- Home
- Conferences
- Conference Proceedings
- Conferences
10th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 23 Mar 1997 - 26 Mar 1997
- Location: Reno, Nevada, USA
- Published: 23 March 1997
41 - 60 of 106 results
-
-
Two-Dimensional Imaging Of Transient Electromagnetic Soundings
More LessTransient electromagnetic (TEM) soundings have been used extensively in geophysical
investigations in connection with environmental problems (Christensen and Sørensen 1994, 1995).
Within the geophysical community there is a growing realization that dense measurements
over large areas are needed to produce the reliability and detail demanded by the severity of many
of the problems addressed in environmental geophysics (Sørensen et al 1995, Sørensen 1996).
This calls for new efficient methods of data collection and fast interpretation procedures.
Consequently much interest has been directed towards rapid approximate interpretation
algorithms - imaging procedures - which will allow a quick overview of the data collected and
provide good starting models for rigorous, iterative inversion procedures.
A number of imaging methods for transient electromagnetic soundings have been suggested
based on the variation of the diffusion velocity with conductivity, e.g. Eaton and Hohmann (1989).
-
-
-
3D Electromagnetic Inversion For Environmental Site Characterization
Authors David L. Alumbaugh and Gregory A. NewmanA 3-D non-linear electromagnetic inversion scheme has been developed to produce images
of subsurface conductivity structure from electromagnetic geophysical data. The solution is
obtained by successive linearized model updates where full forward modeling is employed
at each iteration to compute model sensitivities and predicted data. Regularization is applied
to the problem to provide stability. Because the inverse part of the problem requires the
solution of 10’s to 100’s of thousands of unknowns, and because each inverse iteration
requires many forward models to be computed, the code has been implemented on
massively parallel computer platforms. The use of the inversion code to image
environmental sites is demonstrated on a data set collected with the Apex Parametrics
‘MaxMin 1-8s’ over a section of stacked barrels and metal filled boxes at the Idaho
National Laboratory’s ‘Cold Test Pit’. The MaxMin is a loop-loop frequency domain
system which operates from 440 Hz up to 56 kHz using various coil separations; for this
survey coil separations of 15, 30 and 60 feet were employed. The out-of phase data are
shown to be of very good quality while the in-phase am rather noisy due to slight
mispositioning errors which cause improper cancellation of the primary free space field in
the receiver. Weighting the data appropriately by the estimated noise and applying the
inversion scheme is demonstrated to better define the structure of the pit. In addition,
comparisons are given for single coil separations and multiple separations to show the
benefits of using multiple offset data.
-
-
-
3D Ert Applied To The Monitoring Of Joule Heating
More LessThis paper describes the application of three-dimensional Electrical Resistivity Tomography (ERT) to monitor
in-situ soil six-phase ohmic-heating (SPOH) at the Integrated Demonstration for VOCs in Soils and Groundwater at
Non Arid Sites. The ERT data sets, collected between five pairs of adjacent wells using an array of electrodes
located in 4 boreholes, have been previously interpreted with a 2D algorithm (Ramirez and Daily, 1994) to map the
changes in conductivity in a clay layer located in the vadose zone over a period of approximately 2 months (before,
during and after electrical heating). The objective of this work is to show the usefulness of a new 3D ERT
interpretation technique for a complete monitoring of a real 3D underground process. Heating causes moisture
content, temperature, and fluid conductivity changes, all of which affect electrical resistivity, and the successful
application of ERT provides a three-dimensional view of the subsurface resistivity or conductivity distribution.
This information helps us to understand the interaction between heating and the heterogeneous subsurface, the
effects of heating on fluid flow and on contaminant removal. The reconstruction of both the background resistivity
and of the changes over time, caused by the combined effects of moisture redistribution and heating caused by
SPOH and vapor extraction, is enhanced by the 3D interpretation.
