Case Study in the Santa Susana Mountains: Observing Discontinuities Using Non-invasive Surface Wave Methods

Abstract:

A case study was performed at a former industrial site in the Santa Susana Mountains in Canoga Park, California in order to observe discontinuities such as faults and fractures that potentially affect groundwater flow and contaminant transport within the study area. The active multi-channel analysis of surface waves (MASW) was performed using a landstreamer and weight drop source along a service road spanning approximately 1.4 miles. From previous geologic investigations, it is assumed that at least two faults cross the seismic profile and that one end of the profile is within a shear zone. The subsurface is composed of brittle to competent sandstone with intermittent shale beds that make up the Chatsworth Formation. Collected data were used to construct 2-D shear-wave velocity profiles and for backscatter analyses. Noise tests from an additional reflection survey along the same road was used as passive MASW to supplement the active data where low frequency data was absent. The goal of this case study is to observe the presence and orientation faults and fractures that could not be determined from surface mapping or downhole geophysical logging.

Higher mode surface waves contaminated much of the data providing evidence of the complex geologic structure potentially caused by an abundance of faults, fractures, and changing topography throughout the profile. The passive MASW provided low frequency dispersion information which helped dispersion curve picking where the active data was not sufficient. The backscatter analyses observed several events that coincided with geologic features seen in the MASW results as well as in areas where the higher mode contamination was too great for MASW analysis.

As a result, the combination of active MASW, passive MASW, and backscatter analyses provided velocity information at depths to 30 m. Several discontinuities were observed in the velocity profiles and backscatter analyses. These low velocity features may provide information regarding the presence and orientation of faults and fractures after further investigation. Only 51% of the stations were used for characterizing the subsurface before the signal-to-noise ratio and the risk of artifacts and aliasing in the data became too great for extracting reliable results.