Dr. Samuel T. Peavy


PhD Geophysics, Virginia Polytechnic Institute & State University, 1997

MSc Geophysics, Memorial University of Newfoundland, 1985

BS Physics, McNeese State University, 1983


Research Interests


Research Interests

My main interest is the integration of data from a variety of geophysical techniques in the investigation of geologic problems at all scales. Other interests include but are not limited to the development and application of signal processing techniques for data enhancement in the geosciences, and the development and practical application of high resolution '4-D' geophysical methods in the engineering and environmental sciences.



Recent Abstracts


Peavy, Samuel T., Dept. of Geological Sciences, Rutgers University, 195 University Ave., Newark, NJ 07102, peavy@andromeda.rutgers.edu; Sayer, Suzanne, College of Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, ssayer@vt.edu.

New River terrace deposits near Pembroke in southwestern Virginia were discovered to contain an antiform and several faults during the excavation of an embankment in 1992. The central portion of the antiform was found to contain a narrow graben striking ENE to NE. The location of Pembroke within the Giles County Seismic Zone led to a consideration that these young faults might be associated with an active fault in the basement. However, the terrace has numerous sinkholes from dissolution of the underlying Ordovician carbonates, and the faults could be the result of the collapse of a deeper karst feature. A geologic and geophysical investigation was undertaken with the purpose of finding the root cause of the younger faulting.

Seismic refraction, roll-along electrical resistivity, terrain conductivity, magnetometer, and gravity surveys near the embankment and in a large hay field across US Highway 460 from the embankment were performed. The seismic refraction results indicated that as much as 134 feet of terrace deposits overlie the carbonate basement, and the roll-along resistivity was able to track the trend of the graben into the hay field. Initial analysis of the terrain conductivity, gravity and magnetic data indicated that the trend established by the resistivity surveys might extend more than 1000 feet from the embankment. However, this trend appears to be terminated by an east-west alignment of anomalies. No clear link to possible basement faulting could be established, however.

Further analysis of the gravity and magnetic anomalies was undertaken using a combination of gradient methods known collectively as potential field attributes (PFA), which more clearly delineates the established trends allowing for better interpretation of the results. PFA may prove useful in the analysis of other data sets of similar nature.  [Note: Further analysis indicates that the collapse of a karst-related structure is most likely responsible for the graben structure -- see below] {Related paper accepted to Geophysics}

{Presented at Northeastern section meeting, GSA, Portland, ME 1998}

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Above: Calculated tilt angle values for gravity field, Pembroke, Virginia.   Northeast trending negative (blue) values in tilt angle indicate a low density zone interpreted to be a karst related collapse, probably associated with the antiform and graben structure across the street.


Samuel T. Peavy, Dept. of Geological Sciences Rutgers University, 195 University Ave., Newark, NJ 07102, and David W. Valentino, Dept. of Earth Sciences, State University of New York at Oswego, Oswego, NY 13126

Electrical resistivity measurements were made to determine the variability of surficial deposits, the depth to bedrock and to characterize the distribution of groundwater at the Rice Creek Field Station near Oswego, New York. The field station is underlain by drumlin deposits and ablation till associated with Pleistocene glaciation. These deposits reside on Ordovician quartz sandstone of the Oswego Formation that outcrops within 1500 m of the study site. Locally the Oswego Formation contains subvertical fractures with an average spacing of less than 0.5 m. Twenty offset Wenner electrical resistivity surveys were conducted in June and August of 1998 along trails and across an open field within the field station grounds. Analysis of pseudosections and simple 1-D modeling and 2-D least squares inversion indicate the following: 1) low resistivity zones associated with perched water tables within the chaotic drumlin deposits; 2) highly variable and resistive near-surface measurements along Rice Creek indicative of large (>1 m diameter) glacial erratics as observed in the creek bed; 3) a transitional zone below ~250 ft elevation of subcircular highs separated by relatively low resistivities that continue into the deepest portions of the data, which is coincident with the projected depth to bedrock beneath the field station and is interpreted to be an undersaturated zone within the fractured Oswego Sandstone; and 4) low resistivities below an elevation of ~190 ft are interpreted to be the top of the saturated domain within the fractured bedrock.