-
-
-
Effective Design Of A Horizontal Air Sparging Well
Authors Doug LaBrecque, Gianfranco Morelli and Paul D. LundegardIn air sparging, in-situ remediation is accelerated by injecting air in the saturated zone. The
success of air sparging depends on the size and geometry of the region of air flow. This study
used ERT surveys to observe changes in saturation, and thus air flow for a horizontal sparge
well. The site is in relatively homogeneous, well sorted fine to medium grained dune sands at
the Guadalupe Gil field. The horizontal sparge well was 4.7 m below the surface. The well was
constructed of stainless steel and perforated along 15 m interval. ERT images were made along
a pair of east-west trending planes near the north and south ends the well.
ERT images showed that the zone of influence of the horizontal sparge well is relatively small.
During the early times, the width of the zone of influence is between 1.5 and 2.5 m. Later, the
width expands slightly but never exceeds 3.3 m.
There were some differences between the north and south planes. The north plane had a slightly
narrower zone of influence but a higher peak saturation, exceeding 50% during the test. An
increase in pressure created a small increase in the saturation and size of the zone of influence.
The ERT images could not be reconstructed until the metal well was included in the FEM mesh.
The well was found to be located slightly further from the center ERT cables than expected.
The background resistivities are consistent with those of clean sands. Although there is some
variability in the resistivity of the background, it is small compared to the variability of the
image reconstructions and can be ignored.
-
-
-
Three-Dimensional Georadar Studies Of Deformation In Deltaic Sediments
Authors Milan Beres, Andre Pugin and Alan GreenThree-dimensional ground-penetrating radar (georadar) data collected across a 4-m-high hill in the Rhone
River delta (western Switzerland) contain prominent reflections from faulted and folded bedding. Crosssections
and time slices extracted from the georadar data reveal a number of faults that strike parallel to the hill
slopes. Several circular-shaped uplifted blocks together with intervening graben-like features are observed at
various depths. A similar-sized graben-like structure with step faults that converge with depth has been mapped
at a nearby outcrop. Thin layers of silt and fine sand that were intersected in boreholes correlate with strong
georadar reflections and are interpreted as elastic intrusions. Our results are consistent with the hypothesis that
the hill and its internal structures were formed by a large rockfall.
-
-
-
High Resolution 3-D Seismic Experiment Across The Oquirrh Fault
Authors David M. Morey, Chengjun Wu, Yue Wang, Zhaojun Liu, Jing Chen and Jerry SchusterA 3-D seismic data set was recorded across the Oquirrh fault in May of 1996
by University of Utah personnel. The data (112,896 traces) were collected on a
fine grid in order to image the shallow (SO’-100’) detailed features of this fault.
Previous trenching studies to a depth of about 20 feet along this fault reveal
offset bedding caused by large magnitude earthquakes. The goal of this study is
to explore the potential for complementing trenching studies with 3-D seismic
imaging, with the hope of correlating earthquake recurrence intervals to bed
offsets in the seismic image.
The processing of this data set includes 3-D tomography and 3-D prestack
migration. Results from 2-D tomography show a clear imaging of the shallow
subsurface fault to a resolution of about 10 feet.
-
-
-
Characterization Of The Hydrogeologic Framework At Aberdeen Proving Ground, Maryland, Using Marine Seismic Reflection Profiling
More LessHigh-resolution, single-channel, marine seismic reflection profiles were collected in the shallow waters of
the Chesapeake Bay estuarine system surrounding the Edgewood Area of Aberdeen Proving Ground, MD. This
data set furthers our understanding of the hydrogeologic framework by identifying paleochannel locations,
delineating paleochannel geometries, and characterizing relationships between multiple generations of
paleochannels.
