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Two reflection seismic transects, one across the central Appalachians in Virginia and the other across the southern Appalachians in Georgia, reveal a significant contrast in mid- and lower crustal reflectivity from east to west. Data from east of the Blue Ridge geologic province in Virginia and to the east of the Inner Piedmont in Georgia show a highly reflective crust extending from the near-surface to the Moho, including zones of east-dipping reflections, a sub-horizontal reflection signature at ∼7 seconds, and a west-dipping Moho. Reflection seismic data from west of the Blue Ridge in Virginia and Inner Piedmont farther south are characterized by reflector geometries related to deformation above a master decollement, leading to classic ‘thin-skinned’ tectonic structures in the overlying allochthon, and few if any apparent structures in the underlying basement. The location of the Iapetan rifted margin, the preexistence of favorably oriented structures to the east of this point, and sub-horizontal weak zones within the lower Paleozoic shelf strata have played critical roles in the distribution of seismic reflector geometry. Seismic reflection signatures seen in the southeastern United States are a result of multiple episodes of deformation from the early Paleozoic through the middle Mesozoic. Oblique stresses during late Paleozoic time produced transpression that manifested itself as predominantly strike-slip faulting to the east of the Blue Ridge/Inner Piedmont. Onlapping lower Paleozoic shelf strata responded to tectonic stresses through thin-skinned deformation above a master decollement during the late Paleozoic Alleghanian orogeny, aided in part by sub-horizontal zones of weakness in the strata. This partitioning of strain was supported via tectonic buttressing provided by Precambrian continental crust that was little deformed in the Taconic orogeny. During the Alleghanian orogeny, the variations in Valley and Ridge deformational style between the central and southern Appalachians were controlled by the original shape of the continental edge. Further deformation during Mesozoic extension occurred to the east of the Precambrian rift margin in the region where favorably oriented faults were reactivated, leading to the rotation of the fault zones from more steeply dipping initial orientations, the merging of the mid-crustal reflection zone with the Moho, and the formation of Mesozoic basins and antiformal reflections in the seismic sections.

The objective of this case study is to predict whether 5 years of water‐flood production from a thinly layered Gulf of Mexico reservoir will change its seismic amplitude‐variation‐with‐offset (AVO) response in a detectable manner. Density and velocity profiles were computed from in situ wireline logs for 100% oil, gas, and brine saturations and for a 5‐year prediction that was based on a fluid‐flow and production simulation. Analytical AVO curves for simple half‐space models did not match AVO curves extracted from synthetic seismograms computed with a full‐waveform layer‐stack algorithm. Several different amplitude corrections were tried to reduce the AVO curves from the synthetic data to the analytical ones, but, ultimately, none was deemed satisfactory. Instead, AVO change attributes based on relative changes, polarity changes, or ratios were used. Attributes based on the change of AVO gradient were perceived to be most diagnostic of the water flood, but they were also overly sensitive to interference noise and amplitude correction errors. For field data from the study area, a large decrease in intercept magnitude may be the best indicator of the waterfront.

The Savannah River Corridor is defined herein to extend from the Brevard zone in South Carolina to just northwest of Charleston, South Carolina. The length of the Corridor is 400 km, the width 300 km, and the strike is S 40°E. The Corridor includes (1) the Savannah River Site, which is the locus of the most detailed seismic reflection data set in the eastern United States, and is the keystone for the extrapolation of regional reflection seismic data away from the Site, and (2) the South Carolina–Georgia seismic zone. The principal data used for the conclusions of this study are reflection seismic data acquired by the Consortium for Continental Reflection Profiling (COCORP), the Appalachian Ultra-Deep Core Hole (ADCOH) project, and Conoco, Inc. (SRS). Relatively higher seismicity appears to be associated with a crustal scale antiform that has been imaged in the Savannah River Corridor as well as to the northeast along strike in Virginia. An increase in earthquake activity occurs in that part of the crystalline allochthon that overlies and conceals a foreland-dipping imbricate stack that has developed in early Paleozoic rocks beneath the overthrust Piedmont crystalline allochthon. To the southeast, hypocenters of two SRS earthquakes (1985 and 1988), projected along the strike of the Pen Branch northwest-bounding fault of the Dunbarton Triassic rift basin plot on a highly reflective southeastward-dipping shear zone that underlies the basin. Here, the proximity of the hypocenters to the border fault of the Dunbarton basin suggests that syntectonic structures associated with the Pen Branch border fault might be involved. Some of the seismicity is probably associated with reactivation of older Paleozoic and Mesozoic structures.