{Presented at SAGEEP meeting, Oakland, CA, 1999}

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Above:  Resistivity map along entry road, Rice Creek Field Station, Oswego, NY.  L shows zones of low resistivity that continue into the subsurface; P is the location of a perched water table.

Location and Delineation of Subsurface Tar Contamination Using Electrical Methods

GRANGER, Elizabeth, and PEAVY, Samuel T., Dept. of Geological Sciences, Rutgers University, 195 University Ave., Newark, NJ 07102

Electrical resistivity and induced polarization (IP) measurements were made to explore the variability of electrical parameters within tar-contaminated soils at a vacant lot on the floodplain of the Oswego River in Fulton, NY. The site was a shingle manufacturing facility operating from 1936 - 1960. Subsequently, the property was divided into several partitions and used for the disposal and storage of asphalt and roofing shingles. Degradation of the buried shingles with time generated diapirs of tar which eventually extruded onto the surface, creating self-replenishing "tar boils" at the site.

Data from an original Wenner electrical resistivity survey conducted in December, 1997 were used to delineate highly resistive areas believed to be associated with the tar. Subsequent trenching and drilling revealed tar in some of the identified areas. However, there were also large (> 2m diameter) boulders of Oswego sandstone in the subsurface, believed to be related to the building of the Oswego Barge Canal. A second Wenner electrical resistivity survey using a 1-m offset was conducted in December, 1998 to locate remaining areas of potential tar contamination. These locations were the focus of IP surveys conducted during May and June of 1999. These surveys were conducted to test the ability of IP parameters to distinguish between organic and inorganic resistive zones in the subsurface. Recent publications of laboratory results have indicated that spectral IP has the potential to identify contaminants through variations in the phase and amplitude spectra. Derivation of these spectra can be done in the field by fitting time domain IP data using a Cole-Cole model, then deriving the spectral parameters.

{Presented at National GSA meeting, Denver, CO 1999}

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Above:  Image of IP data collected in Fulton, NY in 1999 showing chargeability in mV/V.


4-D Imaging

Recent publications have shown the utility of 4-D or time-lapsed seismic imaging in the evaluation of oil and gas reservoirs around the world (see recent issues of The Leading Edge, Geophysics, or AAPG Bulletin). The ability to track the migration of fluids in the subsurface seems promising given recent results. However, there are problems, including those associated with the use data sets of differing vintages in the 4-D analysis. Amplitude anomalies notwithstanding, true quantification of the results still awaits.

In the environmental industry, there have been attempts to do similar imaging of subsurface contaminant plumes using ground penetrating radar (GPR), electrical, and electromagnetic methods. Images seem to indicate changes in the subsurface which in some cases can be associated with contaminant movement. However, variability and seasonal changes of physical properties in the near-surface have made the quantitative use of this information difficult to nearly impossible. Victoria Hover and I plan to pursue a study combining geochemical, geologic, and geophysical information over a relatively small area in an attempt to reach more quantitative conclusions from a 4-D study using electrical resistivity.

Induced Polarization Studies

With the able assistance of graduate student Mrs. Elizabeth Granger, we have undertaken a series of IP measurements at the site of a former shingle factory in Fulton, NY.   "Tar boils" -- presumably derived from buried shingles on-site -- appeared at the surface in several places in a vacant lot.  These were investigated initially by resistivity measurements, and later by trenching.  Zones of high resistivity correlated with subsurface tar and with large boulders of Oswego sandstone, possibly derived from the building of the nearby Oswego River Barge Canal.   Therefore, we returned to the site to collect IP data in the hope that the data would show substantial differences between the sandstone boulders and tar diapirs.   Data were collected in the late spring and summer of 1999.  The results show that IP data can resolve the tar diapirs.  These results were presented at GSA in Denver last month (see recent abstracts, above).  Further research into Cole-Cole model parameters and their relationship to the tar are being explored at this time.

