Browsing by Author "Costain, John K."
Now showing 1 - 20 of 26
Results Per Page
Sort Options
- Amplitude balancing in [tau]-p domainGuo, Mu (Virginia Tech, 1994-05-10)An approach to balance the amplitude of seismic data in the Ï -Ï domain is introduced in this study. The idea of amplitude balancing technique is based on the following observation: In the Ï -Ï domain, direct wave, ground-roll, primary reflection, multiple and refraction arrivals are located at different regions. These regions can be viewed as signal region and noise region. By increasing the amplitudes in the signal region and suppressing the amplitudes in the noise region, so called amplitude balancing in T-Ï domain, the signal-to-noise ratio of seismic data can be improved. The Ï -Ï domain amplitude balancing scheme is tested and calibrated on synthetic seismic data using AIMS®' package. The modeled data is also used to illustrate transformation (slant stacking) to and from Ï -Ï domain. The signal-to-noise ratio enhancement using amplitude balancing in Ï -Ï domain is illustrated. This general discussion also includes aliasing effect of slant stack and deconvolution in Ï -Ï domain. After the calibration with syn1hetic data, the amplitude balancing in Ï -Ï domain is applied to real seismic data recorded on the Atlantic Coastal Plain near Richmond, Virginia and Aiken, South Carolina to explore the possibilities of enhancing the quality of seismic data. Processing of synthetic and real data is carried out on VAX 11/785 and Sun Sparc 10 workstation at the Regional Geophysics Laboratory at Virginia Polytechnic Institute and State University using DISCO@2 seismic data processing package. The results suggest that Ï -Ï domain amplitude balancing can be combined into conventional seismic data processing sequence to improve the signal-to-noise ratio and thus give a better imaged seismic section. Extensive tests carried out indicate that choice of ray parameter range, the degree of amplitude change, are important aspects of the processing in Ï -Ï domain. In this study, a complete data processing was carried out to generate a stack section of NRC line 2 in Virginia while the amplitude balancing in Ï -Ï domain was incorporated into a conventional processing scheme. The Ï -Ï domain processing of NRC line 2 improved the data quality. The signal-to-noise ratio enhancement obtained by the amplitude balancing in Ï -Ï domain led to test the method to improve weak reflections from within the Dunbarton Triassic basin on SRP line 2EXP in South Carolina. After the application of amplitude balancing in Ï -Ï domain, CMP gathers showed enhanced signal-to-noise ratio, although the improvement became almost indiscernible after stack.
- Analysis and interpretation of compressional (P-wave) and shear (SH-wave) reflection seismic and geologic data over the Bane Dome, Giles County, VirginiaGresko, Mark J. (Virginia Polytechnic Institute and State University, 1985)Approximately 37 km of predominantly 24-fold P-wave Vibroseis data and 16 km of 24-fold SH-wave Vibroseis data were acquired in the southern portion of the folded Appalachians near the Bane Dome in Giles County, Virginia. Data processing techniques included the application of newly developed methods for crossdip removal as well as the determination of statics solutions in the case of time variant shifts within the data traces. Minimum-phase filter deconvolution was also applied for the removal of reverberating energy and multiples recorded on the SH—wave lines. Vp/Vs ratios were used to aid in the determination of lithologies in the absence of bore-hole data. Interpreted thickening of the Lower Cambrian to Upper Precambrian sequence beneath the Bane Dome appears to represent Eocambrian rifting. Faults generated at that time may now be reactivated by the present stress regime, causing earthquake activity in this area. Interpretation of the seismic data supports a duplex structure proposed for the Paleozoic rocks of the Bane Dome Complex within the Narrows thrust sheet of southwestern Virginia.
