A refraction seismic survey was
conducted to measure compressional-wave seismic
velocities to aid in the evaluation of the rippability
and/or excavability of the subsurface. Refraction
seismic data were acquired along five lines selected by
the client. Stakes were placed at the end of each
seismic line and stations along the lines were marked
with flagging, but land surveying to the client’s
coordinate grid was not performed, therefore no location
map is included with this letter report.

The 24-channel refraction seismic
data were acquired with 30-Hz geophones, and a 16-pound
sledgehammer source. The geophones were located 10 feet
apart and source impacts were made at various distances
offset and along the seismic profile. The geophones were
located on a straight line and distances were measured
with a tape. Relative elevations were surveyed with a
level and stadia rod. The seismic data were stacked,
nominally, eight times at each source point to increase
the signal-to-noise ratio. Stacking, or signal
enhancement, involved repeated source impacts at the
same point into the same set of geophones. For each
source point, the stacked data were recorded into the
same seismic data file, or record, and, from each impact
and thus was enhanced while noise was random and tended
to be reduced or canceled. Overall, the quality of the
seismic data was excellent and easily identifiable first
breaks (first arrival of seismic energy) were present.

The refraction seismic data were
processed and interpreted. The general processing and
interpretation flow consisted of the initial selection,
or "picking", of the seismic first breaks, creation of
data files for input into the interpretation program,
and interpretation of the data using modeling and
iterative ray-tracing techniques. The program uses the
delay-time method to obtain a first-approximation depth
model, which is then trimmed by a series of ray-tracing
and model-adjustment iterations to minimize any
discrepancies between the picked arrival times and
corresponding times traced through the 2.5-dimensional
cross-sectional model. For the direct arrivals through
the first layer, the velocity is computed by dividing
the distances (relative to elevation and horizontal,
versus slope, distance) from each source point to each
geophone by the corresponding arrival times. These
individual velocities are averaged for each source
point, and a weighted average is computed. For layers
beneath the first layer, velocities are computed by two
methods: 1) Regression, in which a straight line is fit
by least squares to the arrival times representing the
velocity layer and average velocities are computed by
taking the reciprocals of the weighted average of the
slopes of the regression lines, and 2) the
Hobson-Overton method wherein velocities are computed if
there are reciprocal arrivals from two opposing source
points at two or more geophones. The final velocities
are computed by taking an average of the two methods.

Figures 1 through 5 are the
relative elevation versus distance refraction seismic
depth models, with annotated average velocities for each
layer, for lines 1, I extension, 2&5, 6, and 7,
respectively. Figures 1 through 5 were constructed using
the depth model data, and the estimated total depth of
investigation was computed by simply subtracting 60 feet
from the relative surface elevation. In refraction
surveys, depth of investigation is related both to the
length of the surface spread of geophones and source
points, and the expected subsurface velocities. Since
basement in the survey area probably consists of
relatively fast velocity material (assumed greater than
8000 feet/second), the first geophone to "see" a
refraction from that layer would be at a distance of 3
to 4 times the expected depth (if 60 feet is assumed,
then that geophone would be at 180 to 240 feet along the
spread, but probably closer to the 180 feet because of a
relatively large velocity contrast between the basement
and overlying sediment velocities). Since a refraction
was not apparent within the data from a third layer
along any of the lines, only an estimate of depth of
investigation can be made. For the figures, a
conservative estimate of 60 feet was chosen (total
spread length of 240 feet divided by 4), but the depth
of investigation could be deeper (i.e., 80 feet or 240
divided by 3). Again, however, without a refraction from
the third layer, the depth is only an estimate.

**Figure 1**

As discussed above, only two layer
refraction seismic depth models were computed for each
line since no refraction from a third layer is present
within any of the data. The layer I velocities range
from 1500 to 1928 feet/second, which is consistent with
that expected from unconsolidated sediments, while the
second layer velocities range from 3390 to 4237
feet/second, which is indicative of the Gila
Conglomerate. However, due to the averaging nature of
the computation of the seismic velocities, as previously
discussed, and minor changes in the surface or
near-surface (a few feet), it would be more
geophysically correct to state that the first layer
velocities are about 1700 feet/second, and second layer
velocities are around 4000 feet/second. Using these
geophysically estimated velocities for the subsurface in
conjunction with tables prepared by the Caterpillar
Tractor Company, it should be possible to rip to the
estimated depth of investigation with a D9, D8, or D7
ripper. However, marginal rippability occurs for
conglomerates at about 4500 feet/second for a D7 ripper,
so it would probably be more cost effective to use a D8
or D9 since the velocities of the second layer approach
the limits of the D7 ripper.

**Figure 2**

**Figure 3**

**Figure 4**

**Figure 5**