Fine-grained bottom sediment resting in deeper, low-energy waters were found to block high-frequency
acoustic wave transmission; therefore, data acquisition is limited to areas near-shore were water depths are less
than three meters. Approximately 120 km of shoreline were surveyed yielding 50 km of high quality data. Three
major reflectors are identified as erosional unconformities, and are interpreted to be Pleistocene paleochannel
surfaces. Paleochannels are found to be spatially and physiographically associated with all modern subaerial
lowlands and their local drainage systems. The marine seismic reflection data provide strong evidence that
present-day lowlands are underlain by an aquifer dominated by paleochannels. Paleochannel configurations,
together with paleochannel-fill models, indicate these paleochannels have the potential support vertical and
horizontal contaminant migration.
-
-
-
Mapping A Near Surface Variable Geologic Regime Using An Integrated Geophysical Approach
Authors Noel T. Rogers, Stewart K. Sandberg, Peter Miller and Greg PowellAn integrated geophysical approach involving seismic, electromagnetic, and electrical methods was employed to
map fluvial, colluvial and bedrock geology, to delineate bedrock channels, and to determine fracture and joint
orientations that may influence migration of petroleum hydrocarbons at the Glenrock Oil Seep. Both P (primary)-
wave and S (shear)-wave seismic refraction techniques were used to map the bedrock surface topography, bedrock
minima, stratigraphic boundaries, and possible structure. S-wave data were preferred because of better vertical
resolution due to the combination of slower velocities and lower frequency wave train. Azimuthal resistivity/IP
(induced polarization) and azimuthal electromagnetics were used to determine fracture orientations and
groundwater flow directions. Terrain conductivity was used to map the fluvial sedimentary sequences (e.g.,
paleochannel and overbank deposits) in the North Platte River floodplain. Conductivity measurements were also
used to estimate bedrock depth and to assist in the placement and recording parameters of the azimuthal
soundings.
The geophysical investigation indicated that groundwater flow pathways were controlled by the fluvial
paleochannels and bedrock erosional features. Primary groundwater flow direction in the bedrock and collwial
sediments was determined from the azimuthal measurements and confirmed by drilling to be N20-4OW along the
measured strike of the bedrock and joint orientations. Joint/fracture orientations were measured at N20-4OW and NlO-
30E from the azimuthal data and confirmed from measurements at a bedrock outcrop south of the site. The bedrock has
an apparent NlOE anisotropy in the seismic velocity profiles on the old refinery property that closely match that of
measured joint/fracture orientations and may have a minor effect on groundwater flow.
-
-
-
Application Of Electromagnetic Methods To Locate Shallow Water Well Sites In A Fractured Aquifer
Authors Kendrick Taylor, Matt Chesley, Korblaah Matanawi and Tim MinorLow permeability fractured bedrock makes it difficult to select productive locations
for shallow water wells in the Atebubu district of Ghana, West Africa. Drilling has
demonstrated that productive and nonproductive sites are frequently separated by less than
50 meters. The drilling success rate is approximately 25 percent when experienced
hydrogeologists use only surface visual observations to select drilling sites. A combination
of remote sensing and geophysics was used to improve the success rate. SPOT satellite
imagery and stereo aerial photography were used to identify areas where suspected
fractures cross drainages. These areas were typically 100 to 150 meters in diameter. The
frequency domain electromagnetic method was used to locate zones in these areas where
the electrical conductivity increased with depth. These were interpreted as zones of deeper
weathering and fractured bedrock, and were selected as favorable drill sites. Use of this
exploration strategy increased the success rate 35 f 10 percent when the best site within
2 kilometers of the target community was selected. This doubled the success rate and
reduced the number of holes drilled to provide wells to these community by more than a
factor of two. When SPOT satellite imagery and aerial photography were used without the
frequency domain electromagnetics, the success rate only increased 8 f 8 percent. The
methods are cost effective when they are implemented by Ghanaian personnel and in areas
that have a low drilling success rate.