- Applications of Roll-Along Electrical Resistivity Surveying in Conjunction with Other Geophysical Methods for Engineering and Environmental Site CharacterizationSayer, Suzanne (Virginia Tech, 1996-04-29)Roll-along electrical resistivity surveying was used with seismic refraction, magnetometer and gravity surveying in geophysical characterization of sites with a specific environmental or engineering problem. Three examples are presented where resistivity surveying provided vital constraints on acquisition and interpretation of other data in chaotic terrane. A commercially resistivity meter was used with prototype equipment designed, assembled, and tested at Virginia Tech. The equipment included a multiconductor cable consisting of interchangeable segments and a circuit allowing selection of numerous electrode configurations. The Sinking Creek Landfill, a 10-acre site, was used for disposal of municipal waste in the early 1970’s. Roll-along resistivity proved to be the most useful geophysical tool in ascertain its internal structure. Wenner configuration resistivity data, sensitive to both conductive leachate and ferrous metals, showed trenches within the landfill displayed in profile. Magnetic field measurements revealed anomalies over some trenches suggesting a method for discriminating between ferrous metal and leachate. Results of a resistivity survey can help planners of a refraction survey avoid low velocity “blind” layers. The Mid County Landfill borrow area is a 26 acre site situated within the Max Meadows Breccia and used for cover material for an adjacent landfill, The engineering problems were to measure the volume of rippable material, but travel time data were somewhat ambiguous. The refraction data interpreted using a) conventional 3-layer analysis b) horizontal 3-layer analysis of single shots, and c)continuous velocity gradient analysis of single end shots were compared with auger refusal depth. The single end horizontal analysis matched auger refusal depths most closely. Roll-along resistivity pseudo-sections made along the refraction lines proved to be effective for qualitatively imaging pinnacles and megaclasts. Excavation of fill material from a 75 acre river terrace in Pembroke exposed an antiform cut by high angle, near surface faults. Geophysical characterization was undertaken to determine the thickness of the alluvial deposit, and the relationship of the faults with structures in the underlying bedrock. Seismic refraction showed the terrace was as much as 134 feet thick. Resistivity pseudosections revealed a resistivity anomaly associated with the graben could be detected for a horizontal distance of several hundred feet. A gravity gradient paralleling the resistivity anomaly extends the feature more than 1000 ft from the exposed structure. Tenuous evidence of a bedrock escarpment beneath the near surface structure is found in a combination of seismic refraction, gravity, and electrical resistivity data. Roll-along resistivity has proved to be key to geophysical interpretation of these three areas. Images displayed on pseudosections reveal lateral inhomogeneity more clearly than could be discerned from seismic, gravity and magnetic data. Roll-along resistivity data can provide information for efficient siting of additional geophysical studies.
- Broken-formations of the Pulaski thrust sheet near Pulaski, VirginiaSchultz, Arthur P. (Virginia Polytechnic Institute and State University, 1983)Broken-formations (Hsu, 1974; Harris and Milici, 1977) occur in the lower part of the Pulaski thrust sheet and contain some of the most strongly deformed sedimentary rocks in the Valley and Ridge province of the southern Appalachians. Deformation in this zone ranges from grain-scale cataclasis to regional-scale faulting. The broken-formations are distinguished from rocks structurally higher on the sheet and from rocks of the underlying Saltville sheet by (1) a sharp increase in the variability of fold and fault styles, (2) greater ranges in fold plunges and dips of axial surfaces, (3) a low degree of preferred orientation of folds and faults, (4) an increase in the frequency of mesoscopic structures, and (5) the presence of Max Meadows tectonic breccia. Structural analyses indicate that deformation in the broken-formations is Alleghanian in age and that the deformed zone formed under elastico-frictional conditions, possibly under elevated fluid pressures with temporally variant stresses and that lithology may have played an important role in localizing the broken-formations along the base of the Pulaski sheet.
- Computation of pseudosonic logs in shallow fresh/brackish water wells: a test case in Brunswick, GeorgiaAllen, Nancy J. (Virginia Tech, 1989-11-13)Due to the usefulness of sonic logs in formation evaluation, efforts have been made to develop a method for calculating pseudosonic logs for wells in which sonic logs were not originally obtained. These efforts attempt to use electrical resistivity data in the calculation of pseudosonic logs by means of empirical scale functions. The purpose of this study is to examine ways of applying these relationships in relatively shallow wells where the principal formation fluid is fresh or brackish water. Data from four wells situated in Brunswick, Georgia were used in this study. Conventional focused resistivity logs are sensitive to beds as thin as one foot and can provide detail similar to that seen on sonic logs. Focused resistivity logs should be best for conversion to pseudosonic logs in shallow wells, where invasion is minimal and the water used for drilling fluid has electrical resistivity close to that of formation water. Sonic and resistivity logs from a representative well are needed in the procedure for finding an empirical relationship between sonic transit time and resistivity. Values of transit time plotted versus resistivity are read from corresponding depths on both types of logs. The graphs obtained in this study reveal significantly more scatter than previously published graphs based upon deep well data. An important feature clearly evident in the graphs is the presence of groups of points which me offset from each other. A separate scale function relating transit time and resistivity can be obtained from each group of points. It is noted that the different groups correspond to differences ir1 the chlorinity of the formation water. The results of this study indicate that it is necessary to consider the salinity of the formation water as well as electrical resistivity for purposes of calculating pseudosonic logs. In previous studies three constant coefficients were deterrnined experimentally in order to obtain an empirical scale function. The present study suggests that it may be possible to replace these constants with chlorinity dependent coefficients. The final results of this study indicate that reasonably reliable pseudosonic logs can be obtained only by using high quality focused resistivity logs from wells where information about the salinity of the formation water is also available.