Reprocessing of line PR3 proprietary seismic reflection data has delineated Grenvillian, Paleozoic, and Mesozoic structures within the Appalachian foreland, Blue Ridge, and Piedmont of the central Appalachians in Virginia and West Virginia. The eastern portion of PR3 can be correlated along strike with the western portion of line I-64, reprocessed earlier at Virginia Tech. The combined seismic reflection data image the crust from the eastern Valley and Ridge, Blue Ridge, Piedmont, and Atlantic Coastal Plain provinces. Within the Piedmont, large (as much as 10 km wide) reflective structures imaged on both lines PR3 and I-64 are interpreted to be thrust sheets that might be composed of deformed Catoctin, Evington Group, and possibly younger metamorphosed rocks. A concealed extension of the Green Springs mafic mass intrudes a thrust sheet imaged along the PR3 profile. The Blue Ridge-Piedmont allochthon was transported north-west along the Blue Ridge thrust, which ramps upward ∼12 km east of the surface exposure of the Mountain Run Fault. Westward along line PR3, the Blue Ridge thrust maintains an undulating geometry; the maximum thickness of the Blue Ridge allochthon is interpreted to be ∼4.5 km. The Blue Ridge allochthon is generally acoustically transparent and overlies lower Paleozoic shelf strata. The maximum thickness of these strata is ∼8 km. Shelf strata are interpreted to extend in the subsurface 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 has not experienced the same degree of deformation as observed in the eastern provinces. A basement uplift ∼45 km wide is imaged beneath the Valley and Ridge province and is interpreted as having formed prior to Late Cambrian time. Farther west, reflections imaged beneath the Glady Fork anticline in the Appalachian Plateau are interpreted as a positive flower structure associated with wrench fault tectonics. Relatively few deep (>9 km) crustal reflections are imaged along line PR3. The majority of reflections that do exist at these depths are observed beneath the Piedmont and eastern Blue Ridge. The high reflectivity associated with the Grenvillian basement in these areas might be the result of the Paleozoic orogenies and extension related to Late Proterozoic and Mesozoic rifting.

Interpretive reprocessing of seismic reflection data has elucidated Paleozoic and Grenvillian structures in the southern Appalachians. The seismic data include a 7500-km² grid of ADCOH, Seisdata, and COCORP reflection profiles that traverse the Blue Ridge and Inner Picdmont geologic provinces of North Carolina, South Carolina, and Georgia. Surface geology and potential field data were used to constrain the interpretation. The reprocessed seismic reflection data have delineated the internal and external geometry of the crystalline Blue Ridge-Inner Piedmont allochthon, including the locations of the Blue Ridge master decollement, Haycsville fault, and Brevard fault zone. On the basis of the reprocessed data, all of the major faults within the allochthonous upper crust sole in the Blue Ridge master decollement. Reflections extending to the southeast from beneath the surface location of the Hayesville fault to the Blue Ridge thrust might be the seismic signature of a high strain zone. This implies that internal deformation of the Blue Ridge allochthon associated with the Alleghanian orogeny might have occurred farther to the west than has been previously documented from field studies. Relative amplitude seismic data enabled the discrimination between Blue Ridge-Inner Piedmont crystalline rocks and underlying lower Paleozoic shelf strata, thereby delineating the Blue Ridge thrust. The interpreted geometry constrains the top of the shelf sequence beneath the Blue Ridge to depths of less than 3 km. This relatively shallow depth of the shelf strata together with the presence of duplex structures and bright spots that are imaged within the sequence might imply favorable conditions for hydrocarbon exploration beneath the Blue Ridge. Midcrustal reflections from within the upper-to-Iower crust are interpreted to originate from preserved Grenvillian structures that were reactivated at the basement surface during Late Proterozoic-Early Cambrian extension. Reflection continuity is occasionally disrupted by interpreted post-Grenvillian, pre-Early Cambrian low-density intrusions. Topography at the basement surface, possibly caused by the intrusions, is interpreted to have controlled the formation of some of the structures within the overlying allochthon, including Blue Ridge and Brevard fault zone ramps. Correlation of seismic time-structure contour maps with available gravity data and two-dimensional gravity modeling suggest that anomalies in the gravity field can be attributed to low-density sources within the autochthonous crust. Discontinuous reflection packages from depths of 36–42 km are interpreted to originate from the Mohoroviĉiĉ discontinuity. The reflectors trend about N15°E with a true dip of approximately 15°NW.