-
-
-
Monitoring Of A Pumping Test In An Unconfined Aquifer With Ground Penetrating Radar
Authors Anthony L. Endres, David L. Rudolph and William P. ClementThe temporal and spatial response of an unconfined aquifer at CFB Borden to a pumping
test was monitored by means of ground-penetrating radar (GPR) profiling. The reflection
caused by changing water content in the transition zone between the overlying residually
saturated material and the water saturated aquifer below was used to estimate the location of
the top of the water saturated zone (TWSZ). Observed depth variations in the TWSZ
reflection differed significantly from the water table drawdown measured in the piezometers.
The GPR results are consistent with the in-situ water content measurements of Nwankwor et
al. (1992) and support their proposed delayed drainage mechanism. Diffractions along the
TWSZ reflection indicate that differential drainage due to small scale variations in aquifer
properties occurs. Arrival time variations of reflections from underlying stratigraphic
horizons are accounted for by changes in the overlying velocity structure caused by the
lowering of the TWSZ.
-
-
-
Geophysical Monitoring Of A Groundwater Tracer In The Gray Delta, Maine
Authors Stewart K. Sandberg, Charles R. Fitts and Roelof VersteegSurface geophysical measurements were successful in monitoring a groundwater tracer
experiment within the municipality of Gray, Maine. Low levels of volatile organic compounds
have been detected in the town’s water-supply well field, which is located 900 meters away from
the municipal landfii. In order to discover the relationship between the well field and la&ii, a
tracer experiment was conducted at an accessible location between them. The tracer, consisting of
minor amounts of salt mixed with water recovered from each injection point, was injected at two
locations 10 meters apart within a sand pit; one injection was in a monitoring well screened from
3.4 to 6.4 m below ground surface, and the other was a surficial injection, within a pit dug into the
saturated zone of the aquifer approximately 1 m below ground surface.
Geophysical methods tested included spontaneous polarization (SP), resistivity, induced
polarization (IF’), frequency- and time-domain electromagnetics (EM), and ground-penetrating
radar (GPR). Resistivity methods included the dipole-dipole array, radial array, and mise-a-lamasse.
The radial array was a modified pole-dipole array where one transmitter electrode was
down the injection well at the screen and the other was some distance away. Potential measuring
points were located at n-spacings of 1,2,3, and 4, using an a-spacing of 5 m. IP data were also
acquired along with resistivity data. GPR data were collected using both 200 MHz and 100 MHz
antennas.
Initially, the most effective and efficient geophysical method for location of the tracer was
the SP method. Radial array resistivity, GPR, and n-&e-a-la-masse methods also proved effective
in detecting the tracer. Frequency-domain EM (terrain conductivity) measurements proved
excessively noisy. This paper describes SP, radial array resistivity, and GPR methods, since they
appear at this stage of our analysis to be the most informative.
-
-
-
Resistivity Measurements And Modeling In The Vicinity Of A Trench Well
Authors Paul J. Wolfe and Sara B. HodlOn North Andros Island, Bahamas, an extensive array of trench wells has been dug to collect
fresh water from the surface (unconfined) aquifer for export to Nassau. There is concern that salt
water will upcone below the trenches if water is withdrawn too rapidly. Resistivity soundings are
a common method for determining depth to the fresh-salt water interface. Where the soundings
are most important, the normal assumptions of resistivity sounding interpretation methods fail
because of the presence of the open, water-filled trench.
To interpret the soundings in the vicinity of the trench, the finite differences ground water
modeling program, MODFLOW, was reinterpreted to model the electrical flow. The resulting
models and several forms of resistivity soundings are compared and evaluated to assess their
usefulness in this unusual setting.
