Seismic Reflection Surveys
At North Haiwee Dam
AGI is currently performing a multi-phase seismic reflection investigation at the Los Angeles Department of Water's North Haiwee Dam to investigate subsurface faulting. In Phase 1, deeper seismic reflection profiles were recorded along two 1.5 mile-long survey lines designated as Lines 1 and 2. These lines were setup north of the existing dam to cross the anticipated north-northwest trend of faulting. A swept frequency, vibroseis energy source (outputting 20,000 pounds of force into the ground) was used used to record reflections from basin layering to depths exceeding 3,000 feet. The reflection profiles for Lines 1 and 2 were initially evaluated for faulting. Based on the fault patterns detected on Lines 1 and 2, additional survey lines were positioned in Phase 2 to further investigate the trend of this faulting. Lines 3, 4 and 5 were setup to record higher-resolution, near-surface reflection and refraction tomography profiles of the upper 800 feet. The migrated reflection profiles for Lines 1 and 3 are shown below. AGI completed this reflection data processing using the Vista 2D/3D data processing software.
The resulting seismic profiles were used to interpret the locations of younger fault planes extending upward near the ground surface. These profiles were used to prepare a a preliminary mapping of fault traces near the surface in the vicinity of the proposed upstream dam. Additional seismic surveys are planned to complete this investigation of fault trends.
Seismic Reflection & Refraction Surveys
Large Land Development
The surveys were recorded with 10-foot spaced shot points and receivers to prepare higher-resolution, 24-fold common midpoint reflection profiles. The 1,800-foot long reflection profile shown below shows subsurface layering in the upper 500 feet. Several paralleling profiles such as this were evaluated for breaks in subsurface layering associated with faulting. These results were used to map the trend of several potentially active faults forming a "pull-apart" structure beneath the former lake basin. Our clients conducted further investigation of these seismic fault locations using trenches and boreholes to estimate their age of last movement. The seismic profiles and resulting geologic data were used to prepare a final investigation of faulting for the Riverside County Building Department.
AGI provided a "turn-key" seismic data acquisition through data evaluation program. We used our own multi-channel seismic recording equipment and completed all seismic data processing and geologic evaluation of the seismic profiles in house.
& Refraction Surveys
AGI and Norcal Geophysical Consultants teamed together to completed seismic reflection and refraction profiles along seven 2D seismic survey lines located across an onshore area where a shaft and tunnel heading beneath the San Francisco Bay were proposed. The total lineal coverage of these 7 seismic lines was 17,000 feet (3.2 miles).
The surveys were recorded with various source and receiver configurations to prepare higher-resolution, 24 to 30-fold common midpoint reflection profiles. The reflection profiles were used to provide images of subsurface layering in the upper 700 feet. The profile below shows younger Plio-Pleistocene mudstones on lapping the older Franciscan formation. These profiles were used by our client Fugro West to map the subsurface topography of a key mudstone unit. This mudstone unit was the preferred tunneling formation.
AGI supervised the seismic data recording operations and supplied its own 120-channel seismic recording system. AGI also completed all data processing of the seismic reflection profiles and worked with Fugro West to integrate these profiles into their SMT 2D/3D Pak interpretation work station.