The split-step Fourier method is developed and applied to migrating stacked seismic data in two and three dimensions. This migration method, which is implemented in both the frequency-wavenumber and frequency-space domains, takes into account laterally varying velocity by defining a reference slowness (reciprocal of velocity) as the mean slowness in the migration interval and a perturbation term that is spatially varying. The mean slowness defines a reference vertical wavenumber which is used in the frequency-wavenumber domain to downward continue the data across a depth interval as in constantvelocity phase-shift migration. The perturbation term is used to define a "source" contribution that is taken into account by the application of a second phase shift in the frequency-space domain. Since the method does not include the effects of second and higher order spatial derivatives of the slowness field, the method theoretically is accurate only when there are no rapid lateral slowness variations combined with steep angles of propagation. However, synthetic and real examples indicate that good results are obtained for realistic geologic structures.

New multichannel seismic reflection data collected over the Nankai Trough image the accretionary complex in two areas: the International Program of Ocean Drilling leg 87 transect area (western area) and the region of upcoming Ocean Drilling Program leg 131 (eastern area). The incoming Shikoku Basin sedimentary section consists of hemipelagic clays and thin terrigenous turbidites. The basin section is overlain by a trench wedge that is 12-16 km wide and 350-750 m thick at the thrust front. Accretionary deformation begins in a protothrust zone that is characterized by thickening and seaward tilting of the trench wedge. The zone in the western area is 4.5 km wide and is characterized by "kink" folds; the zone in the eastern area is only 2.5 km wide and does not exhibit such folds. The frontal thmsts in each area are imaged as fault plane reflections and ramp upward from within the basin hemipelagic section. The overthmsting sediments form fault-bend folds over these ramps. Thrust spacing at the toe of the slope is 1.5-2.5 km. The second thrust cuts up from an inferred dtcollement within the Shikoku Basin sedimentary section. In the eastern area, a reflection marking the top of the basin pelagic sediment section changes from normal to reversed polarity about 6.3 km seaward of the thrust front and underlies the entire protothmst zone. This reflector continues with reversed polarity under the accretionary complex and is at the level of the basal dtcollement. The underlying basin pelagic section is apparently thmst undisturbed beneath the accretionary prism. The reversal of polarity indicates a change in reflection coefficient that is due to a combination of decreasing seismic velocity and density across the interface. This decrease in velocity and density may indicate that the dtcollement is a zone of high porosity due to fluid expulsion from deeper within the accretionary prism. The reflections from the first and second thmsts are also reversed polarity, possibly indicating that they also are pathways of fluid expulsion. The critical wedge taper of the westem area is greater than that of the eastern area, an observation that is consistent with the existence of an overpressured dtcollement in the eastern area. MOOREET AL.: STRU• OFTHEN••OU• 8755 Am., 66, 117-133, 1986.

Providing an up-to-date overview of the most popular global optimization methods used in interpreting geophysical observations, this new edition includes a detailed description of the theoretical development underlying each method and a thorough explanation of the design, implementation and limitations of algorithms. New and expanded chapters provide details of recently developed methods, such as the neighborhood algorithm, particle swarm optimization, hybrid Monte Carlo and multi-chain MCMC methods. Other chapters include new examples of applications, from uncertainty in climate modeling to whole earth studies. Several different examples of geophysical inversion, including joint inversion of disparate geophysical datasets, are provided to help readers design algorithms for their own applications. This is an authoritative and valuable text for researchers and graduate students in geophysics, inverse theory and exploration geoscience, and an important resource for professionals working in engineering and petroleum exploration.

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