A parametric study of borehole influence in resolving seismic velocity in well-to-well tomograms

Abstract

The data collected in a well-to-well tomography experiment are inherently incomplete even when augmented by VSP data. The nature of the experiment suggests a geometric limitation to the resolution of any central structure. In particular the Backus-Gilbert method shows that only poor horizontal resolution can be expected (Menke, 1984). Another major constraint can now be identified within the experiment, namely the borehole itself. A parametric model has now been developed in which the borehole size, transducer standoff, damage-zones, and the degree of velocity attenuation, were examined using a range of host velocities and hole separations. It was found that there are significant velocity variations caused by random perturbations in borehole size. These errors are particularly significant for boreholes with large diameters and small transducer offsets. For an altered zone, errors in both the dimensions and the degree of velocity alteration, gave measurable velocity variance, particularly with large boreholes. In all cases it is observed that the variance in velocities increases as hole separation is decreased.