The objective of this case study is to predict whether 5 years of water-flood production from a thinly layered Gulf of Mexico reservoir will change its seismic amplitude-variation-with-offset (AVO) response in a detectable manner. Density and velocity profiles were computed from in situ wireline logs for 100% oil, gas, and brine saturations and for a 5-year prediction that was based on a fluid-flow and production simulation. Analytical AVO curves for simple half-space models did not match AVO curves extracted from synthetic seismograms computed with a full-waveform layer-stack algorithm. Several different amplitude corrections were tried to reduce the AVO curves from the synthetic data to the analytical ones, but, ultimately, none was deemed satisfactory. Instead, AVO change attributes based on relative changes, polarity changes, or ratios were used. Attributes based on the change of AVO gradient were perceived to be most diagnostic of the water flood, but they were also overly sensitive to interference noise and amplitude correction errors. For field data from the study area, a large decrease in intercept magnitude may be the best indicator of the waterfront.

Two problems that commonly affect shallow seismic data are narrow bandwidth and interference from ground roll. Deconvolution techniques are often attempted to overcome these problems and thus improve resolution, but cannot generally improve resolution without losing signal-to-noise ratio. In this paper, we demonstrate the application of two simple techniques for enhancing shallow seismic data. The first, Vibroseis whitening is accomplished simply by application of an automatic gain control (AGC) to swept source data before they are correlated with the sweep. The second, stretched automatic gain control (SAGC), involves convolution of impulsive source data with a synthetic sweep and application of an automatic gain control before correlation with the sweep. We compare the effectiveness of these techniques with conventional deconvolution techniques, using data acquired during the 1993 non-invasive seismic source comparison at Oak Ridge National Laboratory. Our results indicate that these methods are capable of improving the quality of processed shallow seismic data from both swept sources and impulsive sources.

DNAG Transect E-3. Part of GSA’s DNAG Continent-Ocean Transect Series, this transect contains all or most of the following: free-air gravity and magnetic anomaly profiles, heat flow measurements, geologic cross section with no vertical exaggeration, multi-channel seismic reflection profiles, tectonic kindred cross section with vertical exaggeration, geologic map, stratigraphic diagram, and an index map. All transects are on a scale of 1:500,000.

In addition to seismic. velocities and densities, quality factor Q estimated from seismic data can help in lithologic interpretations. Q estimations as function of travel time can be used to determine lithology and/or levels of fluid/gas saturation because Q may be an order of magnitude more sensitive to changes in saturation or pore pressure than velocity as it has been shown by Winker and Nur (1979) and Toksoz et al. (1978).

Seismic data from the Atlantic Coastal Plain are reprocessed and composite refraction-reflection stack sections produced to investigate basement faults that penetrate upward into Atlantic Coastal Plain sediments in South Carolina. Reprocessing recovered reflections from within the deep crust to the Moho as well as from within thin veneer (300) of the Atlantic Coastal Plain sediments. One of the major objectives of this paper is to discuss the use of shallow refracted arrivals to construct a composite refraction- reflection stack that allows better imaging of the subsurface at shallow depths.

A seismic reflection profile in east-central Maine reveals a steeply dipping fault zone that terminates in diffraction hyperbolae that are associated with an offset of the Moho. When combined with data from geologic mapping, the seismic data imply Mesozoic or post-Mesozoic dip-slip reactivation of the Norumbega fault zone, hitherto interpreted as a locus of mid- to late Paleozoic dextral offset. This finding highlights the value of seismic reflection data in determining the history of fault displacement and warns of the complexities that arise in attempts to seismically characterize faults in polygenic tectonic regimes.

