Between December 6, 1985, and January 9, 1986, the Hawaii Institute of Geophysics Research Vessel Moana Wave surveyed the Manus Basin, north of New Britain, Papua New Guinea. This basin occupies a back‐arc position with respect to the New Britain arc‐trench system and contains an active plate boundary (Figure 1; also Taylor [1979]). Organized under the aegis of the Committee for Coordination of Joint Prospecting for Mineral Resources in South Pacific Offshore Areas (CCOP/SOPAC), the cruise was part of a program to assess the mineral potential of southwest Pacific marginal basins and included a National Science Foundation‐sponsored study of crustal accretion processes in a fastspreading (greater than 100 mm/yr) back‐arc basin. The tectonic fabric of the Manus Basin was mapped and defined with the Sea MARC II sidescan sonar and bathymetric mapping system. Then dredge samples and cores were collected, and a towed bottom camera system was used to search for signs of hydrothermal activity.
A modified version of the Kirchhoff-Helmholtz integral can be used t o synthesize elastic wavefields in niedia for which velocity is a function of range, x, aswell as depth, z. The essence of the method is that rays are traced from both source and receiver to some intermediate surface, S . The field at the receiver is then given by an integral over C, whose integrand is a particular product of the values of the source and receiver wavefields. The surface 2: is not a reflector since the medium is continuous across it. Geometrical ray theory (GRT) is used to calculate the source and receiver wavefields on 2 . When either the source or receiver wavefield has a caustic in C then the GRT amplitude is infinite and, in theory, the method breaks down. However, numerical breakdown can be avoided by parameterizing the GRT amplitudes so that their singularities are integrable and choosing C so that caustics of the source rayfield and caustics of the receiver rayfield d o not intersect on C. We refer t o this alternative as the extended Kirchhoff-Helmholtz (EKH) method. For reasons of economy EKH may be a practical alternative to the more theoretically correct procedure o f using many surfaces: e.g. for two surfaces, tracing rays fr-om the source t o the first surface C, , then from every point on C, , to every point on the second surface C,, then from the receiver to C,, then integrating over the product manifold C, x C 2 .In this paper we give examples of the errors that arise when caustics on C are treated as integrable singularities. First the EKH method is compared with the WKBJ method for a stratified medium, then the EKH method is compared with the ordinary Kirchhoff-Helmholtz method where C intersects no caustics. Errors in the EKH method take the form of small spurious phases which generally arrive later in time than correct arrivals. The arrival times of these error phases can be changed by adjusting C. For some velocity models these phases can be eliminated completely.The EKH method is not as fast as the Maslov (extended WKBJ) method because of the amount of ray tracing needed. However, one of the attractive features o f the EKH procedure is that its underlying theory is very simple.
Deep sounding seismic reflection data show undeformed reflectors at depths down to 11 kilometers beneath the continental rise and abyssal plain and 7 kilometers in basins of the lower slope. Weak reflectors are visible beneath the salt of the Sigsbee Scarp and within salt ridges separating the lower slope basins.
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