Dipping Events Research Articles

Lateral variations in crystal reflectivity beneath the Paris Basin

The application of a new method of automatic extraction of reflections from a stacked Seismic section to the ECORS North of France profile highlights the lateral variations in crustal reflectivity observed beneath the Paris Basin. Five characteristic zones can be distinguished along the 228 km long profile. A complex pattern of spatially interfering dipping events occurs in the lower crust of the transition region between the zones of reflective and transparent lower crust. Modelling reveals that these events are diffractions from discontinuous subhorizontal scatterers concentrated near 8 s. Hyperbolic events at Moho level (12 s) may indicate that the 1 s increase in travel time to the Moho observed across the transition region is accommodated by a discontinuous step. There is no spatial correlation between the lateral variations in reflectivity observed within the upper and the lower crust. Neither is there evidence that any structure within the upper crust continues into the lower crust. We interpret these two observations as an indication of a younger origin of the lower crustal reflectivity and discuss the possibility that the early extensional phase of evolution of the Paris Basin has contributed to the lateral variations of reflectivity observed within the lower crust.

A geophysical study of the Earth's crust in central Virginia: Implications for Appalachian crustal structure

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.

Dip moveout in the frequency‐wavenumber domain

In recent years dip moveout (DMO) has come into routine use in the seismic processing industry. The main benefits of including DMO in the processing sequence are that (1) stacking velocities after DMO are dip‐independent, and (2) “reflection point smear” associated with dipping events is eliminated by laterally shifting the reflection points to their zero‐offset position. These effects of DMO are also beneficial for estimation of stacking velocities by simplifying the interpretation of velocity analysis. Dip moveout also has an inherent dip filtering effect (Bolondi et al., 1984) by lowering the frequency content on the stacked section of steeply dipping aliased events, which leads to reduced migration noise.

Migration of noisy crustal seismic data

A slowness‐weighted diffraction stack algorithm is used to migrate crustal seismic reflection data. Slowness information is extracted from the data using a slant stack (beam‐forming) process. The main purpose of beam forming is to improve the signal‐to‐noise ratio over a given range of slowness values. Localized slant stacks are used to design a coherency‐weighted slowness filter in the time distance domain. Through migration the subsurface structure and the observed seismic data are tied via slownessselective diffraction curves. A combination of nonlinear slowness filtering with migration gives excellent noise rejection and leads to an improved image in the depth domain. The method, which is particularly effective for migrating moderate to steep dipping events with low signal‐to‐noise ratio, is applied to various poststack and prestack synthetic data examples and to a deep crustal data set collected on Vancouver Island as part of the Canadian Lithoprobe project.

Deep events in U.K. South Western Approaches

Summary. Apparently planar dipping events are observed in seismic data off south-west Britain within otherwise essentially transparent upper and middle crust. These are believed to represent Variscan thrusts, some of which were re-activated during the post-Carboniferous phase of extension that affected the southern U.K. They can be seen in two extensive commercial seismic surveys recorded to 6 s two-way-time (TWT) and, where laterally persistent, they have been mapped to reveal their essentially planar nature. Commonly these dipping events are associated with deeper, near-horizontal, or gently convex upwards events with which they often appear to converge. Where ‘real’, these are thought to indicate a complex fault system or possibly the top of the reflective lower crust. The thrusts are seen over the whole area except where granite is known to occur, and commonly exert a major control on the position and subsequent deformation of overlying sedimentary basins.

Results of DEKORP 2-S and other reflection profiles through the Variscides

Summary. The first DEKORP profile, DEKORP 2-S, a 250 km long line perpendicular to the Variscan strike direction, has provided evidence of major crustal shortening during the Variscan orogeny. Sporadic dipping events in a generally transparent upper crust are interpreted as thrust faults, while the highly reflective lower crust fits into the general picture of Palaeozoic provinces. Correlations are established between certain reflectivity patterns and rheology. Moho depths and reflecting lamellae are considered to be post-Variscan.

Seismic reflection measurements behind the Hikurangi convergent margin, southern North Island, New Zealand.

