William P. Iverson

Master decollement root zone beneath the southern Appalachians and crystal balance

A root zone of the southern Appalachian master decollement is postulated to exist beneath Kings Mountain belt, on the basis of an alternative interpretation of COCORP seismic profiles, palin-spastic restoration of the entire crust, and earlier geological and geophysical interpretations. Mildly deformed Cambrian metased-imentary rocks beneath the decollement end beneath the Piedmont in the vicinity of Danielsville, Georgia. Reflections farther southeast are probably from ultramylonites. Weak subhorizontal reflections continue southeasterly to the Kings Mountain belt, where a series of east-dipping (30°) reflections dominate the section to a depth of 20 km beneath the Charlotte belt. Reflections southeast of the root zone are possibly all related to the structure in an accreted island arc.

V p V s from mode-converted P-SV reflections

P-SV reflections are generated by a compressional‐wave source and result from P waves that are converted to shear (SV) waves upon reflection. Recording both the P and SV components yields compressional and shear data simultaneously. Verifying that the easily detected events really are P-SV reflections is accomplished by noting the good correlation of surface CDP data with vertical seismic profile (VSP) reflections. Stacking velocities from P-SV CDP gathers determine the VpVs product when source‐to‐receiver offset is less than the depth of the reflector but data from synthetic models show that P-SV reflections are nonhyperbolic for shallow reflections or when source‐to‐receiver offset is too large. Shear velocity (Vs) can be calculated from P-SV reflections by one of two techniques: comparison of stacked section P-P and P-SV reflection times or by using the P-P and P-SV stacking velocities. Unfortunately, most P-SV reflections on a P-SV seismic section do not necessarily originate from exactly the same dep...

Reprocessing and reinterpretation of COCORP southern Appalachian profiles

Abstract Reprocessing of COCORP southern Appalachian data was focused on basic seismic evidence for continuation of sediments beneath a master decollement. The most important evidence is a nearly continuous series of subhorizontal reflections extending from the Valley and Ridge province into the Piedmont province. Continuity of subhorizontal reflections becomes tenuous in the Inner Piedmont. Careful reprocessing has yielded evidence for termination of strong reflections beneath the allochthon and the beginning of a relatively weak and complex series of “events”. Termination of sedimentary rocks beneath the Piedmont is interpreted from true amplitude seismic data. A zone of detachment continues southeast of the sediment termination as far as the master decollement root zone. Research on stacking velocities has indicated that complex velocity structures could create apparent low stacking velocities. This phenomenon may occur in the Charlotte belt of Georgia. Bouguer gravity can be modeled as a former craton of normal density with an accreted margin of very slightly higher density. Variation in crustal thickness also contributes to the Bouguer gravity gradient. No continuous large-scale overthrust is needed southeast of the interpreted master decollement root zone located beneath the Kings Mountain belt and Charlotte belt.

Reprocessed COCORP southern Appalachian reflection data: Root zone to Coastal Plain

COCORP (Consortium for Continental Reflection Profiling) seismic reflection data from the Atlantic Coastal Plain and eastern Piedmont in Georgia have been reprocessed and interpreted. Dipping events correlated with the master decollement root zone may be the only features that can be correlated with other reflection lines. Farther southeast on the Coastal Plain the shallow section is improved to produce reflections corresponding to the base of the Coastal Plain. A Triassic basin is confirmed to be more than 2 km deep and is the only major basin accurately revealed by the seismic lines. Improvement of the shallow seismic data is important to understanding the deep data. Some apparently deep reflections are probably produced from within the shallow Coastal Plain. Even though these shallow data have been improved, the Augusta fault is not interpreted as a major reflective feature.

Apparent azimuthal shear anisotropy from normal-moveout differentials

A simple normal-moveout (NMO) correction can move common-depth-point (CDP) reflection times up or down a few milliseconds when the stacking velocity is varied a small fraction. Different stacking velocities on two orthogonal shear components are often required, such as in the cases of azimuthal anisotropic media (Thomsen, 1986) or transversely isotropic media (Levin, 1979). The difference between these two cases is that CDP reflection times are different on S1 and S2 in the case of azimuthal anisotropy, while CDP reflection times are equal for transverse isotropic media. The similarity is that both cases require different stacking velocities on the S1 and S2 components.

Two kilohertz reflection data from 6,000 feet

High resolution geophysics has expanded in our industry as the need for detailed studies has grown both in exploration and production environments.