Lower Mississippian ramp-slope deposits (Paine Member) of southwestern Montana are composed of thin, rhythmically interbedded limestone and argillaceous limestone (argillite). Millimeter-thick graded layers typical of limestone beds represent distal storm deposits, whereas argillite layers containing abundant whole, delicate fossils represent quiet-water deposition during times of little or no storm activity. Spectral analyses of the fluctuating insoluble-residue content (quartz, muscovite-illite, organic matter) indicate a dominant periodicity of 0.6-2.85 ka in the ramp-slope deposits; no spectral peaks corresponding to typical Milankovitch-type periods ({approximately}20-100 ka) were observed. Similar {approximately}2.5 ka paleoclimatic periodicities are recorded in Quaternary continental and alpine glaciers, Quaternary deep-sea sediments, C variations in Holocene tree rings, and Permian deep-water evaporite varves. These short-term paleoclimatic fluctuations may represent one of several harmonics of the precessional (19-23 ka) or obliquity (41 ka) orbital cycles or may be related to variations in solar activity.

Summary. Vibroseis reflection data across the Blue Ridge and Piedmont provinces in Virginia, acquired in 1981 by the United States Geological Survey, have been reprocessed at Virginia Tech. The 12-fold, 14–56 Hz vibrator data were originally 8 s in length, but were extended to 14 s during correlation. Interpretation of the data was further improved by using a well-defined crustal velocity model derived by previous workers from earthquake and blast analyses in the area. Low reflectivity areas on the seismic section are interpreted to represent Grenville-aged crust on the section, in contrast to highly reflective allochthonous units. These regions of reflectivity allow for mapping of the gross crustal structure. Strong, subhorizontal arrivals at 9 to 12 s in the reflection data are interpreted to be from lower crustal layering just above the Mohorovicic Discontinuity (Moho) defined by earlier refraction work. This layering may be reflective only beneath regions of deformed crust and not at the base of the undisturbed craton. The depth to these reflectors is approximately 35 km beneath Richmond, Virginia and increases gently westward until the reflections disappear beneath the Blue Ridge Mountains about 70 km west of Richmond. These thicknesses are in agreement with earlier refraction work which also indicated greater Moho depths to the west. A rethickening of the crust near the Atlantic coast is also interpreted from the refraction data but is not evident on the reflection line, probably due to an acquisition or energy penetration problem. The deep reflections consist of a subhorizontally layered package about 5 km in thickness, thinning slightly toward the craton. Their base coincides with the crust-mantle boundary determined from refraction data, indicating that the reflections lie within the lower crust. Lower crustal reflections were not recorded west of the Piedmont on the multichannel data. PmP arrivals recorded by earlier workers from beneath the Blue Ridge, however, are consistent with a Moho reflector about 8 km shallower than indicated by the refraction data. If the crust-mantle boundary is a second-order discontinuity (smooth transition zone) there, the PmP arrivals may have been reflected from near the upper boundary of the transition zone and the refracted waves may have travelled along the base. The discrepancy in depths may therefore be a measure of the transition zone thickness.

Summary. Some 180 km of new VIBROSEIS profiles have been acquired in the southern Appalachian Inner Piedmont, Brevard fault zone and eastern Blue Ridge as part of the ADCOH Project site investigation. These data are of the highest quality yet obtained in a crystalline terrane in the US, perhaps in the world, and reveal several conclusions that should have a direct bearing upon the world-wide nature of composite crystalline thrust sheets and their modes of interaction with the platform rocks beneath. Strong reflections previously interpreted as the base of the crystalline sheet are clearly part of the platform sedimentary (clastic rocks) sequence resting upon the autochthonous basement and early Palaeozoic rift basins. This reflection package and related transparent zones are clearly repeated beneath the crystalline sheet indicating a complex of thrusts repeating units within the platform succession. Reflectors (granitoid-amplibolite contacts) in the crystalline sheet in the Inner Piedmont represent recumbent folds of similar wavelengths and amplitudes to folds mappable on the surface. Duplexing of platform rocks beneath the crystalline sheet appears to have resulted in doming of the crystalline sheet. Similarly, duplex formation in the platform was probably controlled by both the thickness of the crystalline sheet and the rheological properties of the platform succession.

