It was interesting to read of processing mundane noise to produce an ultrasound image of the geology of Los Angeles. A big question in the state is the deep structure of San Francisco Bay. Clearly, the bay and the valleys extending to its northwest and southeast form a rift valley. But it’s difficult to observe faulting in the rocks because of the depth of sediment. Maybe the area could be probed ultrasonically.

Charles B. Warren
San Francisco, Calif.