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  1. 1. The term sinkhole receives a lot of use, and equal amounts of abuse, in the popular media. Generally, anytime a hole or depressionformsinthelandsurface,sometimesinashortperiod oftime,itiscalledasinkhole.Sinkholesaregeologicfeaturesformed by movement of rock or sediment into voids created by the dissolution of water-soluble rock. Some sinkholes, such as the infamous Winter Park, Florida, sinkhole of 1981 (Figure 1), capture the attention of society as we view expensive homes and automobiles teetering on a precipice about to dis- appear into the underworld. Subsidence features caused by other processes, such as mine collapse and washouts resulting from broken water and sewer mains, are not true sinkholes, but may be equally as damaging. These features also result from rock or soil moving into a void, but the void was a result of human activities. This paper compares the effectiveness of various geo- physical techniques in lo- cating these two types of subsidence features. For the purposes of this paper, both true sinkholes and anthro- pogenic subsidence features will be referred to as sink- holes in keeping with the ver- nacular usage. Anthropogenic subsidence features. Some of the more costly sinkholes, in terms of resourceandfinancialloss,go largely unnoticed. For exam- ple, the Retsof salt mine near Cuylerville, NewYork (USA) collapsed in March 1994 due to weakened pillar design at a depth of over 335 m in two minor portions of the mine. Two sections of the mine col- lapsed providing a pathway for the massive aquifer of the overlyingGenesseeRiverval- ley sediments to access the open mined section and the remaining unmined sections. The subsequent flooding into the mine approached 76 m3 per minute. This event low- eredgroundwaterlevelsfrom 15 to 30 m in local wells and over 125 m in the area of the collapse. The collapse and flooding resulted in abandonment of this, the second largest saltmineintheworld,plusdestructionoffarmland,highways, and bridges as the collapse worked its way to the surface (Figure 2). This event had an impact on the national supply of road salt and created substantial losses of jobs, but it is not well known outside of the local community or the mining industry. Geophysical surveys, including self potential and seismic reflection, were employed in the early stages after the initial collapse to define the area of significant downward waterflow,collapsedfeatures at depth, and affected regions in the overburden. Similar sinkholes involv- ing salt have happened in Louisiana, when an oil explo- ration well drilled into an open subsurface salt mine (Jefferson Island salt dome) and emptied a surface lake into the mine, and in sections of Kansas where leaky brine disposalwellshavedissolved salt beds and generated sur- face collapse. Loss of property and in- frastructureisnotuncommon in towns such as Rock Springs, Wyoming (USA) where collapse of coal-mine workingshasresultedinsink- holes. In such areas, geo- physical surveys, mainly seismic reflection, have been employed to map existing underground workings, thereby aiding hazard delin- eation studies. Compare these to hun- dreds of times each year when the nightly commute home is blocked off because a backhoe operator broke a water main, and the roadway is diverted around the “sink- hole.” Sinkholes and Florida. For the remainder of this paper, we will focus on examples demonstrating the state of geophysical practice in foren- sic evaluation of sinkhole potential within a well- knownkarst(limestone)envi- ronment of the United States—namely Florida. In Florida, concerns for public safety and minimizing the Geophysical applications to detect sinkholes and ground subsidence THOMAS L. DOBECKI, Shaw Environmental & Infrastructure, Tampa, Florida, USA SAM B. UPCHURCH, SDII Global, Tampa, Florida, USA 336 THE LEADING EDGE MARCH 2006 Figure 1. Aerial photograph of the 1981 (limestone) sinkhole in Winter Park, 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 formed near Cuylerville, New York over a collapsed salt mine section approxi- mately 335 m below land surface.