On the Piedmont of the southeastern United States, seismic reflection statics at different points in a typical common‐depth‐point (CDP) gather can differ from one another by more than 50 msec because of topographic relief in excess of 50 m, variations in the thickness of the weathered zone that can extend deeper than 50 m, and variation of velocity in the weathered zone of more than 100 m/sec. The ABCD method for computing statics is introduced to account for these velocity and thickness variations as well as the topographic effect. This method combines elevations and positions of source‐receiver points with times of first arriving refracted waves read from reflection correlograms or seismograms. It was tested in central Virginia where typical piedmont conditions are encountered. At four locations, ABCD statics are close to control values determined independently from refraction experiments. At 99 source‐detector points, ABCD statics differ by an average of 4 msec, (maximum of 19 milliseconds) from conventional elevation statics that do not account for local velocity thickness variations in the weathered zone. Where source‐receiver points were in line, modified ABCD statics were obtained from first arrival traveltimes without using elevation and position data. In this metamorphic terrane where clear reflections are difficult to record, ABCD statics appear to be more effective than conventional elevation statics for enhancing reflections on a seismic record section.

The conventional procedure used to acquire Vibroseis® seismic reflection data is to sum in the field the contributions from several vibrator sources distributed over the source array. An alternative method of recording the data which provides more flexibility in the processing is to record the output from each pad position in the source array rather than summing in the field. Prewhitening these data before summing can improve the signal‐to‐noise (S/N) ratio. If cancellation of surface waves by a source array is not a requirement, then processing each sweep as a separate source point can result in increased lateral resolution. These procedures were applied to seismic data over a buried rift basin in the southeastern United States. The results demonstrate improvement in the S/N ratio and spatial resolution that enable better interpretation of the complex, internal geometry of the basin.

The southern Appalachian ultradeep core hole (ADCOH) is designed to test modern ideas about the formation of mountain chains along the edges of continents. The faulting, stratigraphy, lithology and out-crop patterns observed during detailed studies of the surface geology in this region have been interpreted in terms of a series of thrust sheets of diverse ages, which were transported northwestward (Fig. 1) over the Iapetan continental margin of North America (Hatcher 1978). Seismic reflection data and interpretations by Clark and others (1978), Cook and others (1979, 1983) and Harris and Bayer (1979) support this interpretation, and reveal a zone of high reflectivity which dips to the southeast beneath portions of the Blue Ridge and Piedmont physiographic provinces. This zone of reflections is correlative with a similar interval known in the sedimentary rocks of the Appalachian foreland some 150 km northwest of the ADCOH study area, and has been interpreted as a regional decollement, serving as the root zone for major thrust faults such as the Brevard.

The principal goal of the Appalachian Ultradeep Core Hole (ADCOH) Project is to study the processes related to the formation and reactivation of large faults in the internal parts of an intact composite crystalline thrust sheet formed by continent-continent collision along an ancient continental margin. Composite crystalline thrust sheets are some of the largest structures in orogenic belts, comparable in size to the large accreted terranes. They are therefore of central importance in the overall processes related to formation of mountain chains and the evolution of continental crust.

common-mid-point gathers to the amplitude and polarity of a trace at a given reference offset and time The reference offset is zero for seismic stacked record sections to obtain interoretable. correct. and accurate amplitude and polarity. Some of the eifects of the offs& dependent reflection amplitudes in critical processing stages, such as velocity analysis and stacking, are presented to emphasize the importance of the correction introduced. The synthetic data examples are presented to exhibit the importance of amplitude versus offset correction in routine data processing and interpretation. Without the Zoeppritz amplitude correction, the stacking process may produce incorrect and inaccurate reflections. In some cases reflections may be totally lost by the stacking process, resulting in incorrect interpretations. Velocity analysis which suggest double velocities for the same reflection time can be explained by the effect of the Zoeppritz amplitudes. If the stacking velocities are affected by the Zoeppritz amplitudes one may prefer to choose the higher velocities over the lower velocities to attenuate multiple energy. This is a case that can alter the polarity of reflections. It is demonstrated that if the seismic parameters are available, one can at-& the Zoeooritz amolitude correction to eliminate these effects. ?‘& seismic p’kuneters’may be obtained by inversion using the Zoeppritz equations or by methods of velocity analysis which use both actual amplitude information and geometrical relations.

