From March 18-23, 2000, UNR's
Geophysical Applications class investigated Neogene
sediment depths north of Reno.
The class profiled Warm Springs and Hungry Valleys in detail.
In addition, they reconnoitered Red Rock and Long Valleys.
Data were also collected across the ore bodies of Virginia City, south of Reno.
Click on each photo for a higher-resolution JPEG file,
240-550 kb each.
Geophysical measurements can separate Neogene sediments
from underlying and older rocks on the basis of their physical property
contrasts.
The granitic basement rocks exposed here on the west side of
Warm Springs Valley have higher density and seismic velocities,
and lower electrical conductivity, than the more recent
sediments that overlie them.
To detect a 100-meter thickness of sediment that may be
0.4 g/cc less dense than basement, the class had to know
the relative elevation of each gravity measurement to
better than 10 cm accuracy.
They employed the differential GPS technique with phase
correlation in post-processing to reach the needed accuracy.
Setting up the GPS base-station receiver on a benchmark of
known elevation also provides excellent control on absolute
elevation. This helps to merge the class's gravity data with
data taken by others.
The roving GPS receiver records signals from the same satellites
at the same time as the base GPS receiver.
Cross-correlation of the digital data each evening could then
pick out the differences in the arrival time of the satellites'
broadcasts, which are related to the distances and elevation
differences between the two receivers.
We observed each gravity point for 10 minutes.
The team member holding the receiver on the pogo pole has
to watch a bubble level on the pole, to keep it vertical.
For a line of gravity stations separated by more than 100 meters,
differential GPS provides all the needed elevation accuracy,
for less time than traditional theodolite surveying.
Although the Lacoste and Romberg model G gravimeter is
not subject to significant instrumental drift, repeated
measurements at base stations such as the GPS base
assures collection of high-accuracy data.
Reconnaisance of even a deep basin requires one part per
million accuracy in measuring Earth's gravitational
acceleration.
The class's aim in the valleys north of Reno was for better
than one part in ten million accuracy, or 0.1 milligalileo (mGal).
Aside from elevation effects controlled by the GPS
surveying, the local topography can be the most
significant source of error to gravity measurements.
At the sides of the valleys the gravity team members
would make several independent estimates of a
terrain correction.
Each valley reconnoitered required the setup of both
GPS and gravity base stations at a nearby benchmark.
Seismic Surveying
Interpretations of basin depth from gravity profiles
depend heavily on the poorly known average density of
the sediment fill.
Seismic refraction profiles offer independent estimates
of basin depth that can help establish average
sediment densities.
Shown here are the elements of a single-phone seismic
refraction receiver channel, one of 48 extending along
a multi-channel cable 720 m long.
First a member of the seismic team digs a hole to
plant the geophone below loose soil and organic debris.
Better coupling improves signal strength and reduces
noise from wind and other surface effects.
This is a sandy area; in sod use a spade to cut the turf
and lever open a pocket you can spike the phone into
the bottom of.
Press the geophone spike into the bottom of the
hole, by hand or gently with a boot heel.
It must remain within ten degrees of vertical,
and the spike must not rattle loosely within the
hole.
Always connect the geophone to the cable takeout at the
same time you bury the geophone.
Be sure the lead wire is not stretched tight between the
cable and phone - that will generate noise.
Now bury the geophone and tamp the fill with your boot.
Observe that the connections are correct (in-line) and
that no wires are stetched or dangling from bushes before
you move on the next geophone.
The hardest part of the seismic survey is unreeling and
then reeling up the two 24-channel, 360-meter-long
cables that carry the geophone signals back to the
recorder. Each cable reel weighs about 35 kg. The
cables carry 48 separate copper conductors, and so
cannot be pulled on hard or stepped on.
With two to carry the reel, one to turn it, and one
to playout cable or adjust tension, cable layout
or pickup can be done in less than a half hour.
In a day, the seismic team could complete one 720-m refraction
survey and one 200-m reflection survey.
Due to the generosity of the W.M. Keck foundation,
the Mackay School of Mines posesses 48 channels of
``grouped'' 100-Hz high-frequency reflection geophones.
Six moving-coil sensors are connected in parallel so
their signals will average electrically.
Each geophone group, 5 meters wide, then plugs into
one cable takeout and one channel.
Thus the center of each of the 48 reflection ``receivers''
is at the center of the 5-m-wide geophone group array,
2.5 m east of the flag and the takeout in this line
in the middle of Warm Springs Valley.
To record reflection data, the seismic crew strikes a 5 kg hammer
against a steel plate on the ground surface ten times at every
other takeout (or flag) location, right through the 200-m-long
receiver line. This procedure yields excellent coverage along
a 100-m-long line of midpoints centered within the receiver
line.
The class's reflection data from both Warm Springs and Hungry
Valleys were surprisingly good.
The 10 hammer hits produced clear reflections from up to
400 m depth in Warm Springs Valley and 700 m depth in
Hungry Valley.
The stratigraphy of the Tertiary lake sediments, underlain
by basement and overlain by Quaternary sands, should be
clear in the results.
If the Tertiary lake sediments are not diatomite-rich, as is
common near Reno, but are instead clay-rich, then
there will be serious problems with the seismic interpretations.
Low-velocity clays cannot be sensed by refraction surveys,
and often give falsely deep depths in reflection surveys.
Clay-rich sediments are very electrically conductive, however,
and a conductivity sounding method such as time-domain electromagnetics
(TEM) would be very sensitive to the depth of the top of a relatively
conductive (clay) layer below a relatively resistive (sand) layer.
Here the EM team stretches out the current transmitter loop, 40 m
on a side.
Pushing several amps through the transmitter loop sets up a
magnetic field that produces complementary telluric current
loops in the ground.
Suddenly switching the transmitter current off causes negating
``eddy currents'' to travel down over the ensuing tens to hundreds
of nanoseconds. These eddy currents, of course, produce their own
magnetic fields, which this receiver loop senses as a function of
time after transmitter shutoff.
The time is roughly proportional to the depth of the eddy current,
and the strength of its magnetic field is greater if conductivity
is higher at that depth.
The proTEM receiver the class used stacks thousands of transmitter
``turn-off'' events to overcome noise.
The stacking process can take up to an hour for each sounding.
The EM team was able to complete more than five soundings each
day they worked.