No abstract

In the seismic exploration domain, the following statements may be said to represent a common understanding. First, a valid earth model with the required properties is crucial for successful pre-stack depth imaging. Second, tomographic inversion is one of the most efficient and appropriate techniques for obtaining such a model. Third, borehole seismic data, because of its acquisition geometry, is very useful for achieving an "all-in-depth" earth model. Finally, integration of different data, such as surface seismic, borehole seismic and well logs, is necessary in order to improve the robustness and reduce the non-uniqueness of the inversion process. However, at the same time, it is known that differences in frequency bandwidth, wave propagation behavior and spatial coverage and resolution mean that such integration is rarely achieved in practice. Integrated tomographic inversion therefore remains a topic of considerable academic and practical interest.In this paper, we present a joint tomographic inversion scheme that brings some hope to resolving the questions above. This technique is demonstrated on a set of different 3D VSP surveys acquired over UNOCAL's Attaka field in Indonesia. The updated 3D model was then used in a 3D pre-stack depth migration. The results were greatly superior to all earlier seismic images obtained over this field.

The borehole acoustic reflection survey (BARS) is a method for imaging near-borehole structures using waveforms acquired by a sonic logging tool: Signals from a monopole or dipole source are reflected from geologic interfaces and recorded by arrays of the hydrophones of the same tool. Due to the nearly 1D configuration of the source and hydrophones, BARS provides only 2D images, in which the dimensions are the axis of the well and the distance from the center of the well. We have developed methods for identifying the azimuthal direction of the reflector, which is not given by the BARS image, by using the differences of the signals recorded by hydrophones at varying azimuths. We have conducted a theoretical analysis for incident plane waves as follows. Analytic solutions in the frequency domain are numerically computed at discrete frequencies and converted to time-domain synthetics using the fast Fourier transform for a newly derived model that includes the effect of the tool in the fluid-filled borehole. For event signals from the monopole and dipole sources, the hydrophones recording the largest signal amplitude are located on the diameter parallel to the wavenumber vector of the incident wave, on the side from which incident wave is coming, and in the azimuth oriented parallel to the incident wave, respectively. Based on the synthetic results, we discuss methods for identifying the azimuthal direction of the reflectors in observed data obtained by the BARS surveys.

S U M M A R YAs the first step towards a general analysis of the stability of optimally accurate predictorcorrector (P-C) time domain discretized schemes for solving the elastic equation of motion, we analyze the stability of two P-C schemes for a 1-D homogeneous case. Letting t be the time step, h be the spatial grid interval, β be the velocity of seismic wave propagation and C = β t/ h be the dimensionless Courant parameter, we find that each scheme has the following stability properties: stability for 0 < C ≤ C 1 , instability for C 1 < C < C 2 , stability for C 2 ≤ C ≤ C 3 and instability for C 3 < C, where 0 < C 1 < C 2 ≤ C 3 . We refer to the region C 2 ≤ C ≤ C 3 as the second island of stability. The values of C 1 , C 2 and C 3 are schemedependent. The existence of a second island of stability in a numerical scheme for solving the wave equation has not, to our knowledge, been previously reported.

A velocity estimation procedure is described for walkaway VSP data. Ray tracing through a heterogeneous 2D model is iterated to optimize a functional based on the coherency along predicted moveout curves for reflected P events. The model is held fixed down to some chosen boundary at or below the receiver array while the velocities in conformable layers are allowed to vary below the boundary. This boundary could, for example, be the base of salt in a deep water offshore borehole seismic survey. Travel time inversion of direct travel times ensures that the overburden model has been calibrated, leaving the velocity profile in the layers belo w the boundary (e.g. base of salt) free. We use conventional travel time tomography to solve for velocity updates but map coherency into travel time so that an anti-coherency functional is minimized rather than residuals with respect to picked reflection times. This velocity inversion technique is aimed at the important problem of predicting velocities ahead of an intermediate drilling depth for input to pore pressure and fracture gradient models or for migration.