Archeological Investigation, Mexico, NY

Geophysical data were collected in the town of Mexico, New York on June 28-29, 1999 in order to determine the location of a prospective tunnel in the subsurface. The potential location for the tunnel was in a driveway between two buildings in the town near the intersection of Highways 104 and 69 at the old Starr Clark place. Starr Clark was a tinsmith with known abolitionist sentiments. Anecdotal and local historical evidence suggest that the houses and the tunnel may have been used by slaves during the time of the "Underground Railroad" to hide from potential captors and affect their escape to Canada and hence to freedom. The finding of a tunnel on the property would confirm this. Geophysical methods are often used in archeological work due to their non-invasive nature.

With the able assistance of David and Rick Valentino, a dipole-dipole electrical resistivity survey and magnetometer survey were conducted.  The resistivity was collected using a 1-m electrode spacing, and the magnetic data at 0.25 m.  We were unable to determine with any certainty if a tunnel exists at the location surveyed.   However, magnetic data indicate a large body of high magnetization and reversed magnetic polarity at a shallow depth.  Models constructed from the data match this interpretation, but do not indicate with certainty what this object(s?) might be.   Further tests and perhaps drilling would be necessary to ascertain the identity of the anomaly source.

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Other Research Interests

Study of the Eastern Virginia Geophysical High (EVGH)
C. Çoruh, J. K. Costain and I have been studying a gravity and magnetic high in eastern Virginia. Known variously as the Salisbury, Sussex-Currioman Bay, Sussex-Leonardtown, and Eastern Virginia Geophysical High (EVGH), the anomaly has been a source of controversy for most of the last 30 years. Two main schools of thought have prevailed on the origin of the anomaly, with one group maintaining that it represents a suture zone of possible Taconic- or Alleghanian-age, and the other group relating its origin to Mesozoic extension. Some authors have suggested that the anomaly trend is Alleghanian, strengthening their argument that the anomaly indicates a Late Paleozoic suture zone. However, as the anomaly's source lies beneath Atlantic Coastal Plain sediments, all geologic evidence collected in the vicinity of the EVGH is a result of basement penetrating wells. The few wells that have penetrated the basement in the vicinity of the EVGH display a variety of lithologies ranging from Triassic-aged strata, phyllite and biotite schist to metavolcanics and possible metamorphosed ultramafics.

The intermittent nature of the EVGH, particularly its magnetic field, argues against a single source along strike, but instead would be most consistent with a narrow belt of multiple sources. Basement refraction surveys yield a velocity of 6.3 km/s in the area of the EVGH. Well data and basement refraction velocities of 4.0-5.2 km/s to the east and west of the EVGH are indicative of possible Triassic strata. The USGS I-64 seismic reflection data show a thickness of up to 6 km for the Toano Basin to the east of the EVGH. The reflection data also show a narrow (~ 9 km) zone of discontinuous reflectivity in the mid- to upper crust coincident with the EVGH, a general lack of reflectivity to the east of the EVGH, and a reflection Moho at ~ 33 km nearby. Time-term analysis of mostly unreversed refraction profiles by James and others (1968) indicated an increase in crustal thickness from ~ 35 km just to the west of the EVGH to over 45 km to the east, a result inconsistent with the the reflection data.

The increase of crustal thickness from the time-term analysis could be caused by the presence of the Toano basin, which was unaccounted for in the original calculations. A recalculation of the depth to the Moho for a 4.5 second time-term using a model that includes 6 km of Triassic strata at 5.2 km/sec produces a thickness of 39.5 km as opposed to the value of 46.2 km from the original model. If the effect of the down-dip shooting geometry of the original experiment is also considered, a reduced time term (0.56 s) results in a total crustal thickness of 34 km -- a result consistent with the reflection data.

