Long-period background earth noise as measured in shallow, hand excavated holes

To facilitate its objective of high-resolution imaging of the Earth's crust and upper mantle, The Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) has initiated studies into developing instrumentation capable of achieving that goal. The requirements include portable sensors capable of resolving seismic signals to 100 second periods.

To test the feasibility of obtaining useful long-period seismic data from a portable array, prototype instruments were installed in hand excavated shallow holes (postholes) at several sites in various geologic settings across the continental United States. Three of the sites were near established seismic vaults and comparisons between posthole installation and vault installation were made.

Results from this study indicate that posthole installation of long-period sensors may indeed be feasible: eight out of the 12 sites occupied had long-period background noise levels low enough to resolve 100-second surface waves generated from a magnitude 5.0 earthquake 30 degrees distant from the recording station. At periods less than 10 seconds, background noise recorded from postholes was no more than 3 dB. higher than that recorded in vaults. At 100 seconds, vertical noise was 11 to 16 dB. higher than that recorded in vaults and horizontal noise was 4 to 22 dB. higher. Across all posthole installations, as compared to Peterson’s Low Noise Model, vertical and horizontal noise at 100 seconds averaged 27 and 45 dB. higher, respectively.

Sites should be located directly on bedrock, where possible. If this is not possible, they should be in well compacted inorganic soil with a low moisture content. Immediately after installation at a potential site, a noise sample should be analyzed in the field to test the suitability of the site.