I and others at SEP (Nichols et al, 1989; Cole 1989) have investigated the possibility of recording the ambient seismic wavefield and attempting to gain knowledge of the subsurface geology from it. This ``passive seismic'' problem suffers from the complications of the drill bit source, but suffers from an additional, obviously important, complication: we don't have a source at some known position in the earth.
These complications arising from the use of unconventional seismic sources are no surprise, and have guided the design of drill-bit and passive experiments from the start. Yet in all experiments performed to date, there have been limitations, usually imposed by constraints on time or equipment. My purpose in this paper is to spell out the requirements for experimenting with novel sources, see how these needs have or have not been met in experiments performed to date, and propose ways in which we any deficiencies in past work could be overcome in future experiments.