SEISMIC IMAGING IN URBAN AREAS: EXAMPLES AND LESSONS LEARNED

For earthquake hazard assessment applications in urban areas we acquire high-resolution seismic data to image faults and sedimentary basin structure, and measure seismic velocity in the 10 to 1000 m depth range. We find that most conventional high-resolution seismic imaging techniques are transferable to the noisy urban setting with some restrictions or modification. For example, the sledgehammer source used for P- and S-wave site characterization up to about 50 m depth works well on quieter residential streets, or at quieter times on noisy streets, with occasional pauses for intermittent auto traffic. For greater imaging depth weight-drop sources up to about 150 kg are suitable, but we have had difficulty preventing source double impacts over long cdp surveys on concrete. Weight-drop sources over 150 kg are rarely suitable except for industrial areas because of loud impact noise that is unwelcome in residential areas. Our preferred seismic source for urban surveys targeting the 50 to 1000 m depth range is a 5000 to 6500 kg vibroseis truck because the energy is spread out over several seconds. On deeper-imaging surveys other problems emerge, including geophone station skips for driveways and street crossings, and signal/noise issues due to long source-receiver offsets. Recording with at least 120 channels helps alleviate these problems. Further reduction of traffic noise can be achieved in data processing through the use of coherent signal prediction filters and stacking. Alternately, traffic noise can be used as a surface wave source to extract S-wave velocities through modeling. (Any use of trade names in this report is for descriptive purposes only and does not imply endorsement by the USGS.)