-
-
-
A Hydrogeologic Framework Of Washoe Valley, Nevada From Joint Gravity And Magnetic Modeling And Terrain Conductivity Data
Authors Ronald Petersen and Robert KarlinGravity, ground magnetic, and terrain conductivity surveys were conducted in Washoe
Valley, Nevada to define subsurface structure and lithology as part of an effort to assess
groundwater resources and model basin recharge in this rapidly growing commum ‘ty between
Reno and Carson City. The gravity results indicate the valley is an asymmetric fault-bounded
half-graben, backtilted to the west where it is filled with nearly 2000 feet of coarse-grained
sediments. A major fault runs beneath the axis of the valley, with considerably shallower
basement beneath the upthrown eastern side. A magnetic high immediately south of New
Washoe City suggests this shallower eastern basement is overlain by a volcanic unit. This unit
is in turn overlain by electrically conductive fine-grained sediments. Together, these results
suggest a more complex eastern subsurface with generally lower transmissivities relative to the
thicker section of coarser-grained sediments filling the western side of the valley and that the
locus of deposition has shifted eastward through time. These findings place important
constraints on ongoing efforts to model groundwater recharge and availability. This study
demonstrates the utility of using multiple geophysical surveys to map the spatial distribution of
interbasin lithologies and estimate their hydraulic properties.
-
-
-
Hydraulic Modeling At Rokballe Test Site By Application Of Geophysical Data
Authors Dorte Dam, Steen Christensen and Niels B. ChristensenThe objective of the present study is to invistigate the by application of geophysical data in hydraulic modeling.
Numerical models have been developed for a 1 km2 Danish catchment using comprehensive field data. The
investigated area concerns a phreatic aquifer in a fluvioglacial formation. Three models are considered. The first is
based solely on bore hole informations and hydraulic head data. The remaining two include transient
electromagnetic data. The bottom of the aquifer is determined as the level of the good conductor, and the electrical
resistivity of the saturated zone provides qualitative information about the hydraulic conductivity.
Zonation of the aquifer based on the electrical resistivity, improves the estimation of the hydraulic conductivities.
The residual is slightly smaller even when only very little geophysical information has been implemented.
-
-
-
Location Of Buried Valley Aquifers Applying Spatially Dense Transient Soundings
More LessThe water supply in Denmark is based solely on groundwater. The abstraction sites are often
located at the rim of the cities next to old dump sites posing a significant risk for
contaminating adjacent aquifers. Furthermore, the industrial development outside cities, and
agricultural activities form another substantial threat of leakage of unwanted substances down
to the aquifers.
It is a common problem that in cultivated areas there is an increasing need for mapping
ground water resources. This concerns the necessity to protect the ground water against
pollution from surface activities, to ensure that the effects of draw downs do not produce poor
water quality by sulfation, mobilisation of salt water or other unwanted water qualities, and to
make sure that the abstraction does not heavily influence on surface waters such as lakes,
rivers, streams etc. (Sorensen et al, 1995). In order to meet these demands it is necessary to
have sufficient detailed and reliable hydrological models from which an optimal strategy for
sustainable abstraction can be obtained. The set-up of these models calls for detailed and
reliable mapping of the aquifers.
In Denmark a considerable number of the aquifers are buried valley aquifers composed of
resistive Quatemary sand and gravel deposits laid down in deep valleys eroded in Tertiary
well conducting clays and covered by an inhomogeneous conductive Moraine clay cap.
Formerly the delineation of these aquifers has been based mainly on drilling information
and has provided a more regional understanding of the extent and interconnection of the
aquifer systems. It has been demonstrated that geophysical mapping in combination with
traditional geological mapping using drilling information and surface observations is a more
powerful and economical tool to achieve reliable mapping on a detailed scale. Within the last
decade the volume of geophysical investigations using electrical and electromagnetic
methods have increased substantially in Denmark. The main reasons for the successful
application of geophysical mapping are the use of newly developed field efficient methods
and the close relationship that exists between hydraulic conductivity’s and electrical
formation resistivities in the Danish buried valley aquifer systems.
This paper presents results from the geophysical mapping i.e. the application of the transient
electromagnetic method used for invastigating the interconnection of aquifers. and for
delineating the composition of the aquifers themselves.
