Draft Report:
USGS-NEES pilot study in Garner Valley, California,
using T-Rex vibroseis source vehicle for high-resolution
P-wave seismic reflection imaging
William J. Stephenson
U.S. Geological Survey
Geologic Hazards Science Center
Golden, Colorado, USA
DISCLAIMER: This draft report is distributed solely for the purpose of unofficial information to
interested parties. Its content is deliberative and predecisional. Because the manuscript has not been
approved for publication by the U.S. Geological Survey (USGS), it does not represent any official USGS
finding or policy.
Executive Summary:
As part of the 2004 Network for Earthquake Engineering Simulation (NEES)
Demonstration at the NEES Garner Valley Instrumentation Site in southern California,
the U.S. Geological Survey (USGS) conducted a pilot study in collaboration with
NEES@UTAustin to demonstrate the application of the NEES vibrator vehicles with
multi-channel high-resolution seismic reflection/refraction acquisition systems for
detailed subsurface imaging. The USGS piggybacked this effort with funding from,
among other sources, the NEHRP Venture Capital project entitled “A Multi-facility
Approach to Solving Problems in Earthquake Hazards” (J. Gomberg was Principal
Investigator). The overall objective of this project was to assess the potential to solve
challenging problems related to earthquake hazards and engineering by marshaling the
combined facilities of the NEES consortium, the Incorporated Research Institutes for
Seismology consortium (IRIS; also supported principally by the NSF), and the USGS
National Earthquake Hazards Reduction Program. The primary focus of this component
of the project was to assess the potential of the T-Rex three-component vibroseis truck as
an imaging source both for high-resolution shallow basin and crustal scale structure
imaging. T-Rex performed admirably as a high-frequency source vehicle for highresolution P-wave seismic reflection imaging of the upper kilometer.
Introduction:
As part of the NEES Garner Valley demonstration project, and with funding from
the National Earthquake Hazards Reduction Program (NEHRP), the U.S. Geological
Survey (USGS) participated in a pilot study using the NEES source vehicle T-Rex for
vibroseis P-wave high-resolution seismic reflection imaging along roadways in Garner
Valley, California (Figure 1). Seismic data processing was generally conventional for
these data, which included gain correction, bandpass filtering, and stacking velocity
analysis. Depth conversion was completed post-stack, but these data have not been
migrated as presented here.
Figure 1. Map of Garner Valley with locations of seismic profiles GV1 and GV2.
Pilot Experiment:
We acquired two-second records using 60 sensors (channels) per source point.
Each source record was correlated with the recorded vibroseis field sweep prior to saving
the field record to disk. The source was swept from 20 to 180 Hz over 8 seconds, and two
correlated source records were stacked at each station. Source station spacing was 10 m.
We used a single 8 Hz resonant-frequency vertical-component geophone at each receiver
station, with each sensor spaced 5 m apart.
Figure 2. T-Rex in operation along seismic profile GV1.
Preliminary Results:
The T-Rex vibroseis truck worked well in generating seismic signal needed for
high-resolution P-wave seismic reflection imaging. The unmigrated two-dimensional
images are shown in Figure 3. The primary feature imaged on these profiles is the
interpreted sediment-bedrock boundary, which is subhorizontal over much of the extent
of GV1 at approximately 120 m depth. This boundary is slightly deeper on the eastern
side of GV2 (about 180 m), and then it deepens significantly to the west. In addition to
resolving the sediment-bedrock boundary, there appear to be several lateral disruptions
along this surface that are interpreted as possible faults. There is no constraint on the age
or timing of faulting from these data.
Figure 3. Preliminary interpretation of unmigrated depth-converted stacked P-wave
seismic reflection profiles GV1 and GV2. Profile GV2 is approximately 500 m south of
GV1, on the west side of state Highway 74.