Abstract:
The borehole sonic measurement in well logging consists of a wideband acoustic source operated in ``pitch--catch'' mode with an offset, linear array of receivers. The source/sensor configuration is moved up the borehole axis at a more-or-less constant speed, and the source fired at depth intervals of approximately one receiver spacing, generating a set of array measurements at each depth. The array processing problem is that of estimating the depth-dependent slowness dispersion of individual modes in the received wavefield. The classical approach to slowness estimation has involved processing each shot array individually, generating a sliding estimator window. The aperture of this measurement is defined by the linear extent of the receiver array. When logging subsurface formations comprised of closely spaced layers, it is often of interest to improve this ``intrinsic'' spatial resolution. A new two-dimensional spectral estimator has shown to be effective for dispersion estimation using single-shot data sets [M. P. Ekstrom and C. J. Randall, J. Acoust. Soc. Am. 98, 2867(A) (1995)]. In this paper, this estimator is extended to hi-spatial resolution logging. The array aperture is ``focused'' to improve vertical resolution by exploiting the measurement redundancy available with multiple, contiguous shot files. Examples of processing-field-acquired data sets will be presented.