- Correlation of seismic reflection data with seismicity over the Ramapo, New Jersey, fault zoneD'Angelo, Richard M. (Virginia Tech, 1985-07-24)Reflection seismic data, mylonite reflectivity, gravity data, and earthquake hypocenters have been integrated into a possible explanation for seismicity in the Ramapo fault area. Seven reflection seismic lines were processed using variations in sorting and residual statics. Single VIBROSEIS sweeps were treated as separate sourcepoints. Compressional velocities and densities were determined in the laboratory. Reflection coefficients and gravity models provide evidence for reflections from mylonite zones. Earthquake hypocenters were projected into the vertical seismic sections. The results suggest a correlation between rock volumes containing hypocenters and rock volumes containing mylonite zones. The seismic line furthest from the Taconic suture displays fewer hypocenters and mylonites, in agreement with an assumed model of mylonite development possibly associated with obduction of continental crust. The mylonite zones in the basement may serve as local areas of crustal weakness for seismic activity occurring in the area.
- Crustal structures and the Eastern extent of the Lower Paleozoic Shelf Strata within the Central Appalachians: a seismic reflection interpretationLampshire, Laura Dermody (Virginia Tech, 1992-06-05)Reprocessing of line PR3 proprietary seismic reflection data (24-fold) has delineated Grenvillian, Paleozoic and Mesozoic structures within the Appalachian foreland, Blue Ridge, and Piedmont of the central Appalachians. The eastern portion of PR3 can be correlated along strike with the western portion of line 1-64, reprocessed earlier at Virginia Tech. The 1-64 seismic reflection data (12-fold) images the crust from the eastern Valley and Ridge, Blue Ridge, Piedmont and Atlantic Coastal Plain provinces. Automatic line drawing displays were produced from both data sets for the purpose of interpreting and comparing subsurface structures. Within the Piedmont, large reflective structures imaged on both lines PR3 and 1-64 are interpreted to be nappes that might be comprised of deformed Catoctin, Evington Group and possibly younger metamorphosed rocks. A concealed extension of the Green Springs mafic mass intrudes a nappe imaged along the PR3 profile. The Blue Ridge-Piedmont allochthon was transported in a northwest direction along the Blue Ridge thrust, which ramped upward beneath the Piedmont province approximately 12 km east of the surface exposure of the Mountain Run Fault. Along line PR3, the Blue Ridge thrust maintains an undulating geometry, and the maximum thickness of the Blue Ridge allochthon is interpreted to be approximately 4.5 km. The Blue Ridge metamorphic allochthon is generally acoustically transparent and overlies parautochthonous Lower Paleozoic shelf strata. The maximum thickness of these strata is approximate1y 8 km. Shelf strata are interpreted to extend as far east as 5 km east of the surface exposure of the Mountain Run Fault, the northeastward extension of the Brevard Fault Zone, where they are truncated by the Blue Ridge thrust at a depth of 10.5 km (3.5 s). Various folds and blind thrusts are imaged beneath the Appalachian foreland; however, the foreland does not appear to have experienced the same degree of deformation as observed in the eastern provinces. A basement uplift approximately 45 km wide is imaged beneath the Valley and Ridge province and is interpreted as having formed prior to Upper Cambrian time. Further west, reflections itnaged beneath the Glady Fork anticline in the Appalachian Plateau are interpreted as a positive flower structure associated with wrench fault tectonics. Relatively few deep crustal reflections are inlaged along line PR3. The majority of reflections that does exist at these depths is observed beneath the Piedmont and eastern Blue Ridge. The high reflectivity associated with the Grenvillian basement in these areas suggests that this crust was deformed during compression related to the Paleozoic orogenies and extension related to Late Proterozoic and Mesozoic rifting.
- Determination of fractured aquifer characteristics from evaluation of pump tests of wells in the crystalline rocks of the Blue Ridge allocthonDawson, James W. (Virginia Polytechnic Institute and State University, 1988)The Precambrian age rocks of the Blue Ridge thrust sheet near Roanoke, Virginia, comprise an igneous and metamorphic assemblage with the predominance of porosity and permeability of the formations attributed to secondary factors. Aquifer characteristics of transmissivity, storage coefficient, hydraulic conductivity and fracture NL) permeability are determined from evaluation of pump tests conducted on ground water supply wells developed in this fractured aquifer. Evaluation of pump test data indicates that aquifer response is similar to a double porosity medium in some instances. Comparison of pump tests at locations close to, and further removed from, the leading edge of the Blue Ridge Thrust Fault indicate little variability in fracture permeability. The data suggest that deep circulation of ground water in the Blue Ridge allochthon may be more likely than previously thought.
