Robert F. Ballard

Time-lapse seismic study of levees in southern Texas

The primary objective of this work was to measure changes in compressional(Vp) and shear-wave (Vs) velocities in an earthen levee during a ponding experiment designed to simulate flood conditions on the Rio Grande in south Texas. Preliminary testing at five levee sites, all within a 10 km radius and each with unique physical, EM, and core characteristics, was completed and a single low-conductivity, highly fractured site was selected for investigation. Several different types of seismic data were recorded. Seismic data analysis techniques appraised included Pand S-wave refraction tomography and Rayleigh surface-wave analysis using multichannel analysis of surface waves (MASW). P-wave methods provided reasonable results, but no change was observed in velocity even after full pool had been maintained against the levee side for two days. The S-wave velocity change was rapid and isolated to one area within the confines of the pool. The reason for the latter effect cannot be uniquely determined; however, it may possibly be the result of an isolated variable expansion of the clay core, a likely explanation considering the preceding years of drought. Alternatively, these changes could be related to mechanical compaction variability and variable material distribution within the levees. Figure 1. Location of the San Juan Quadrangle, Texas, USA.

Time-lapse seismic study of levees in southern New Mexico

The primary objective of this work was to measure changes in compressional(Vp) and shear-wave (Vs) velocities in an earthen levee during a ponding experiment designed to simulate flood conditions on the Rio Grande in south New Mexico. Although similar to such experiment, performed an year earlier on the Rio Grande in south Texas, the levee seismic response results are different. This work was similar to previous Preliminary testing at three levee sites, all within a 1 km radius and each with unique physical, EM, and core characteristics, was completed and a single low-conductivity, highly fractured site was selected for investigation. Several different types of seismic data were recorded. Seismic data analysis techniques appraised included Prefraction tomography and Rayleigh surfacewave analysis using multichannel analysis of surface waves (MASW). P-wave velocity change (decrease) was rapid and isolated to one section within the pool confines, which already had anomalously high velocity most likely related to burrowing animals modification of the levee structure. S-wave velocity change was gradual and could be observed along the whole width of the pond within and below the levee. The results within the levee sand core were consistent with the observations of sand S-wave velocity changed due to saturation.

Tunnel signature prediction for a cross-borehole seismic survey

Seismic location of tunnels or voids with a cross‐borehole survey is examined with field data and theory. The field data were taken at a site with a 2.2-m high by 2.7-m wide, roughly rectangular cross‐section tunnel, using a newly developed 1 to 5 kHz system employing a P‐wave sparker source. The synthetic records were obtained using a 2.5-D boundary‐valued solution for an explosive point source near a cylindrical void, and the solution was evaluated with the method of steepest descent. The synthetic waveforms compared well to the field data; both showed a maximum reduction of amplitude in the tunnel shadow of 8 dB and a maximum first arrival delay of 0.1 ms. Additional theoretical modeling was used to examine the variations of the received signals with tunnel size and frequency and showed amplitude reduction increased with frequency and tunnel size. Calculations showed that S‐waves scattered from the tunnel are more than 20 dB smaller than the primary P‐wave on hydrophones and more than 12 dB smaller on ...

A 2‐C Towed Geophone Spread for Variable Surface Conditions

Routine scanning of the subsurface using surface waves and body wave first arrivals can be done cost effectively and with confidence at relatively high production rates if: consistent time-lapse measurement can be made, high production rates can be maintained, and fixed processing flows can be used on all data sets from a particular area. Development of a towed geophone spread that can maintain consistent coupling on both hard surfaces (such as roads and parking lots) and in lightly vegetated or loose dirt ground cover is critical to high production rates and instrumental in automated difference processing routines. Acquisition tests have proved that the coupling necessary for accurate recording of surface waves can be established and maintained over an extended distance and variable terrain with only pressure contact to the earth’s surface through aggressive contact points. Unlike body wave surveying, where coupling of geophones by invasive “planting” produces the optimum processed data, surface wave energy recorded from planted geophones is equivalent for most applications to surface wave energy recorded from pressure coupled geophones. A towed spread design measured surface waves over a tunnel along the California/Mexico border and in a geologically complex area in southeastern Kansas with fractures and known mine works that appear for most purposes to be equivalent to measurements made with conventionally planted geophone spreads.