The active Australian-Pacific plate boundary passes through New Zealand. In the north, the Pacific plate subducts beneath the Australian plate with an accretionary wedge forming the eastern continental (Hikurangi) margin of the North Island. The structure of the region behind the Hikurangi margin changes from the extensional back-arc basin under central North Island to a postulated crustal downwarp under the southern North Island. A 100 km long multichannel seismic reflection profile was recorded across the region of crustal downwarp. The data show discontinuous coherent reflectors dipping westwards at the east end of the profile, and east dipping reflectors at the west end, from depths of 9 to 15 s two way time. Simple hand migration of these events indicate that the east dipping reflectors, interpreted as the base of the Australian plate crust, abut against the west dipping reflectors which are interpreted as marking the top of the subducted Pacific plate. Detailed earthquake hypocentre locations in the area show a dipping zone of high seismicity, the top of which coincides closely with the west dipping events, thus supporting this interpretation.

Tectonic implications of subthrust structures revealed by seismic profiling of Appalachian‐Ouachita Orogenic Belt

A growing body of deep seismic reflection profiles across the Appalachian‐Ouachita orogenic belt reveals subthrust structures which have been previously interpreted as products of continental rifting, of later thrusting, or of some combination of rifting and thrusting. The subthrust features can be placed into three general categories: (1) small normal faults which offset lower plate shelf sequences and basement in the foreland and have been attributed to thrust loading, perhaps reactivating earlier rifting faults, (2) moderately dipping seismic events which lie entirely below lower plate shelf sediments in the foreland, interpreted in part as strata deposited during the earlier rifting, and (3) hinterland‐dipping reflection sequences beneath more interior portions of the mountain belt, previously interpreted as thrust faults which imbricate sediments and basement related to the earlier rifted margin. These interpretations are evaluated through a comparison of reflection profiles across the orogenic belt to previously interpreted seismic lines across modern extensional and compressional settings. The comparisons suggest that although some normal fault offsets of lower plate shelf sequences may be original or reactivated rifting faults, the dipping events entirely beneath the shelf sequence are more diagnostic of the rifting process. These dipping sequences are therefore consistent with previous interpretations that large portions of the earlier rifted margin are preserved in an undeformed state beneath thrust sheets in the Appalachian foreland. The comparisons also suggest an alternative to interpretations of the hinterland‐dipping sequences as thrust‐related, in that the sequences are similar in seismic appearance to that of volcanic wedges commonly observed in the narrow zone separating continental from oceanic basement on modern passive margins. The implication of this new interpretation is that the actual continent/ocean boundary related to the earlier rifted margin may be preserved on the lower thrust plate beneath interior portions of the Appalachians and Ouachitas.

A SEISMIC MODEL OF FAULTS IN THE COOPER BASIN

A synthetic seismic section was modelled to help in the interpretation of Cooper Basin seismic lines which cross major faults and exhibit shadow zones.A major fault bounding the northwest flank of the Packsaddle Structure in the Merrimelia-Innamincka Farmout Block in South Australia was selected for modelling. A geological cross-section postulated on the basis of wells on either side of the fault was fed into the seismic modelling package AIMS (Advanced Interpretive Modelling System — licensed by Geoquest International Inc.) to produce a synthetic seismic line. This synthetic line provided a realistic match with an actual seismic line across the fault. Pre-stack migration of the actual seismic data is suggested to provide additional evidence for the reliability of the model.The shadow zone in the synthetic section is caused by dipping events in the fault shadow zone created by compaction of the Toolachee and Patchawarra Formations along the hanging wall of the fault plane. The dipping events cause reflected energy to be detected outside the fault zone. The large component of compaction within the Permian section is largely ascribed to thick coal horizons. The possibility of petroleum traps in the hanging wall of the fault is inferred and drilling is recommended.

Prestack partial migration

Conventional seismic data processing can be improved by modifying wide‐offset data so that dipping events stack coherently. A procedure to achieve this improvement is proposed here, which is basically a “partial” migration of common offset sections prior to stack. It has an advantage over full migration before stack in that, in the case of the latter, the final product is a migrated section. However, the prestack partial migration provides the interpreter with a high‐quality common midpoint (CMP) stacked section which can be subsequently migrated. The theory of prestack partial migration is based on the double square‐root equation, which describes seismic imaging with many shots and receivers. The double square‐root operator in midpoint‐offset space can be separated approximately into two terms, one involving only migration effects and the other involving only moveout correction. This separation provides an analysis of conventional processing. Estimation of errors in the separation yields the equation for prestack partial migration. Extension of the theory for separable approximation to incorporate lateral velocity variation yields a significant term proportional to the product of the first powers of offset, dip, and lateral velocity gradient. This term was used to obtain a rough estimate of lateral velocity variation from a field data set.