomore competent, higher-velocity (redder) materials. Theposition and magnitude of the surface collapse is indicatedby the depression in the surface topography.

A unique characteristic of refraction tomography that isenticing as a future development is broadside shooting, orundershooting, where geophone spreads are extended alongone side of a house and the seismic sources are deployedalong the other sides. This would enable generation of avelocity map under the house. Invasive testing through floorslabs can be done (minidrills or cone penetrometer systems),but methods are limited as to where they can be applied, andthey are very damaging to the floor, as one would imagine.So this potential noninvasive means to look under houseswould be an attractive development. Undershooting wouldalso offer an advantage over borehole-to-borehole seismictomography because it would sample in a horizontal planeand thereby maximize the chance of shooting through a ver-tical, low-velocity column. Such a vertical column could falloutside of the vertical plane defined by the two boreholes.Also, boreholes are always expensive (relative to the cost ofgeophysical surveys of this type) and represent a potentialpathway or breach in a confining layer that should be avoidedwhen possible.

Another seismic technique that has had limited applica-tion but offers great promise for defining ravel zones is theMASW (multichannel analysis of surface waves) technique.

This technique also maps S-wave velocity variation but hasthe additional capability of detecting and mapping velocityinversions (decreases with depth) and, so, may have a uniqueability to detect incipient ravel zones (i.e., sinkholes in theformative process that have yet to reach land surface).

Reflection surveys have typically been applied, on a spo-radic basis, to the deep sensing of depressions on the top ofrock surface or detecting cavities within the rock mass itself.With the development of tomographic imaging techniquesthat will better define structure in the overlying sediments,there may be increased application of reflection methods forsinkhole delineation.

Summary. Sinkholes, then, regardless of their underlyingcause, represent hazards that can be extremely costly and dis-ruptive to society in general, are ubiquitous in where theyoccur, and happen routinely. Geophysics provides importanttools that can be used to:

• Predict where and within what limits sinkholes are likelyto occur (planning; risk assessment).

• Determine the underlying cause of an existing or form-ing subsidence or depression (forensic evaluation).

• Evaluate the success or failure of ground improvementprograms (grouting, compaction, mechanical stabiliza-tion) intended to “fix” sinkhole conditions (remediation).

The sinkhole remediation aspect is taking on renewedemphasis as communities and agencies realize that the sink-hole often offers a rapid, high-permeability pathway to intro-duce surface contaminants into the groundwater system. Sowhile the surface depression offers a tempting area to dis-pose of old vehicles and trash, there are severe consequenceslurking within such activities.

Suggested reading. “Sinkholes, west-central Florida” byTihansky (in Land Subsidence in the United States, USGS Circular1182, 1999). “The Retsof Salt Mine Collapse” by Kappel et al.(in Land Subsidence in the United States). Mitigating Losses fromLand Subsidence in the United States (Commission on Engineeringand Technical Systems, National Academy Press, 1991). TLE

Corresponding author: tom.dobecki@shawgrp.com

MARCH 2006 THE LEADING EDGE 341

Figure 9. P-wave refraction tomography section acquired adjacent to a ground depression/sinkhole that formed adjacent to a salt dome structure inTexas. Interpreted ravel zones are indicated by the two lobate features of decreased seismic velocity.

Copyright © 2006, The Society of Exploration Geophysicists