Two- and three-dimensional gravity and magnetic modeling show that the EVGH can be explained by a near-vertical, mafic intrusive complex over a westward dipping Moho with no increase in crustal thickness . This model is best explained by dike swarms of Mesozoic age associated with the opening of the Atlantic Ocean and post-dating the formation of the associated Triassic basins to the east and west. These mafic dikes may have taken advantage of a pre-existing, near-vertical Alleghanian structure that may have been associated with transpression during that time. Gates and others (1988) and Valentino and others (1994) have provided geologic evidence for the existence of a substantial component of strike-slip during the Alleghanian. With lateral displacements of up to 150 km, some of the larger strike-slip motions may have been associated with through-crustal zones of deformation. These zones may have been used during the latest phase of Mesozoic extension as conduits for the mafic material producing the EVGH and other similar anomalies in the southeastern United States. So in essence, the EVGH, while technically Mesozoic in age, may indeed be a direct consequence of Alleghanian strike-slip motion.

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Study of the COCORP GA-16 seismic line

In a similar study to that above, it was noted by Dr. C. Çoruh that COCORP GA-16 in southeastern Georgia exhibits a similar reflection and potential field signature to that in eastern Virginia. The tectonic setting is different, however, as wells in south Georgia have penetrated Paleozoic sedimentary rocks of African origin, indicating that a real suture of Alleghanian-age probably exists in this area. The geophysical anomaly seen in COCORP GA-16, while somewhat different in appearance to the EVGH, can also be modeled effectively using a near-vertical source for the gravity anomaly. It is our hypothesis that the origin of the anomaly source is also Mesozoic, but once again relies on a pre-existing structure to act as a conduit for the mafic material. The difference in appearance can be explained by a more compressional mode of tectonics in Southeastern Georgia, forming a suture, versus a strike-slip mode in eastern Virginia. The signature of an injected suture should be different from that of an injected, through-crustal strike-slip zone.

Structural dip limitations on migrated seismic data

A study of structural dip limitations in migrated seismic data was undertaken with the purpose of evaluating the resolution of seismic data in general and deep-crustal seismic data in particular. For the study, GeoQuest International's AIMS seismic modeling system was used to 'acquire' CDP data over a simple model of the crust. The model contained a series of dipping reflectors with dips ranging from 0 to 70 degrees at a depth of 15 km along a 100 km long profile, and a flat Moho reflector at 33 km depth. A 14-56 Hz tapered Klauder wavelet was used along with a 5 km maximum offset with an end-on spread and 48-channels. These values are typical of most deep crustal vibroseis data sets. In addition, full wave equation modeling was used to better simulate real seismic data, and noise was added to the final gathers. The resulting nominal 24-fold CDP gathers were transferred to CogniSeis' DISCO seismic processing package and stacked using the model velocity of 6 km/s and recording times of 13, 16 and 25 seconds. The stacked data clearly indicated that the 13- and 16-second stacks were not 'seeing' all the dipping reflectors, in particular the 60 and 70 degree reflections were either entirely or partially missing from the stacks. Even the 25-second stack was missing a portion of the 70 degree reflector. The Moho reflector was distorted slightly by the mid-crustal structure, with obvious breaks and layering seen in the stack, most likely a result of the complex wave propagation through the mid-crust. Stacks were also made using a variable maximum offset of 2.5 to 5.0 km. for all the different recording times.

The stacks were migrated using two different finite-difference algorithms and a Kirchhoff algorithm. The shorter data lengths proved to severely limit the dip resolution of the final migrated section, with the 25-second data giving the best results. In addition, the finite-difference algorithms produced poorer results for dips > 40 degrees, even though the dip limit was set at 70 degrees. The explanation lies in the way finite-difference migrations work and the lack of aperture, or number of traces, during migration. Dipping reflectors are distorted during CDP processing by being lengthened and spatially and temporally displaced, with higher dip angles being the most affected. The aperture limitation became the major factor determining the effectiveness of the finite-difference algorithm. For the Kirchhoff algorithm, this was not a problem, as the aperture is the entire seismic section. Some dispersive effects were noted in the results for Kirchhoff migration at the higher dip angles, most probably due to aliasing of the migration operator caused by a spatial sampling interval (50m) that was too large for those higher dips. This could be cured by recording at a finer CDP interval, though this has not been attempted to date. In addition, the time-migrations used in this study are not able to correct for the complex wave propagation that produced distortions of the planar Moho reflector. Only pre-stack algorithms can successfully attack this problem, but the drawback then becomes the accurate determination of velocity at depth, a very daunting task. Limiting the offset proved to be a minor factor in dip resolution, with more dispersion occurring at shorter offsets for all the algorithms, possibly due to the limitation of a different type of aperture - the spatial aperture. Limiting the spatial aperture limits the frequency band, and larger bandwidths are migrated more successfully (see Berkhout's books on seismic resolution and seismic migration).