- Estimation of seismic parameters from multifold reflection seismic data by generalized linear inversion of Zoeppritz equationsDemirbağ, Mustafa Emin (Virginia Tech, 1990-05-05)An inversion method is developed to estimate the P- and S-wave velocities and density ratio of two elastic, isotropic, and homogeneous media separated by a plane, horizontal boundary from P-wave reflection amplitude-versus-offset (AVO) data recorded at the surface. The method has for its basis the inversion of the plane wave Zoeppritz. equations by generalized linear inversion (GLI) and bootstrapping. The Zoeppritz equations are converted into the time-offset domain by using Snell’s law, common mid-point (CMP) geometry, and two-way travel (twt) time. The equations in the time-offset domain have five independent variables that enable estimation of P- and S-wave velocities and density ratio for the upper and lower layers. The linearity and uniqueness of the inversion are investigated by residual function maps (RFMs). The RFMs show closed elliptical contours around the true values of the seismic parameter pairs except in the case of S-wave velocity pair for which the open contours imply a linear correlation. However, the RFMs of S-wave velocities with the other model parameters show well defined minima, indicating the uniqueness of the inverse problem in the absence of noise. The estimation of seismic parameters is constrained by physical considerations and the results are enhanced statistically by bootstrapping to obtain the most likely solutions, i.e., the mode values of the distribution functions of solutions, and the confidence limits of the most likely solutions. The inversion method is tested using model AVO data with and without random noise. The tests show that the model parameters are exactly recovered when offset-to-depth (O/D) ratio 1s about 2 or larger, depending on the contrast among the seismic parameters of the media. The results for small O/D ratios (< 1) diverge from the true values, especially for S-wave velocities, and indicate the importance of the O/D ratio in the AVO data inversion. The parameters are not recovered correctly in the case of noisy model AVO data because of the degrading effect of noise in the inversion. However, the model parameters fall into the confidence limits of the estimated parameters when tight constraints are imposed on the solutions, and the signal to noise (S/N) ratio is high. The inversion method is sensitive to auxiliary parameters such as the root-mean-square (rms) velocity and zero-offset twt time which are used in the adjustments of observed or calculated reflection amplitudes to compensate for the effects of wave propagation. Because the plane wave Zoeppnitz equations define the variation in reflection amplitude with offset for a single boundary, the method is limited to isolated reflections in the CMP gathers. The AVO inversion is applied to field data from the Atlantic Coastal Plain in South Carolina to show the feasibility of the method. The first example is from Charleston, S.C. where the estimated seismic parameters from adjacent CMP gathers are in close agreement demonstrating the stability of the AVO inversion. The second example is a data set that crosses the border fault of the Dunbarton Triassic basin, S.C. For this data set common offset stacked CMP gathers are used to increase the S/N ratio and minimize the surface coupling effects. The inversion results show that the seismic parameters are greater north of the border fault indicating crystalline basement while smaller parameters to the south represent the Triassic basin. P-wave velocities estimated for the crystalline basement (6.4 km/s) and the Triassic basin (4.8 km/s) are in good agreement with the computed refraction velocities and support the interpreted location of the Dunbarton Triassic border fault.
- Failure of asperities by hydraulically induced fatigue: a model for the generation of intraplate seismicityNeedham, Daniel L. (Virginia Tech, 1987-08-18)A model is proposed in which intraplate earthquakes are triggered by the tensile failure of asperities by subcritical crack growth. The principle subcritical crack growth mechanism is fatigue driven by cyclic tensile stresses within an asperity. Cyclic stresses result from cyclic pore pressure variations which propagate from the Earth's surface because of annual variations in the height of the water table. Asperities are postulated to be porous and permeable masses of saturated host rock hydrologically connected to fluids within an open fracture. Porosity and permeability within asperities are assumed to be due to microcracks within the rock matrix. Tensile stresses within an asperity are due partly to mechanical loads, but pore pressure is the primary Inechanism by which tension is developed and fatigue operates primarily in tension. Fatigue crack growth is enhanced by chemical subcritical crack growth mechanisms such as stress corrosion and these mechatusms are proposed to act in unison. Faulting may be initiated when a population of asperities is either driven to failure by these mechanisms alone or when it is weakened to the point at which it is vulnerable to small applied stresses. Numerical modeling of the interaction of pore pressures and stresses within asperities indicates that a small transient increase of pore pressure on the order of a fraction of a megapascal will increase the tensile stresses within an asperity by a fraction of a megapascal. Consequently, it may be possible for a small increase in pore pressure, due to elevated water table levels, to trigger asperity failure and result in seismicity.