  2. 2. potential threat of sinkhole damage to commercial and resi- dential properties has fueled the search for noninvasive tools that could be used to warn the public of an imminent sub- terranean threat. Florida is famous for its limestone bedrock platform and manyimpressivesinkholes.Manyofthelarge,roundsparkling lakes found scattered throughout Florida owe their origins to carbonatedissolutionactivity.Thecombinationofgeologically young and shallow limestone, mobile sandy soil overburden, and abundant groundwater yields an almost perfect recipe for limestone dissolution, subsidence, and sinkhole formation. Referring again to Figure 1, it is obvious that sinkholes can form 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 by subsequent depositional cycles of marine terrace deposits. In addition, the limestone is soluble and often acidic shal- low groundwater continues the process of dissolution and formation of new voids in the limestone. Because of these processes, the Florida legislature has included a statutory requirement that homeowner’s in- suranceincludeaprovi- sion that covers sinkhole-related dam- age. The statute cur- rently defines sinkhole activity as the move- ment of soil into voids causedbydissolutionin the underlying lime- stone. It goes on to de- fine sinkhole loss as damage to the insured structure as a result of sinkhole activity. There- fore, losses due to sub- sidence associated with mines, washouts, and other human causes are generally not covered. There are many other ways in which a depression can form or a house may experience damage from move- ment of earth materials. For example, holes, depressions, and/or damage can be related to heaving by tree roots, decaying or compress- ing organic matter (peat soils, buried wood, de- caying tree roots and stumps), raveling of soils into broken sewer lines or septic tanks, leaking or poorly con- structed water wells and swimming pools, buried construction de- bris, shrinking/ swel- ling clay soils, leaking MARCH 2006 THE LEADING EDGE 337 Figure 3. Photograph of a residential-scale sinkhole. The portable drilling rig seen within the depression was in the process of drilling/testing soils within a GPR anomaly area near a home when the drilling activity caused sudden ground subsidence. Figure 4. Example illustrat- ing the sequence of develop- ment of a cover-collapse sinkhole in Florida.
  3. 3. water lines or irrigation pipes, very loose natural soils, and animal burrowing. Estimates of monetary damages caused by sinkhole activity are difficult to quantify because much information concerning property damage and payouts is proprietary. A 2002 investigation by Florida State University reported on an analysis of 877 Florida sinkhole claims filed between 1996 and 2001. Average payout per claim was US$62 628. In 2004, one of the largest of the insurers in Florida reported pay- ing out about $6 million in sinkhole claims, and it estimated that overall owner compensation for sinkhole claims could be as high as $25 million in 2004. In 2005, the Florida Legislature rewrote the law requiring sinkhole insurance. Previously, geophysical investigations were 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 reflect sinkhole activity. Occasionally, the type, location, and degree of damage makes defensible identification of sinkhole activ- ity simple (Figure 3), but these cases are the exception and not the rule. Ground truth confirmation of the components of a sinkhole (as defined) is required, but blind drilling across a property is fruitless because of the low probability of drilling into 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 fallen into a ground depression adjacent to a home. The depression formed during the drilling of an anomalous feature detected during a geophysical survey. Geophysical surveys provide an important 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 that we are in a geologic locale that is conducive to sinkhole gen- eration (e.g., shallow carbonates overlain by mobile sedi- ments).Giventhat,therearethreecharacteristicsunderFlorida statutes that must be present before a site can be associated with 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 downward into 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 as compared 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.
