Very Deep Borehole Installations

May 08

Guralp Systems Ltd. recently delivered and installed two seismic sensors in deep boreholes in California and Japan. In both cases, the downhole package consists of:

  • CMG-3TB very broadband (360s–100Hz) velocity sensor (VBB)
  • CMG-5TB strong motion sensor with response from DC to 200Hz
  • CMG-DM24mk3 7-channel 24-bit downhole digitiser with serial to optical converter
  • single jaw hole lock

CMG-3TB/5TB Sensor Package at SAFOD installation site.

The analogue outputs from the sensors are digitised downhole and the results transmitted via optical fibre to an uphole acquisition computer (California) or a Digital Communications Module (DCM) in the case of Japan.

The California installation is part of a collaborative experiment with the principal investigators of the San Andreas Fault Observatory at Depth (SAFOD) project located near the small town of Parkfield, halfway between San Francisco and Los Angeles. SAFOD is one of the three components of the U.S. National Science Foundation’s Earthscope program, being carried out in cooperation with the U.S. Geological Survey and NASA. The sensor package was deployed into the pilot hole at a depth of approximately 1050m.

The sensor package prior to being lowered into the borehole at SAFOD.

Currently the digitiser is transmitting nine data streams from this sensor package: six VBB velocity data channels sampled at 100 and 200sps respectively as well as the three accelerometer channels sampled at 200sps. The data are part of the SAFOD data mix and will be available to researchers through the IRIS Data Center. For more information on the SAFOD project, please visit the project homepage.

Teleseismic event waveform

Data of a teleseismic event recorded shortly after installation on the SAFOD instrument and a CMG-3TB borehole sensor in UK at Wolverton Vault. The first trace in each pair is from the SAFOD instrument and the second trace is from the UK borehole.
Power spectral density of SAFOD vertical component.

Power spectral density plot of the ground motion for the vertical component of the SAFOD instrument shortly after installation showing the low short period noise at the site.

An identical sensor package was installed in a borehole near the Matsushiro seismological observatory in Nagano Prefecture in the central part of the Japanese main island of Honshu.

The sensor is deployed at a depth of 700m. As in the case of SAFOD, the data are digitsed downhole and transmitted to the observatory via optical cable. The data streams will be incorporated into a national seismic network.

Teleseismic event recorded within a few hours of the Matsushiro sensor installation.

Guralp Systems Ltd. has been building seismometers for deep boreholes since the founding of the company in 1985. The first such instrument was delivered to Japan in the late 1980s and deployed by the crew of RV Joides Resolution at 714m in ODP hole 794 in the Sea of Japan. Since then university researchers, governmental organisations in several countries as well as an international organization have purchased seismic sensors from Guralp Systems for their boreholes. The latest successful deployment underscores GSL’s position as one of the world’s foremost suppliers of seismic sensors for all kinds of applications.