Deep Crustal Reflections in the Northwest-German Basin
G. Dohr, P. Lukic, G.H. Bachmann, 1983. "Deep Crustal Reflections in the Northwest-German Basin", Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces, A. W. Bally
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During recent years the German petroleum industry has recorded long reflection seismic lines up to a two-way time of 14 msec in the course of its routine exploration programs. Most profiles show good to fair crustal reflections that can be attributed to the Conrad and Mohorovicic discontinuities.
As Conrad and Moho turned out to be tectonically much more complex than at first assumed, a dense grid of seismic lines spaced no more than 5 to 10 km (3 to 6 mi) distance is necessary to map these horizons and to deduce a comprehensive picture. As such a dense grid is not everywhere available in Germany, it is desirable to record more long seismic lines in this inexpensive manner in Germany as well as elsewhere.
The first seismic section (Figures 2 and 3) is located southeast of Hannover and runs from north to south. It shows a continuous reflection at the base of the Zechstein (Lower/Upper Permian). This prominent seismic marker can readily be correlated and mapped in the Northwest-German basin and is of utmost importance for the petroleum exploration of the pre-Zechstein (lee Lukic et al, atlas volume 2). The reflection rises from 1.8 secs in the north to 0.5 secs in the south, there forming a broad anticline. Several other reflections are present above the base Zechstein.
The base Zechstein is underlain by a 5 to 6 sec (15 to 19 km; 9 to 12 mi) thick zone with just a few real reflections in the southern part. The next band of strong reflections at about 8 sec (23 km; 14 mi) in the north is attributed to the so-called Conrad discontinuity. Further to the south the Conrad consists of a (up to) 2-sec broad zone with several strong reflection bands, the uppermost rising to about 6 sec (16 km; 10 mi) and thus paralleling the rising Zechstein base. Between shotpoint 391 41 and 43145 the lower Conrad reflection band seems to be faulted. The next strong reflections are at about 10 sec (31 km; 19.2 mi) and represent the Mohorovicic discontinuity (Moho). At shotpoint 41 the Moho is divided into two reflection bands.
The thickness between the upper Conrad and the Moho reflections increases considerably from north to south. Especially at the southern end of the section, abundant reflections are present in the interval.
The second section (Figures 4 and 5) is southwest of Hannover and runs north-northwest to south-southeast. It shows again a good reflection at the base of the Zechstein between 1 sec (2 km; 1.2 mi) in the north and 0.7 secs (1.7 km; 1mi) in the south. Several normal faults are present.
Below the Zechstein base, this section is less clear than the section of Figure 2. Several fair to poor reflections are present at about 4 to 4.5 secs (10 km; 6 mi), which could represent the base of the Paleozoic sediments, and the top of the crystal line Precambrian basement. The Conrad discontinuity is not distinct. It is represented by a set of reflections between 6 and 8.5 secs (17 to 26 km; 10.5 to 16 mi), which seem to be tilted between normal faults. Di&actions are abundant in the interval. The Moho is represented by a more distinct reflection band at about 10 secs (31 km; 19 mi), and seems also to be faulted. The interpretation of, the section (Figure 5) suggests several normal faults, some of which displace the Conrad as well as the Moho and may even have influenced the faults of the Zechstein base. In section (Figures 2 and 3), such faults are not present, or only to a minor extent. The reason for the structural difference in both nearby sections is not known.
The average seismic velocity for P-waves from the surface to base Zechstein is about 3.5 km/sec (2.17 mi/sec).
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Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces
Until a few decades ago, structural and regional geology were traditionally the preserve of field geologists. They usually mapped areas of outcropping deformed rocks and supplemented their work by laboratory studies of rock deformation and by theoretical work. Structural geology became tied to the geology of uplifts, folded belts, and underground mines, all of which were accessible to direct observation. Since World War II we have witnessed a tremendous development of geophysics in oceanography and in petroleum geology. Academic geophysicists in oceanography led their geological colleagues into modern plate tectonics and industry geophysicists developed reflection seismology into a superb structural mapping tool that penetrated the subsurface.
Today we are facing a situation where instruction and textbooks in structural geology are almost entirely dedicated to rock deformation, analytical techniques in detailed field geology and summaries of plate tectonics. Illustrations based on reflection seismic profiles are virtually absent in textbooks of structural geology. These texts illustrate only the parts of the proverbial elephant, together with some conjecture, but without ever offering a glimpse of the whole elephant.
Some of the reason cited for the relative scarcity of published reflection profiles are: 1) the confidentiality of exploration data; 2) difficulties in the photographic reduction and reproduction of seismic profiles for a book format; 3) the two-dimensional nature of vertical reflection profiles; and 4) the obvious distortions in reflection profiles that are typically recorded in time.
The AAPG leadership felt that it was time to attempt to correct the situation and to produce this picture and work atlas. The first volumes, of what may become a series of volumes, are addressing an audience that includes: petroleum geologists concerned with structural interpretations; exploration companies that provide in-house training; the AAPG continuing education program; and academic colleagues interested in updating their curricula in structural geology by inclusion of reflection profiles from the “real world” in their teaching.
The atlas is not meant to be a textbook in reflection seismology (instead we listed some at the end of this introduction) nor a text in structural and/or regional geology. Our intent is simply to provide a teaching tool.