- A finite element analysis of dissolved oxygen drawdown and sulfate production in strip mine spoil dams due to pyritic chemical reactionAmend, Joseph Henry (Virginia Tech, 1975-05-06)The problem of dissolved oxygen drawdown and sulfate production in strip mine spoil dams due to chemical reaction between an excess of pyrite, water, and dissolved oxygen is investigated. A finite element analysis is used to solve the diffusion-convection equation assuming steady state conditions and no internal oxygen sources. Oxygen recharge along the free surface is permitted. The analysis is performed in three steps. Step one calculates the nodal piezometric heads and elemental bulk fluid velocities. Steps two and three determine the nodal dissolved oxygen and sulfate concentrations. Solutions are presented for a wide range of soil permeabilities and dissolved oxygen reaction coefficients. It is found that as the dissolved oxygen reaction coefficients. decreased, the dissolved oxygen deficit decreased and the sulfate concentration increased. From these results it was found that the maximum dissolved oxygen deficit occurs at an internal point in the dam and the maximum sulfate concentration occurs on the downstream face of the dam. The computer program used in the analysis is written in FORTRMi IV computer language and requires a minimum of programming knowledge to implement.
- Geologic framework of gravity anomaly sources in the central Piedmont of VirginiaKeller, Mary Ruth (Virginia Tech, 1983-06-15)Bouguer gravity anomalies at 1870 locations on the central Piedmont of Virginia from 37° 37' N to 37° 52' N and 77° 44' W to 78° 23' W display patterns of variation produced by upper crustal density contrasts and thickening of the crust in a WNW direction. No other deep sources are evident. Upper crustal density contrasts are associated with rock units known from geologic mapping. ‘The subsurface distribution of these rock units interpreted from seismic reflection data was confirmed by measured variations in gravity. A two-dimensional model analysis indicates the following average in situ density values for the principal formations: Arvonia Formation-2.77 gm/cc, Columbia Granitoid-2.75 gm/cc (tonalite) and 2.73 gm/cc (pegmatite), Chopawamsic Volcanics- 2.77 gm/cc (felsic units), and 2.79 gm/cc (mafic units}, Catoctin/ Lynchburg-2.815 gm/cc, Maidens Gneiss-2.775 gm/cc, Grenville Basement- 2.71 gm/cc. Gravity and seismic data are consistent with the existence of a major thrust fault at depths between 9 km and 16 km that separates Grenville Basement rocks from younger Catoctin/Lynchburg rocks. The slight eastward dip of this thrust fault beneath the western part of the area increases significantly east of 78° 05' W. Gravity anomalies suggest the existence of several mafic inclusions within the Columbia Granitoid that were not identified by geologic mapping.
- A geophysical characterization of New River terrace deposits in Giles County, VirginiaCyrnak, Jennifer Sue (Virginia Tech, 1996-06-05)A series of geologically recent faults was discovered within a folded succession of stratified alluvial sediments, commonly referred to as New River terrace deposits, near the town of Pembroke in Giles County, southwest Virginia. Geological and geophysical investigations were utilized to characterize the terrace deposits and investigate the nature of the observed faults. Geologic mapping of the underlying carbonate bedrock provided orientation measurements of fault, bedding, and joint planes; however, none of the features mapped within the bedrock could be directly correlated with the observed faults. The results of geophysical studies suggest significant variations in lithology within the alluvial sediments. Seismic velocities (P-wave) obtained from reversed seismic refraction profiles range from 900-1700 meters/second, while apparent electrical resistivity values vary from 300-2000 ohm-meters. A 75 meter wide, east-west trending low resistivity zone extending across the center of the study area is juxtaposed against an extremely high resistivity zone which is present to the north. The results of seismic reflection and electrical resistivity data analysis are interpreted to suggest that several small and large scale extensional faults may exist throughout the sediments, possibly reaching depths of up to 30 meters. An isopach map of preserved terrace thickness indicates that the faults lie within an east-west trending zone of maximum terrace thickness (35-40 meters thick), which corresponds to a topographic rise in land surface. A structure contour map of the bedrock surface reveals an area of lowest bedrock elevation beneath this zone, implying inverted topography. Results indicate that the evolution of the terrace might be related to ongoing karst processes within the bedrock.
