What Lies Beneath: UH’s La Marque Geophysical Observatory
Wavelets Article in GSH Journal
By Li Chang, Robert Stewart, and Azie Aziz
What was once a military base, Camp Wallace some 15 miles north of Galveston, has become an educational and research preserve of the University of Houston. As part of this 950-acre UH Coastal Center, the Allied Geophysical Lab and the Department of Earth and Atmospheric Sciences have established the La Marque Geophysical Observatory. The mission of the Observatory is to provide a site and capability for advanced geophysical training (Figure 1), research, and monitoring.
Figure 1: The research team from the Allied Geophysical Lab of University of Houston (E. Kocel photo).The Coastal Center, as a whole, hosts a producing oilfield (some 300 BOPD), atmospheric sounding and sampling facilities, and botany and biology labs. As part of developing the Geophysical Observatory, two, shallow (450 ft) wells were drilled and cased at the site (Figure 2). The wells are currently used to conduct borehole geophysics research and teaching. High electrical resistivities on the logs are indicative of freshwater saturated sands.
Figure 2: Drilling rig and resulting gamma ray and electrical logs at La Marque Geophysical Observatory.In addition, a number of objects (drums, containers) have been buried to become targets of near-surface exploration and remediation surveys. The legacy grid of oyster-shell roads at the site provides excellent access for surface seismic training surveys.
In March 2012, we acquired a vertical vibe (Figure 3) line along the east-west entrance road. We were pleasantly surprised by the quality of the reflection data where we could see events down to about 2 s on the raw data (Figure 4).
Figure 3: The AGL’s minivibe at the La Marque Geophysical Observatory, Texas.The high-quality P-wave data motivated us to try recording pure shear-wave seismic reflection data. In early October 2013, we returned to the site with our IVI mini-vibe now configured as a horizontal shaker. The S-wave test used the minivibe shaking in a crossline direction with a 10 -150 Hz sweep over 12 seconds. Eight 24-channel Geodes recorded a total of 192 horizontal (cross-line) geophones. Vibe points were on the ½ station at 5 m intervals. In Figure 5, we show a quick brute stack of the resultant data. We can see S-wave reflections down to about 3 s!
Figure 4: Vertical vibe shot gather at La Marque filtered with a 25-35-100-120 Hz bandpass. The receiver spread consisted of 192 geophones spaced at 5 m.We are excited about future developments and use of the Observatory for long-term measurements of geophysical phenomena as well as exploration surveys, tests, and teaching. And we’d be pleased to collaborate!
NOTE: This article first appeared in the December 2013 issue of the Geophysical Society of Houston’s GSH Journal. Republished with permission from GSH.
Figure 5: S-wave stack using horizontal vibe and geophones.