Ocean-bottom seismograph tomographic experiments－a consideration of acquisition geometries vs. resources and real experiment
Mern-Ru Wu
Abstract
Over the last 20–30 yr numerous seismic images of the Earth’s crust have revealed details of its gross structure, including intra-crustal layering, the geometry of that layering and its composition. As more and higher quality studies are undertaken it is becoming apparent that identified structures have a greater degree of 3-D variability than first anticipated. Thus, the methodology of crustal imaging by seismic means has also developed into the third dimension with a tomographic approach now being widely adopted, particularly so in the marine environment. Such surveys not only focus on mapping the finer scale 3-D structural variability, they also aim to achieve sufficient density of azimuthal coverage and resolution to address preferential orientation patterns of features such as porosity, fracturing and faulting. Recent developments in technology, and consequently cheaper construction and deployment costs of instruments, have resulted in an expansion in the number of instruments available in ocean-bottom seismometer pools. Consequently, individual experiments are being designed to accommodate the maximum number of instruments available and this, coupled with dense grids of shot profiles, significantly impacts on survey cost. In this paperwe consider a variety of approaches to achieving the best resolution of detail for minimal associated cost of acquisition, and for instrument pools of various sizes. A number of different geometries are compared, including example grid designs in current use. Comparison of resolution tests and relative costings for a range of acquisition geometries suggest that, if instrument numbers and/or funds are limited, the most cost effective ways of achieving the desired target resolution may be by (1) shooting additional shot profiles at the expense of deploying more instruments and (2) multiple, overlapping deployments of a small geometry, tailored in shape to the target structure and depth.
References
Perice, C., 2002 Ocean-bottom seismograph tomographic experiments—a consideration of acquisition geometries vs. resources, geophys. J. Int., 151, 543-565.
McIntosh K., 2000 Large aperture seismic imaging at a convergent margin: Techniques and results from the Costa Rica seismogenic zone. M. geophysical Res., 21. 451-474
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