- A geophysical investigation of geologic structure and regional tectonic setting at the Savannah River site, South CarolinaDomoracki, William Joseph (Virginia Tech, 1995-05-11)Time-structure, isochron, and reflection amplitude maps were constructed from 270 km of reprocessed seismic reflection data recorded at the Savannah River Site (SRS), South Carolina. These maps indicate the presence of three major basement blocks bounded by northeast trending faults that penetrate upward from basement into the Atlantic Coastal Plain sediments. Most of these faults are interpreted to be Paleozoic and Mesozoic structures reactivated under compression. The northernmost fault block is bounded on the southeast by the Tinker Creek fault (TCF), a high-angle southeast dipping reverse fault, that appears to be a reactivated splay from a major decollement (Augusta fault?) at 5 km depth. The TCF can be traced with certainty for a distance of 15 km across SRS where it offsets the top of basement and shallower reflections. The throw of the fault, as does its upward penetration into the sedimentary section, increases to the northeast where the shallowest reflections (≈ 20 m) appear to be disturbed. The northwest boundary of this block is unknown. The central fault block, 9 km wide in a NW-SE direction, is bounded on the northwest by the Tinker Creek fault and on the southeast by the Pen Branch fault, the northwest border fault of the concealed Dunbarton Triassic basin, now reactivated as a southeast dipping high-angle reverse fault within the Coastal Plain sediments. The Pen Branch fault (PBF) is the main throughgoing fault in a 3 km wide, 25 km long, fault zone consisting of subparallel fault segments that are commonly down-to-the-northwest north of PBF and down-to-the-southeast south of PBF. The offset of time horizons by the PBF increases to the northeast and the shallowest resolvable horizons appear in time to be deformed across its length. The Steel Creek fault, an antithetic fault to PBF, controls the drainage of Pen Branch, a tributary stream to the Savannah River. The third fault block is bounded by the Pen Branch fault on the northwest. The southeast boundary of this southernmost block is unknown, but could be defined by the Martin fault, a down-to-the-northwest fault located south of SRS that appears to be a major structure. Interpretation of isochron maps and analysis of trend surfaces fitted to reflection time horizons suggest that faulting was ongoing through the Late Cretaceous and was accompanied by tilting and horizontal rotation of the fault blocks. Fault block movements in the Tertiary are uncertain as shallow time horizons cannot be correlated areally with confidence; however, isochrons and reflection horizons correlated across faults reveal deformation, but not offset, of the shallowest resolvable events. The comparison between time-structure and topography reveals areas of positive correlation that suggest either near surface velocity variations or possible Cenozoic uplift. The reprocessed seismic data elucidate the geometry of the Dunbarton basin and the underlying crustal structure. The Dunbarton basin is essentially a half-graben bounded on the northwest by the near vertical Pen Branch fault. The maximum thickness of the basin fill, as determined by seismic modelling, is between 4 and 5 km. Minor intrabasinal faults are imaged within the basin as well as bright discontinuous reflections that probably represent sills. The crystalline crust below SRS is highly reflective and is distinguished by zones of southeast dipping reflections that are correlatable between seismic lines. Two major crustal reflections, tentatively identified as the Augusta fault and a mid crustal decollement, appear to correspond to events imaged in seismic lines collected by COCORP 60 km to the southwest where the latter event was interpreted to be the Appalachian Master decollement. The continuity of deep reflection events along geologic strike from Georgia to South Carolina supports models for evolution of the Appalachian orogen that incorporate large scale, presumably Paleozoic age, thrusts beneath the Piedmont and under the Atlantic Coastal Plain.
- A geophysical study of the earth's crust in central Virginia with implications for lower crustal reflections and Appalachian crustal structurePratt, Thomas L. (Virginia Polytechnic Institute and State University, 1986)Reprocessing of the United States Geological Survey's seismic reflection profile along Interstate 64 (I64) across Virginia with a data extension to 14-sec two-way travel time has provided a stacked section suitable for an integrated interpretation of refraction, earthquake, and blast analyses done by previous workers as well as gravity modelling done in this study. The seismic reflection profile shows a highly reflective upper crust which is consistent with an allochthonous Blue Ridge Province, possibly with underlying thrust sheets, and a basal decollement at about 9 km (3 sec) depth. Immediately east of the Blue Ridge province, the Appalachian structures plunge to up to 12 km (4 sec) depth where their interpretation on the section becomes ambiguous. The Evington Group, Hardware Terrane, and Chopawamsic metavolcanic rocks exposed in the Piedmont Province correspond to numerous reflections which appear to overlie a shallowly (10° to 15°) west-dipping, highly reflective zone dipping from 1.5 sec beneath the Goochland Terrane to 5 sec beneath the Evington Group rocks. Some of the overlying reflections apparently root in this zone which is therefore interpreted to include decollement surfaces along which the overlying rocks were transported. Grenville basement rocks are interpreted to underlie this zone and form autochthonous basement beneath the entire western portion of the profile because relatively few reflections originate from within this region. The Goochland granulite terrane is interpreted as a nappe structure which has overridden a portion of the Chopawamsic metavolcanic rocks. The Goochland terrane is bounded on the east' on the section by a broad zone of east-dipping (20° to 45°) reflections which may penetrate to Moho depths and are possibly correlative with similar events seen on other Appalachian lines. The 164 section contains a layered sequence of reflections at about 9 to 12 sec extending about 70 km west from Richmond, Virginia whose base coincides almost exactly with the Mohorovicic Discontinuity (Moho) interpreted from earlier refraction work. The deep reflections are thus believed to be lower crustal layering forming a 5 to 10 km thick Moho transition zone which is believed to persist across the state. The density contrast of 0.25 gm/cm³ between the lower crust and upper mantle derived from gravity modelling, the seismic transition zone, and the presence of intrusive rocks of lower crust-upper mantle origin at the surface are consistent with partial melting and contamination of the lower crust with upper mantle material. The refraction data and gravity modelling are consistent with a crust which thins from about 52 km beneath the Appalachian mountains to about 35 km beneath Richmond, Virginia, and rethickens by up to 10 km beneath the zone of east-dipping events east of Richmond. The pervasiveness of the zone of east-dipping events on other seismic reflection lines and the continuity of the adjacent Piedmont gravity high suggest that a similar crustal profile occurs along the length of the Appalachians.