We offer the following in response to Peter Maliphant’s discussion of our paper.

The subject of single‐sweep Vibroseis recording has been a source of discussion among U.K. acquisition personnel for some time.

A regional seismic reflection line (I-64) across the Virginia Piedmont has provided a stacked section suitable for an integrated interpretation of geophysical data in the region. A highly reflective upper crust, an allochthonous Blue Ridge Province, underlying thrust sheets including the Blue Ridge master decollement, and a basal decollement at a depth of about 9 km (3 s) are confirmed on the seismic data. Immediately east of the Blue Ridge Province, Appalachian structures plunge to as much as 12 km (4 s) depth. The Evington Group, Hardware terrane, and Chopawamsic metavolcanic rocks (Carolina terrane) crop out in the Piedmont Province, and numerous eastward dipping reflections originate from these rocks in the subsurface. These eastward dipping reflectors overlie a gently west dipping (10°–15°), highly reflective zone that varies in depth from 1.5 s (4.5 km) beneath the Goochland terrane to 4 s (12 km) beneath the rocks of the Evington Group. Some of the overlying eastward dipping reflections apparently root in this zone. The zone may include decollement surfaces along which the overlying rocks were transported. Relatively few reflections originate from within autochthonous Grenville basement at the western end of the profile. The Goochland granulite terrane is interpreted to be a westward thrust nappe structure that has overridden a portion of the Chopawamsic metavolcanic rocks. A broad zone of east dipping (20°–45°) reflections bounds the Goochland terrane on the east. These reflections may originate from deformation zones and continue to Moho depths. They appear to be correlative with similar events seen on other Appalachian lines. The pervasiveness of the zone of east dipping events on other seismic reflection lines and the continuity of the adjacent Piedmont gravity high suggest continuity of crustal features along the length of the Appalachians. A major conclusion of this study is that crustal thinning is responsible for the main components of the gravity field in Virginia, that is, the Appalachian gravity gradient and the Piedmont gravity high. The crust thins from about 52 km beneath the Appalachian mountains to about 35 km beneath Richmond, Virginia, and then rethickens by up to 10 km beneath the zone of east dipping reflections (mylonites?) east of Richmond. The I-64 seismic data also contain a sequence of reflections at about 9–12 s, indicative of lower crustal layering; the base of this zone of reflections coincides almost exactly with the Mohorovicic discontinuity interpreted from earlier refraction work. The layering extends about 70 km west from Richmond, Virginia, and is interpreted as a lower crustal transition zone that is believed to persist across most of Virginia.

A correlation between earthquake hypocenters and seismic reflection data in central Virginia has become apparent on an automatic line drawing (ALD) display of seismic reflection data. With the reprocessed Virginia I-64 reflection Vibroseis data extended to 14 s, reflectors are imaged from the lower crust as well as from the upper crust. Specifically, the improved resolution and data quality of ALDs have produced an image of an antiformal structure bounded by mid-crustal reflections on the bottom and by major thrusts at the top. The reflections that define the roof of the antiform are most prominent from about 6 s (18 km) on the east near Richmond under the Coastal Plain sediments, to 1-1.3 s (3-4 km) between Richmond and Charlottesville, and to 3.5 s (10.5 km) on the west. Seismic signatures that can be followed downward from the surface between Charlottesville and Richmond appear to be truncated at the roof of the antiform. The dominant reflections that define the roof correlate with the seismic signature of the transported Taconic suture on the west flank and mylonites on the east flank. The distribution of hypocenters in the area shows an excellent correlation with the westward-dipping reflections that form the roof ofmore » the antiform on its western flank. Earthquake activity in this locale may be related to reactivation of the thrusts defining the roof and/or faults above the antiformal structure; however, distribution of the easternmost and deepest set of hypocenters appears to be related to an extensive near-vertical diabase dike swarm of Mesozoic age.« less