Background noise recordings. The fundamental problem is
one of differentiating between the drill-bit energy and other energy in
the background wavefield. It would be very helpful to have a recording
of the background field, without the drill bit, in order to identify
the ambient noise sources.
Assessment of previous work:
Drill bit experiment: NO. Such recording requires very little
extra work, and might be very helpful in the processing stage.
The background noise field could be analyzed, and then we would know
what noise sources to expect in the drill-bit data.