  4. 4. a void — voids are ubiquitous so they do not define a sinkhole. Certaintypesofdamagetoastruc- ture can be construed as an indication of ground movement. Therefore, a filed damage claim identifying spe- cific types of structural damage rep- resents a testable argument that the surface is moving or has moved. Subsurface evidence of raveling, therefore, is generally the most impor- tant criterion to be resolved. The final word on raveling is determined by geotechnicaltesting(i.e.,standardpen- etration tests or cone penetrometer soundings), but the key role in apply- ing geophysics is, as usual, determin- ing the proper or optimal locations for such tests. The role of geophysical investiga- tions. “Raveling” is the process by which water transports soil down- ward into voids/cavities in underly- ing strata. The ravel zone is, therefore, a volume of soil that is looser (higher porosity) and has less subsequent strength than nonraveled soils. So the targets for subsurface testing are areas where soil layers are depressed, discontinuous, or truncated and where 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 activity include ground-penetrating radar (GPR), two-dimensional electrical resistivity tomography (ERT), and seismic methods (refraction tomography, reflection, surface wave inversion). Microgravity surveying has been used for sinkhole detection but, because of the ease and success of the other mentioned techniques, little has been done in Florida. The following will describe the application and advantages/disadvantages of each method using real data examples. GPR. GPR is the overwhelming geophysical method of choice for sinkhole investigations. This stems from Florida’s famous dry, high-resistivity surficial sands, which translate into relatively deep GPR penetration (typically 5-10 m). GPR provides the opportunity for dense data coverage (includ- ing 3D coverage), high resolution, and very good penetra- tion with quick turnaround and relatively low cost.Another plus 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 >400 MHz) that may be used to shoot through the slab. Difficulties arise when a site has shallow clay or shallow, salty ground- water. However, when the target of the investigation is to establish evidence of subsidence, the presence of clay is less of an issue than might be expected. If all we can image is the top of the clay and it can be shown that the clay is mov- ing or is locally disrupted due to raveling, the GPR section still provides useful information. A ravel zone can also be viewed directly on GPR sec- tions as a near-vertical zone of discrete scatterers and local increases in reflection amplitudes in the shallow section because of localized increases in soil porosity. Also, if fines (clay) are removed by the raveling process, such features are 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 a strong reflection. This GPR reflection correlates with a thin clay layer identified in local drill holes. The clay horizon shows 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 the single section enabled mapping of the top of this clay hori- zon (Figure 6). The surface map of this single layer exhibits a 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 the shallower soil horizons suggest that this may be an inactive (paleo) sinkhole, but follow up geotechnical drilling would be required within the feature to determine its potential activity. However, geophysical survey results ensure that such invasive testing is performed in optimum locations. ERT. 2D electrical resistivity tomography profiling (sur- face electrode arrays) is also used commonly for sinkhole investigations 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 ravel zone 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-resistivity anomaly (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 replaced by 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 legal definition) 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 generally more affected and, therefore, influenced by cultural noise sources (utilities, piping) and the limited areal space avail- able for surveying (considering the maximum dimension of a typical residential lot may be 20 ǂ 30 m or less and the MARCH 2006 THE LEADING EDGE 339 Figure 6. Surface representation of the top of the same deep clay horizon in Figure 5 as constructed from an orthogonal grid of GPR transects. The depression seen in the section view exhibits a nearly circular pattern in plan view, also suggesting sinkhole conditions.
  5. 5. adjacent house may be only 3 m away). Also, space limita- tions mandate ERT lines be acquired so close to houses or structures 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 least one major insurer in Florida suggests a combination of GPR and 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 the typesofanomaliesroutinelyencoun- tered and confirmed as sinkholes (Figure 7). In the area where these sample data sets were acquired, dolomitic carbonates are known to exist in the upper 10 m, and expo- sures (quarries) in the area show significant karst development (dis- solution features, pinnacles). The ERTsectionsshowamantleoflower- resistivity silts and clayey sands overlying higher-resistivity dolo- mite.Anomalous areas are observed where we see penetration of lower- resistivitymaterialsintothedolomite indicating potential dissolution fea- tures that have been filled with surfi- cial sediments or areas with extensiveweatheringofthedolomite (e.g., along fractured volumes). ERT sections acquired in an urban setting (within 4 m of a house foundation) clearly possess lower signal-to-noise in comparison with sections acquired in undeveloped areas (Figure 8). We note the very high-resistivity surficial sands (a common Florida feature) and an anomalous area near the 11-m mark on the left side of the section where we see a thickening/deepening of the high-resistivity layer, possibly indicative of raveling. However, the section between 9.1 and 27.4 m is consistent with where the ERT tran- sect abuts against the footprint of the house (also indicated on Figure 8), and anomalous areas coincide with that interval. This, too, is a common observation when conducting ERT adjacent to a house foundation. Anomaliesgenerallyoccuratthecor- ner of a foundation. This suggests that the house itself, in the example of Figure 8, could be influencing the inversion results (3D effects on a 2D inversion process). However, com- plementaryGPRprofilesinthesame area plus a visible depression that was forming beneath the slab of the house (both shown on Figure 8) sug- gest that this ERT anomaly is actu- ally a raveling anomaly. Subsequent drilling confirmed this interpreta- tion.Thisexamplealsoindicateswhy multiple 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 anomalous area where reduced seismic velocities are commonly expected and observed. In areas of shallow water table depths, this would mean that shear-wave-based seismic 340 THE LEADING EDGE MARCH 2006 Figure 7. Two examples of electrical resistivity tomography (ERT) images acquired in a rural setting within 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 both examples the apparent breaches in the bedrock indicated by finger-like projections of lower-resistivity materials into the bedrock. Figure 8. Example data sets (ERT and GPR) acquired in close proximity to a house. The ERT shows a possible ravel zone anomaly (circled) near the 11-m mark; however, that location also corresponds to the corner of the house foundation (indicated above the ERT section). The GPR section shows a broad depression 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.
