A Fullband Seismometer for Installation in Slim, Deep, and Tilted Boreholes.
Abstract
Despite cultural and environmental noise suppression advantages, borehole seismometers have not had wide application to seismological networks. The obstacle has been the expense of drilling large diameter boreholes to overcome instrument-bandwidth limitations and borehole tilts. In this presentation we describe a fullband and slimline sonde: a 6-component borehole seismometer for installation in deep boreholes with moderate tilts.
This capability is achieved by combining a triaxial broadband seismometer and a triaxial 4.5 Hz geophone in a high-pressure sonde. The broadband seismometer's response is flat to acceleration between 200 s to 1000 Hz; the 4.5 Hz geophone response is flat to velocity from 4.5 to 90 Hz. Field tests of this instrument suggest a usable bandwidth of 40 s to 400 Hz - hence the term fullband. For standard 76 mm PQ-inner diameter slim boreholes, a 72 mm fullband sonde allows for 15o of borehole tilt. This compensation is achieved with a fixed broadband seismometer and a gimbled 4.5 Hz geophone. For 60 mm BQ slim boreholes, a 52 mm triaxial broadband sonde can tolerate this same tilt or, combined with a fixed triaxial 4.5 Hz geophone fullband 52 mm sonde is limited to 3o. We illustrate the operational aspects of the fullband sonde with data from both two 600 m deep PQ diameter boreholes near a major active fault and one associated with a hydraulic-fracturing test in a mine. The two fault installations made use of abandoned PQ boreholes - the 72 mm sonde diameter being the key to their use. The mine experiment used a similar bore but drilled horizontally. Tilt tolerance was achieved by swapping vertical and horizontal sensor positions in the sonde. Recordings from regional, local, and mine events show these sondes reported the expected ground motions. In the case of the mine it also gave evidence of ground tilts.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.S43E0656P
- Keywords:
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- 7294 Seismic instruments and networks;
- SEISMOLOGY