This album shows some of that work, to give a sense of geophysical work in the field. John Louie coordinated the field exercises and took these photographs. The class is indebted to Mike Widmer of the Washoe County Public Works Department for providing access to some of the sites, and a wealth of background information.
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To transmit seismic waves more than a few hundred meters requires a
powerful source of energy. Here the class is preparing a blast hole for
one pound of a two-component explosive. The explosive is a mixture of
solid ammonium nitrate and nitromethane, prepared by a licensed technician.
The technician, who was also taking the class, prepares the charge by
attaching the bottles with the explosive mixture to a length of detonating
cord, and then setting them in the hole with the end of the cord left above
the surface. An electric blasting cap attached to the cord will detonate
it and the charge below. Feed sacks filled with dirt tamp down the
charge, to direct the most energy possible into seismic waves.
Strings of geophones record the ground vibrations emanating from the
blast. A cable connects them individually to a portable multi-channel
seismograph. The seismic records allow timing the arrivals of several
types of waves at each geophone in the strings, revealing information on
the seismic velocity properties (sound speeds) of the materials below.
Analysis of these data suggested the location of a sub-basin parallel to
the range front fault.
Such a basin may provide an easily-recharged groundwater reservoir,
because it intercepts streams emanating from the Carson Range that provide
all of the recharge on the fan.
Geothermal ground waters are often rich in minerals and highly conductive, so
can produce clear signals on electrical ground conductivity measurement
devices. Here the class uses a frequency-domain electromagnetic (FEM)
instrument to infer shallow ground conductivities, to about 5 m depth.
Source and receiver current loops, held parallel and connected by a phase
cable, induce and measure currents in the ground at 8 kHz audio frequencies.
This instrument can rapidly profile lateral variations in conductivity over
a large area.
The time-domain electromagnetic (TEM) instrument is a more sophisticated
system that allows sounding of conductivity variations with depth as well
as profiling. A crew surveys a square transmitter current loop 25 m on a
side, and places a small receiver loop containing thousands of turns of wire
just outside the transmitter loop.
The transmitter activates a DC current in its loop for a fraction of a
second, then abruptly terminates it.
Over the following few nanoseconds the receiver loop will measure the
magnetic field from the rapidly-dying telluric currents in the ground.
As the deepest induced currents are the last to go, the record of received
currents against time after transmitter cutoff reflect the conductivities
of progressively deeper layers.
At the nascent Redfield Campus, the class showed that although conductive
geothermal waters appear at the surface only in the topographically lowest
areas, they also underlie the entire site.
Higher areas are simply raised up above a fairly constant geothermal
``water table.'' None of the areas surveyed shows strong vertical channeling
of geothermal waters.