- Gravity profile evaluation of geological cross-sections through the Southern Appalachians in Frederick County, VirginiaMason, Michael Randolph (Virginia Tech, 1989-06-01)The geology of Frederick County, Virginia is known mostly from surface mapping. Based on this work are interpretations of the subsurface geology including cross sections which have been constrained by the surface geology and by the method of area balancing based upon palinspastic reconstruction. With the intent to further constrain these cross sections. gravity measurements were made at 422 sites in Frederick County. Then, gravity anomalies were compared with theoretical gravity profiles calculated from two dimensional models of density distribution based upon the cross sections. Using the cross section geometry and densities published for the known and inferred rock units, the theoretical gravity profiles did not compare favorably with corresponding Bouguer gravity profiles. However, by modifying the geometry of the model units and adjusting the model unit densities, a reasonable fit between theoretical and Bouguer gravity profiles was obtained. Although the geometrical modifications adhered to the structural style, no attempt was made to area balance these modifications within the model.
- An Integrated Geophysical Study of the Central Appalachians of Western Virginia and Eastern West VirginiaPeavy, Samuel Thomas (Virginia Tech, 1997-07-18)Over 700 km of industry seismic reflection data in the central Appalachians were reprocessed using both conventional and newly developed processing schemes. A new processing sequence, called dip projection, is introduced. The technique projects crooked-line processed CMPs onto a straight line oriented in the general dip direction for the area. The new stacked sections more closely approximate a dip line and hence are more migration-friendly and interpretable than the crooked-line stacks. Methods of determining the lateral continuity of subsurface density contrasts were also applied to gravity data from the study area. Known collectively as potential field attributes, the analytic signal, the tilt angle, and the gradient of the tilt angle (the potential field wavenumber ) proved valuable in the analysis of the gravity data. Comparison of reflection seismic data from the southern and central Appalachians revealed a dichotomy of seismic reflectivity from east to west. A highly reflective crust beneath the Piedmont in both the central and southern Appalachians contrasts with a general lack of reflectivity beneath the Blue Ridge and Valley and Ridge provinces where coherent reflections are restricted to the upper 3-4 seconds of the data. This difference in reflectivity is interpreted as a fundamental difference in the location and orientation of preexisting zones of weakness between the different crustal regions with respect to the tectonic events affecting the Appalachians since the early Paleozoic. The combination of the results of new methods of seismic and potential fields processing with deep well and geologic information allowed the lateral continuity of two major structures in the central Appalachians to be examined. The Blue Ridge in Virginia was found to overly a duplex of Cambrian-Ordovician carbonates formed in response to stresses during the Alleghanian Orogeny. A large thrust sheet of similar carbonate rocks was interpreted beneath the Nittany Anticlinorium in West Virginia. To the south in Virginia, this thrust sheet is replaced by imbrication of the carbonate package. The change in structural style may be related to the existence of a lateral ramp or it may reflect the overall change in structural style from the central to southern Appalachians.
- Interpretation of refraction and reflection stack data over the Brevard fault zone in South CarolinaLaughlin, Kenneth J. (Virginia Tech, 1988-01-27)Near surface structures across the Brevard fault zone are studied using the refraction and reflection arrivals recovered from the Appalachian Ultradeep Core Hole (ADCOH) regional seismic Line 1. In using refracted arrivals, a new processing approach is introduced that translates refracted first arrivals from multifold seismic data into a refraction stack of two-way delay time sections. Reprocessing of reflected arrivals has improved shallow reflectors and allowed better imaging of the Brevard fault zone. Following processing of refraction and reflection arrivals independently, both data sets are combined into a composite stack section. The composite stack section displays one bright refractor interpreted as the boundary between the weathered layer and high velocity crystalline rocks. This refractor is continuous in the Inner Piedmont with occasional vertical offsets. The continuity of the refractor diminishes across the Brevard fault zone. In the eastern Blue Ridge, the refractor is discontinuous with high angle truncations. On the composite stack section, the Brevard fault zone can be traced from the surface to 6 km (2 s) where it appears to splay from the Blue Ridge thrust. Different from previous interpretations, the Brevard fault zone is imaged as having both an upper and a lower boundary surface as well as a group of reflectors within the zone. This reflection package initially thickens to 2 km at 3 km depth, then thins as it reaches the Blue Ridge master decollement. The Blue Ridge thrust is as shallow as 1.5 km (0.5 s) at the northwest end of the Line l. A deeper decollement is interpreted below the Blue Ridge thrust. The depth of this deeper thrust is 3 km (1 s) at the northwest end of the line, and also joins to the Blue Ridge thrust at 6 km depth making the structures below the Brevard fault zone more complex than previously published.