AVO For Thin Sand Bed Detection

Shelf-deposited sand ridges enveloped in marine shale are common oil traps in the upper Cretaceous sedimentary section of the Powder River Basin, Wyoming. Seismic detection of this class of oil trap is difficult because the reservoir sands average less than 10 m in thickness and they commonly occur at depths of 2500 m or more; they are therefore too thin to image given the frequency content of most seismic data sets. The presence of the sand ridge sequences can be detected however, by means of Amplitude-Variation-With-Offset (AVO) analysis. The AVO effect is a result of the difference in Poisson’ s ratio between clean, fluidfilled sand and marine shale. Both synthetic seismograms and field data recorded over Hartzog Draw field in the Powder River Basin, Wyoming show that reservoir sands that are less than 10 m in thickness produce an observable amplitudevariation-with-offset effect on the reflected wavefield. The results of this work show that amplitude-variation--with-offset techniques can be used as an exploration tool for thin sandsdthat are enveloped in marine shale. The technique also has potential uses for development of newly discovered reserves that are trapped in such sand-shale sequences. Amplitude-Variation-With-Offset (AVO) analysis is a seismic technique by which the lithology of the subsurface is studied by analyzing seismic reflections at various offsets in CDP gathers (Ostrander, 1984). The technique is based on the physical principle that the reflection coefficient at the interface between two rock layers varies with increasing angle of incidence when the ratio of P-wave velocity, Vp, to S-wave velocity, Vs, of one of the layers is significantly different from the Vp/Vs ratio of the other layer (Koefoed, 1955). In order to take full advantage of the AVO technique, the seismic data must be carefully processed so that any variation in amplitude is due solely to reflection coefficient changes and not some physical process such as attenuation or the result of a data processing step (Yu, 1985).

Combining attenuation by Q and spherical divergence

A general correlation of cross‐well seismic data and surface seismic data is attempted simply by examining a combination of the two major mechanisms of seismic wave attenuation, anelastic Q, and spherical divergence. High‐frequency cross‐well seismology can be hindered by the assumption that an order of magnitude increase in frequency is accompanied by an order of magnitude decrease in propagation distance such that the anelastic attenuation (described by quality factor Q) remains constant. Such a comparison, however, neglects the effects of geometrical spreading, which is independent of frequency. Through the consideration of both Q and spherical divergence, it is demonstrated that the total attenuation of 2 000 Hz energy at a distance of 613 m is equivalent to the total attenuation of 20 Hz energy at a distance of 7 000 m. Applications of kilohertz cross‐well seismic surveys between wells spaced by over 600 m could be possible with present dynamic‐range capabilities in seismic recording systems. Such ap...

Termination of Southern Appalachian Overthrust: ABSTRACT

Autochthonous sediments equivalent to Valley and Ridge formations are interpreted to exist beneath crystalline rocks of the entire Blue Ridge and western portion of the Piedmont. Although hydrocarbon potential has not been determined, we have defined the southeastern extent of sediments beneath the allochthonous southern Appalachians. Truncation of underlying sediments by a subsurface fault occurs at 10 km (6 mi) depth approximately 55 km (34 mi) southeast of the Brevard zone. Farther southeast, a basal detachment might exist, but it would be within mylonitized Grenville basement. Blue Ridge basement rocks probably originated from this area. The interpretation is primarily based on detailed analysis of reprocessed COCORP seismic data, modeling potential fields, and the un isputable fact that Grenville rocks must be cut by a deep fault someplace beneath the southern Appalachians. All previous theories which attempted to correlate with the original interpretation of a regional detachment underlying the entire southern Appalachians should be reexamined. Although detachments and major thrust faults undoubtedly exist throughout the southern Appalachians, they do not form one continuous overthrust sheet and are not underlain by sediments deposited on the ancient continental margin of North America. Geophysical studies of the southern Appalachians have determined them to be largely allochthonous. Seismic data show that a continuous master decollement underlies the Valley and Ridge, Blue Ridge, and Piedmont provinces. Other forms of geophysical data are consistent with the hypothesis of an extensive overthrust system. The question remains, however, how far east can we define a continuous master decollement? This question has been debated in many conferences in recent years where two extreme positions have developed. One side believes that the Brevard zone represents a cryptic suture continuing to depth beneath the inner Piedmont and terminating the master decollement at depth. The alternative position draws a detachment continuous from the western Valley and Ridge, beneath the ent re Appalachians to the southeast, deep beneath the coastal plain, and possibly out to the edge of the continent in the Atlantic Ocean. In this paper, a moderate position is presented which places a sloping master decollement root zone beneath the eastern Piedmont. The adjoining province of island arc assemblages is thus regarded as an accreted terrain which is generally regarded as autochthonous. Evidence for such a model is primarily from reflection profiles across Georgia. Additional support comes from gravity, aeromagnetics, magnetotellurics, and surface geology. Figure End_of_Article - Last_Page 487------------

The state of petroleum engineering education

Many of Ken Larner’s recent comments on geophysics education (July 1991 TLE) are also true of petroleum engineering. Perhaps the largest difference between the two disciplines is the distribution of students. Petroleum engineering has traditionally been an undergraduate program. For example, the University of Wyoming presently has 13 graduate students and 43 undergraduates in petroleum engineering. A bachelor’s in PE normally leads to a good job (average salary for inexperienced entry level is over