by Caverdale » Nov 2, 2005 1:40 pm
Paul -
There are two main kinds of seismic surveys - reflection and refraction. In reflection surveys,
sound waves are sent vertically into the earth. When the wave front encounters an interface the
wave is reflected back to the surface where seismometers (specialized microphones) pick up the
sound and the travel time is recorded. This method is worthless for cave detection. In refraction
seismic surveys, the sound waves that travel downward at an angle to the surface. When these
waves encounter a rock layer that conducts sound waves with a higher velocity, the wave is
refracted (meaning the direction of travel changes) and travels essentially horizontally along the
top of the rock layer. The wave is also refracted back to the surface where several seismometers
record the signal. Using the times of travel, geophysicists can deduce what lies beneath. (This is
a greatly simplified explanation - experts please don't jump on me.)
I believe your professor is talking about using some modification of refraction seismology to
detect caves by sending sound waves horizontally beneath the surface. Theoretically, this would
work. Practically, I can think of three problems. First, the sound wave you generate will travel
along the surface and reach the seismometers first, because it is the shortest path. In small
dimension surveys, such as what would be used in looking for caves, this initial hit on the
seismometer may still be ringing when the wave that went through the suspected cave arrives.
Separating the two events might be difficult, if not impossible. Secondly, the void would have to
be quite large to obtain a difference in travel times that can be discerned from those waves that
did not go through the void. Thirdly, the earth surrounding the void is usually not homogeneous,
and may introduce extraneous results that are difficult to interpret (again, if not impossible).
On the practical side, the learning curve to interpret the results for lay people would be too much
to bother with. When you rent the equipment to do this survey, you would also have to hire a
consultant that has experience with the method. There are better ways to geophysically prospect
for caves. The best practical method IMHO, and fastest and cheapest, is resistivity. For shallow
caves, resistivity equipment can be built for a nominal cost, and with certain electrode arrays,
interpretation is comparatively easy. A magnetometer works great for most lava tubes, and if the
void is large enough, could be used with shallow limestone caves. This method is fast, easy to
interpret even for a lay investigator, and the instrument is cheap, compared to other geophysical
instruments, like $5,000 new. eBay has had them for $1K - $1.5K; don't know how good they
are. (Mine was free as compensation for extra time.) Most other methods work, but still require
a specialist to interpret the results, can be very time consuming, and use very expensive
instruments.
Dale Green
NSS 3669FE