-
-
-
Reflection Seismic Mapping Of Caverns And Collapsed Caves, Carthage Missouri
Authors Jesse A. Baker, Mike L. Shoemaker, Mike Roark, Neil Anderson and Ron HindsInterest in locating a cave under the city of Carthage, Missouri was inspired by several pre- 1930 newspaper articles
wherein people claimed to have explored a large cavern system comprised of interconnected chambers, some with
lateral dimensions of 12Om x 60m and ceiling heights of 12m. One problem however, was the articles varied in their
descriptions of the entrances to this system and the subsurface location of the caverns.
Based on these historical accounts, the city of Carthage, in the interests of locating and developing a sizable cavern for
public use, asked the Department of Geology-Geophysics, University of Missouri-Rolla, to help locate the rumored
feature using the shallow reflection seismic technique. A total of twelve reflection seismic lines were acquired in the
northwest section of downtown Carthage and on city property near the eastern edge of town. Line locations were
selected based on the historic literature. Since the target feature was expected to be large and the city had a fixed budget
for exploration, a geophone spacing of twenty feet was used in the interest of maximizing coverage. Several prominent
hyperbolic ditEaction/reflected retraction patterns were imaged on the reflection seismic data, however none were as
laterally extensive as hoped. The city had also budgeted money for drilling, therefore test holes were drilled in order to
further investigate the findings of the reflection seismic data.
-
-
-
Gravity Inversion Using A “Shape-Of-Anomaly” Mfxhod: Developments In Three-Dimensional Modeling And Application To Gravity Over An Abandoned Underground Limestone Mlne In Southeastern Indiana
By R.M. RenéA method is developed for threedimensional modeling of gravity measured at the earth’s surface. The model
is built of many rectangular parallelepipeds of constant horizontal dimensions. Each prism is assigned an allowed
density contrast; however, all density contrasts are of the same sign. An initial model of filled prisms, or “seed”,
is specified. Additional prisms are filled automatically, one for each pass through an algorithm that minimizes the
difference between the shapes of the modeled and observed gravity. This “shape-of-anomaly” criterion was
previously developed and applied to two-dimensional modeling of gravity profiles (Rene, 1986). Other criteria for
controlling the model’s compactness and depth to center of gravity are developed here. Applications are first
demonstrated using synthetic gravity. For the gravity of a buried sphere, the inverse model closely approximates
a sphere if the depth to center of gravity is unconstrained. Some models are generated by specifying allowed ranges
for depths to center of gravity. To treat random noise, including measurement errors and high spatial-frequency
“geologic” noise, a filter is applied to both the observed and modeled gravity. Effects of bias are also shown. An
inverse model is also derived from a negative 143-pgal gravity anomaly of the flooded underground Gheen’s Mill
Cementville Mine in southeastern Indiana. The anomaly is defined by gravity at 837 stations in nineteen profiles
with a 3-m (lo-foot) station spacing. The model is constrained by 17 boreholes that penetrated cavities and 39
boreholes that did not find cavities.
-
-
-
A Ground-Penetrating Radar Study Of Karst Features In Soutekentral Missour
More LessKarst features (cavities and sinkholes) in south-central Missouri generally occur where the
Gasconade Dolomite and Roubidoux formations are exposed to a near-surface environment.
Karstic cavities develop as carbonic acid (derived from atmospheric carbon dioxide and rainwater)
percolates through these carbonate rocks along permeability pathways (e.g., fractures, faults, etc.)
from ground level to the water table. The carbonic acid gradually dissolves the carbonate country
rock and forms cavities. If the roof rock above a developing void becomes structurally unstable,
the cavity will collapse. Collapse features expressed at the surface are referred to as sinkholes. In
south-central Missouri, subsurface cavities and sinkholes are common and constitute a significant
engineering hazard.
To elucidate the internal structure of sinkholes in south-central Missouri, ground penetrating
radar surveys were conducted at eight sites on the Fort Leonard Wood Military Reservation. The
sinkholes studied developed in the upper Gasconade Dolomite and lower Roubidoux Formation.
