NEEDS PARTICULAR TO PASSIVE EXPERIMENTS

Some needs are particular to passive experiments only, and I have listed them here:

• Source at known position for comparison. The best measure of success is to be able to reproduce, even to a very limited extent, results obtained using a known source. This could be a Vibroseis or dynamite source from a conventional reflection experiment, or a source such as a drill bit.

  Assessment of previous work:

  Passive experiment: NO. We did have blasts set off in a quarry 25 km from the array which have been very useful, but a local source would have been more helpful.

• Specialized recording/processing hardware. Passive experiments mean long recording times. Whether we are waiting for relatively strong events, or recording many weak events and combining their results in processing, the volume of data produced by even a modest array would quickly become overwhelming. One solution is to collect as much data as possible and process it as best we can, but an alternative is to use specialized hardware. We might use a recording system that triggers on events of interest and records only at those times, or we might design a system that is processing the data in real time and accumulating a result, rather than storing all the data on tape. This second type of equipment, if available, might also be useful in drill bit work.

  Assessment of previous work:

  Passive experiment: NO. We recorded data for a total of approximately 30 minutes. Our array filled six large 9-track magnetic tapes in that short time. If we tried to record continuously for a long time, we couldn't keep up at the processing end.