From the above study, the following conclusions can be made:

  • The primary factor determining dip resolution in crustal-scale seismic reflection data is the recording time. The 'short' recording times of most crustal data sets will limit the resolution of the steepest dips to the shallowest parts of the section.

  • Steep dips deeper in the section require longer and longer recording times to be resolved, therefore typical crustal data sets (13-16 seconds) probably contain no information on dips greater that 40 degrees below 15 km (5 seconds), and almost no dip information at Moho depths, resulting in a time-dependent, dip-filtered version of reality. As the old saying goes, "If you don't record it, you can't migrate it."

  • The aperture limitation of finite-difference algorithms make it impossible to migrate steep dips successfully for any part of the section below 5 seconds. The Kirchhoff algorithm is much slower (19 hours vs. < 2 hours on a Sun Sparc10 Workstation), but the results are more accurate for steeper dips, and hence should be used exclusively for deep crustal data sets. Any time migration algorithm, however, cannot correct for time distortions caused by complex wave propagation, and hence will be unable to give a definitive answer, especially in structurally complex areas. This requires accurate velocity determination at depth -- a problem as yet unresolved. The question then becomes: Is the layered Moho seen in the model and on most deep-crustal reflection data a reality, or an artifact of complex wave-propagation through the mid- and upper crust?

  • Spatial aperture (maximum offset) limitations made little difference in the migrated results, with a slight increase in dispersion noticeable. This is a direct consequence of the direct connection between spatial and temporal frequency, with a limitation of the spatial aperture producing a band limitation in the frequency domain and leading to greater dispersion.

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3-D gravity modeling algorithms

Dr. C. Çoruh and I have spent some time working on 3-D gravity modeling algorithms for Mathematica and a spreadsheet. Both apply spatial filters on layers of density to calculate the gravitational attraction of a small 3-D cube at depth, with the difference being that the Mathematica program works in the frequency domain via FFT's, and the spreadsheet program works in the space-domain. Both algorithms work well, though the 2-D spatial filtering of the spreadsheet modeling program could take a substantial amount of time depending on the desired accuracy of the solution. Dr. Çoruh is currently working on converting the original spreadsheet modeling program into a Microsoft Excel application.

Short-Time Fourier Transform (STFT) study

Another project which has shown some promise is the application of the Short-Time Fourier Transform (STFT) or f-tau transformation in the processing and interpretation of Vibroseis and well-log data. The STFT works by calculating the Fourier amplitude spectrum over small portions of a data trace, i.e. windowing. The process is repeated over the entire length of the trace by shifting the window by small increments. A criteria I use to determine if the window width and increment are chosen properly is to reconstruct the data trace. The parameters that best allow a proper reconstruction of the original trace are the ones used. The original idea was to use the STFT to remove correlation artifacts from vibroseis data in a manner similar to Okaya and others (Geophysics, July 1992). However, as I delved deeper into the project, I discovered that their method was probably removing a portion of lower frequency signal from the data, in addition to the harmonic energy. I decided to try and find a better way of filtering the data. A paper by Li and others (Geophysics, March-April 1995) on the removal of correlation artifacts from recorded sweeps has given me some ideas about removing such artifacts from recorded data. Unfortunately, the main dissertation project has intervened and prevented me from pursuing what appeared to be a fruitful line of research.

