Very broadband record from the new SAFOD borehole seismograph

A very broadband seismometer was installed in the SAFOD pilot hole on March 6, 2008 at a depth of 1039.5 m. Instrumentation includes a CMG-3TB very broadband (360 s to 100 Hz) velocity sensor, a CMG-5TB strong motion sensor (DC to 400 Hz), and a CMG-DM24Mk3 7-channel 24-bit down-hole digitizer. The experimental deployment is a collaborative effort between Guralp Systems Ltd. and SAFOD.

The data are digitized at 200 sps and telemetered in near-real time to the Northern California Earthquake Data Center (NCEDC) at the U.C. Berkeley Seismological Laboratory. The waveforms are archived at NCEDC (http://www.ncedc.org/safod/) and IRIS DMC (www.iris.edu/data) with network code SF. For spectrograms and helicorder plots go to http://quake.usgs.gov/cgi-bin/helipark.pl.

Spectrograms (Figure above) are particularly useful for identifying the wide range of signals detected by the deep sensor, including episodes of nonvolcanic tremor, local earthquakes and teleseisms. This spectogram shows vertical ground velocity (0 - 100 Hz) recorded on the very broadband seismometer on August 22, 2008 from 19:00 to 21:00 UTC (noon to 2 pm local time). The very broad frequency range observed for two local events (horizontal lines) contrasts with the narrow band observed for regional events. Several episodes of nonvolcanic tremor (light blue areas) are visible with strongest two marked. Note the very high frequency content of the tremor (>30 Hz).