Data were acquired with both a 500 MHZ antenna and a 120 MHZ antenna. The 500 MHZ data
proved to be most useful in terms of imaging the internal structure of the sinkholes. Depths of
investigation varied between one and four meters.
On the 500 MHZ radargrams, the internal collapse structure of the sinkholes is imaged by laterally
discontinuous events which reflect both the dip and dislocated nature of the collapsed strata.
These data are important because they demonstrate that ground-penetrating radar can be used to
map sinkhole features and potentially identify void space to a depth of 4m.
-
-
-
New Wales Sinkhole Remediation
Authors Sam Sahebzamani and Vishwas SatheThis paper presents the Florida Department of Environmental Protection’s (FDEP) investigation of a sinkhole at the
IMC-Agrico Company (IMC) New Wales chemical plant, in Polk County, Florida.
In June 1994, a catastrophic condition occurred at the New Wales chemical plant. A sinkhole had opened under
their industrial waste storage area (phosphogypsum stack) creating a depression within it. The depression was
observed to be 185 feet deep and 120 feet wide, within the 200 foot tall phosphogypsum stack. Further investigation
revealed the existence of a 480 foot long conduit that extended from the top of the stack to the upper Floridan
aquifer system. Remedial alternatives for a sinkhole of such magnitude and potential harm to the potable water
source could not be determined without a thorough understanding of the extent of the sinkhole formation.
Exploratory work involved cross-hole seismic surveys together with the drilling of boreholes at strategic locations.
Remediation consisted of an extensive grouting program using angle drilling to gain safe access into the erosion
cavity. The Phosphate Management Program (PM) of FDEP was responsible for review, evaluation, and approval of
every technical aspect of the proposed exploratory and remedial activities, to ensure that an effective long term
solution was implemented. The Program also received valuable comments concerning this project from an
interdisciplinary technical advisory committee representing local, State, and Federal agencies.
IMC expended approximately $7 million toward sinkhole exploration and remediation, and is required to continue a
monitoring program to verify success of the remedial effort. The event made national headlines and was broadcast
over television networks such as CNN News. Inquiries were received from other countries on the technical aspects
of the remediation.
-
-
-
Effectivenessofgroundpenetratingradarin Detectingcavesinkarstlimestoneofthe Yucatanpeninsula,Mexico
Authors Hans F. Schwaiger, Craig Murray and Larry BrownAlthough ground penetrating radar (GPR) is a promising technique in detecting subsurface cavities,
experience with this technique in limestone is still limited. To further evaluate the effectiveness of GPR
in detecting caves, several surveys were recently conducted in the karst terrane of the Yucatan Peninsula
in Mexico. The surveys were conducted near three cenotes (dolines), Kantun, Ponderosa, and Corral,
located approximately 1 mile inland from the Caribbean coast of the Yucatan Peninsula near Playa de1
Carmen. Cenotes expose the cave system and geometry of the cave roof, providing basic geometric control
for the GPR data. Most of the surveys were oriented radially with a typical length of 150 ft, beginning
at the cenote’s edge then traversing beyond the intersection of the roof of the cave with the water table.
One survey was conducted along a 1500 ft road connecting two cenotes and another conducted over a
passageway connecting Ponderosa cenote and Corral cenote.
A 100 MHz antenna was chosen to insure adequate penetration. The velocity in limestone was
determined from expanding spread CMP profiles to average 0.34 ft/ns, corresponding to a wavelength
of about 3.4 ft at this frequency. The limestone-to-air interface at the roof of the cave was expected
to produce strong reflections. Distinct reflections, however, at the expected times were rarely observed,
perhaps due to side scatter. A few surveys did provide a traceable reflection at the appropriate time
and with a dip that mimicked the shape of the roof of the cave. In addition, one survey also contained
a second reflection which, when depth corrected, revealed a flat horizon at a depth corresponding to the
water surface. These preliminary results suggest that, despite ambiguities, GPR profiling can be effective
for direct cave delineation in Yucatan karst.
-