
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 clientâ€™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