- Moment-radius-stress drop relations and temporal changes in the regional stress from the analysis of small earthquakes in the Matsushiro region, southwest Honshu, JapanDysart, Paul S. (Virginia Polytechnic Institute and State University, 1985)Moment-radius-stress drop relations and composite focal mechanisms were determined for 33 small, shallow, strike slip earthquakes (1.7$MLS4.3) in the Matsushiro region of southwest Honshu, Japan during the period August 1971 through September 1974. Estimates of the scalar moment derived from deconvolved SH displacement amplitude spectra range from 1018 to 1022 dyne-cm. The apparent stress ranges from 0.001 to 3.00 bars and the relationship between apparent stress and moment is approximately linear with a slope of 1.
- The New York-Alabama Magnetic Lineament: its reflection character and relationship to the Grenville frontHopkins, Debbie L. (Virginia Tech, 1995-07-15)The source of the New York - Alabama Magnetic Lineament (NY AML) is revealed on newly reprocessed seismic reflection data, and the Grenville Front Tectonic Zone (GFTZ) is imaged beneath it in eastern Tennessee. Industry data, correlated to lower crustal depths, image a wedge-shaped block beneath the shelf strata of the Cumberland Plateau and Valley and Ridge provinces of eastern Tennessee. Two dimensional gravity and magnetic modeling corroborate the interpretation that the contrast in density and magnetic susceptibility between the wedge and the adjacent crust produces the Lineament. The boundary across which the contrast is generated dips approximately 30° northwest. East-dipping reflections imaged below 7 seconds can be extended northwest to the surface where they align with the position of the Grenville Front. The reflections are interpreted as evidence of deformation related to the GFTZ in Canada. The mid-crustal band of reflectivity visible on most of the reflection profiles lies above the east-dipping reflections and is interpreted to delineate the eastern margin of the GFTZ. The crust southeast of the wedge-shaped block exhibits high reflectivity with well-developed west-dipping events. The west-dipping events might correlate to those reported in Ohio on COCORP data, suggesting that they are pervasive in the basement throughout the eastern United States. The fabric is interpreted to have formed during the continent-continent collision of the Grenville Orogeny. The absence of west-dipping reflections within the wedge suggests that the wedge is younger than the development of the fabrics recorded by the reflections. Vertical dike swarms are interpreted to intrude the west-dipping fabric. The swarms model as felsic and appear on migrated data to be older than uplift, erosion, and deposition of the shelf strata. Crustal thickness estimates by previous authors of over 45 km are corroborated with interpreted images of the Moho on two deeper reflection profiles. The thick crust might be the locus of anatectic melting following Grenville collision. The emplacement of granitic or granodioritic magmas provides an explanation for the density, magnetic susceptibility, and difference in reflectivity of the wedge-shaped block. The New York - Alabama Magnetic Lineament diverges from the location of the Grenville Front north of the study area. The position of the NYAML can be interpreted to represent the axis of anatectic melting following collision, and indicates that the thickest part of the crust formed farther east of the Front in Canada than in Tennessee. Pseudomagnetic field investigations permit the distinction between the source of the New York - Alabama Magnetic Lineament and adjacent high susceptibility sources to the northwest. The sources to the northwest appear from the modeling to be mafic intrusions that might be related to the Norris Lake peridotite. Earthquake locations in the Eastern Tennessee Seismic Zone (ETSZ) are aligned along the southwest edge of the gradient of the NY AML, and fall within the crust characterized by strong west-dipping reflections. Because the contact between the wedge and the region of west-dipping reflections is dipping to the northwest, the relationship between the NYAML and the ETSZ is not clear. More accurate hypocenter locations are necessary to clarify whether the earthquakes are restricted to the region of the crust typified by west dip. If not, the relationship between the earthquakes and the NYAML might be coincidental. A velocity model that considers the dipping boundaries in these reflection data should result in hypocenter locations that can constrain the relationship.