  6. 6. methods would be particularly sensitive to the detection and mapping of such loose soil volumes. Further, because of the typically 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 sinkhole definition has been seismic refraction tomography (SRT) using S-waves, when possible. Tomographic processing of the refraction data set is required because the ravel zone will represent 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 more classical ray-trace interpretation methods. Figure 9 is a P- wave refraction tomography section acquired not in Florida but in Texas where a road failure occurred associated with drilling near a salt dome structure. The section clearly shows development of a pair of lobe volumes of reduced strength/ lower-velocity materials that are extending downward into more competent, higher-velocity (redder) materials. The position and magnitude of the surface collapse is indicated by the depression in the surface topography. A unique characteristic of refraction tomography that is enticing as a future development is broadside shooting, or undershooting, where geophone spreads are extended along one side of a house and the seismic sources are deployed along the other sides. This would enable generation of a velocity map under the house. Invasive testing through floor slabs can be done (minidrills or cone penetrometer systems), but methods are limited as to where they can be applied, and they are very damaging to the floor, as one would imagine. So this potential noninvasive means to look under houses would be an attractive development. Undershooting would also offer an advantage over borehole-to-borehole seismic tomography because it would sample in a horizontal plane and thereby maximize the chance of shooting through a ver- tical, low-velocity column. Such a vertical column could fall outside of the vertical plane defined by the two boreholes. Also, boreholes are always expensive (relative to the cost of geophysical surveys of this type) and represent a potential pathway or breach in a confining layer that should be avoided when possible. Another seismic technique that has had limited applica- tion but offers great promise for defining ravel zones is the MASW (multichannel analysis of surface waves) technique. This technique also maps S-wave velocity variation but has the additional capability of detecting and mapping velocity inversions (decreases with depth) and, so, may have a unique ability to detect incipient ravel zones (i.e., sinkholes in the formative 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 of rock surface or detecting cavities within the rock mass itself. With the development of tomographic imaging techniques that will better define structure in the overlying sediments, there may be increased application of reflection methods for sinkhole delineation. Summary. Sinkholes, then, regardless of their underlying cause, represent hazards that can be extremely costly and dis- ruptive to society in general, are ubiquitous in where they occur, and happen routinely. Geophysics provides important tools that can be used to: • Predict where and within what limits sinkholes are likely to 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 improvement programs (grouting, compaction, mechanical stabiliza- tion) intended to “fix” sinkhole conditions (remediation). The sinkhole remediation aspect is taking on renewed emphasis 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. So while the surface depression offers a tempting area to dis- pose of old vehicles and trash, there are severe consequences lurking within such activities. Suggested reading. “Sinkholes, west-central Florida” by Tihansky (in Land Subsidence in the United States, USGS Circular 1182, 1999). “The Retsof Salt Mine Collapse” by Kappel et al. (in Land Subsidence in the United States). Mitigating Losses from Land Subsidence in the United States (Commission on Engineering and Technical Systems, National Academy Press, 1991). TLE Corresponding author: 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 in Texas. Interpreted ravel zones are indicated by the two lobate features of decreased seismic velocity. Copyright © 2006, The Society of Exploration Geophysicists