TOWARD TUBULAR TOMOGRAPHY

Within ray theory, travel times are sensitive to the medium only along the ray path. If the arrival time is defined to be the point where th e observed seismogram crosses a nonzero threshold, time domain computa tions using Born scattering in a homogeneous unbounded medium show tha t the sensitivity is large within a considerable volume, and most inte nse near the source and receiver. Details of this volume or ''tube'' d epend on the threshold value, the source time function and the instrum ent response. Generally, as the threshold value gets larger, the volum e of sensitivity and magnitude of sensitivity get larger. If the sourc e time function is oscillatory, the region of highest sensitivity can be quite complex. Due to the high sensitivity near the endpoints, ray- theoretical tomography is likely to alias near-source and near-receive r structure to produce spurious structure along the ray path.