Refraction Seismic Survey to Determine
Rippability
by
Jim Hasbrouck
NOTE: The following is a summary of the Results of the Geophysical Investigation.
This paper can be found in its entiretyj at the National Technical Information Service
(NTIS) under the identification number DOE/ID/1258437.
A refraction seismic survey was conducted to measure
compressionalwave 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 24channel refraction seismic data were acquired with
30Hz geophones, and a 16pound 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 signaltonoise
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
raytracing techniques. The program uses the delaytime method to obtain a
firstapproximation depth model, which is then trimmed by a series of raytracing and
modeladjustment iterations to minimize any discrepancies between the picked arrival times
and corresponding times traced through the 2.5dimensional crosssectional 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 HobsonOverton 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 nearsurface (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