Application of the STFT technique to well-log data has been moderately successful. My work with Anna Balog and Mike Pope (former students of Dr. J.F. Read) on pr imarily gamma ray logs has shown that the STFT may have some utility in regional correlation of units or in the identification of cyclicity in carbonate/shale sequences. We were able to show that the Triassic strata in Hungary exhibited cyclicity throughout the preserved record, leading Anna to a model in which a change in climate and not sea-level fluctuations might explain the change from dolomite to limestone in her basin. Mike Pope and I discovered pronounced zones in cyclicity in well logs from the Ordovician strata of the Appalachian Basin. If these zones can be correctly correlated with outcrop or well-lithologic information, then it may be possible to perform regional correlations of those units using STFT's of gamma ray logs.

Recently (July, 1996), I have been using the short-time Fourier transform technique to convert stacked seismic data into a 3-D cube of amplitude and phase information. The multi-trace STFT algorithm allows one to view the variability of amplitude and phase with time for not only a single trace, but along an entire line. Time, or frequency slices can be made using a companion program, although I plan on making it part of the total package.

Pseudomagnetic field constraints on potential field anomalies

Constraints on the source of potential field anomalies can be found by using pseudomagnetic fields. The technique was developed by Dr. E.S. Robinson as a alternative to the pseudogravity method of Baronov. A 2-D spatial filter converts gravity data into magnetic data using Poisson's Relation. If the gravity and magnetic fields are produced by the same body, the pseudomagnetic field will match the real magnetic field. If the sources are not the same, then the fields will not match. Along with the development of a more efficient algorithm for determining the pseudomagnetic field, my development of a a 2-D map normalization and comparison program was instrumental in the application of the technique by Debbie Hopkins and I to three well-known geophysical anomalies: the Mid-Continent Rift, the Eastern Virginia Geophysical High (EVGH; discussed above), and the New York - Alabama Magnetic Lineament Anomaly (NYALA). The results showed that the gravity and magnetic fields of the Mid-Continent Rift and the EVGH had the same sources as expected, but the NYALA has been a more difficult task, particularly since the source of the anomaly lies in the sub-thrust basement, and attempts at residualization have been met with mixed results. Dr. Hopkins has been working on this particular problem, and she I are currently working on a paper about our results.

Dip-projection of reflection seismic data

One of the major problems in reflection seismology is processing and interpreting data collected along crooked profiles -- especially in mountainous regions. In addition to the geometry and statics problems, the crooked-line section generates areas of both low and high fold, and a finished section that is difficult to interpret as various parts of the line are oriented in different directions. Migration algorithms work best on seismic data that is exclusively down-dip, and the algorithms have difficulty moving the reflections back to their proper space-time location.

Dr. Costain developed the idea of "strike-binning", which involves reprocessing crooked-line data by projecting the data onto a line that in the dip projection. The technique involves completely reprocessing the data from scratch after the new geometry is set up. While this is effective in getting the data onto a dip line, the fold is still highly variable, with fold in the former 'strike' areas often > 200.

What I have done is to streamline the process by projecting pre-processed CDP's gathers -- filtered, deconvolved, and statics corrected -- onto the new, straight, dip-directed CDP line. A second inovation is the use of small bins -- 1/5 the size of ordinary CDP bins (e.g. 10m instead of 50m). The smaller bin size solves two problems: it reduces the fold in the strike areas and allows the gathering of smaller fold CDP's into CDP's of more even fold. Care must be exercised, however, to make sure that the gathers contain traces that are from shot-reciever pairs that are associated with each other; therefore there are never more than five 10m CDP's gathered at a time. After editing (to remove common offset traces within gathers), stacking, and interpolation (to make the CDP distances equal), the data are gathered once more into 50m CDP's. The finished section is easier to interpret, and should migrate better because it is now a true dip line. Both model and real data have been processed in this manner with good results. {Recently Accepted by Geophysics}

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Samuel T. Peavy
Department of Geology and Physics
Georgia Southwestern State University
Roney 208
800 Wheatley St.
Americus, GA  31709
E-mail: speavy@canes.gsw.edu

Last updated: 08/10/04