GTSM at Southern California

 

High precision and high stability measurements of the horizontal strain field in a region are made in California using a Borehole Tensor Strain Measurement instrument (BTSM). These instruments are installed at a depth of 100-200 metres at 8 sites in California. The sites are shown in red in the accompanying map.

 

 

HISTORY

 

The PFT site was originally occupied in 1983 for a comparative investigation of GTSM instruments with 2 Sacks-Everton dilatometers and the UCSD long baseline laser interferometer.  Unfortunately, the Sacks-Everton systems failed before any long term comparison could be made.  Two of the three components of the UCSD laser strainmeter are too unstable to allow direct comparison of long and short baseline instruments for anything but tidal measurements.    These tidal studies have been very useful in the study of tidal calibration as the instruments agree with each other better than either agree with best tidal models.

 

Only the SE/NW (optically anchored) laser component was available for long term comparison studies. The SE/NW laser data are (supplied by Wyatt and Agnew) compared with the SE/NW strain inferred from the 3 component GTSM borehole data.  For the 10 year period 1988-1998, variation of the measured strain rate of the GTSM (after removal of the exponential and linear hole recovery effects) and shown in red is less than 50 ne/year from the absolute strain rate identified by the LSM (shown in black). This is an important and fundamental result which confirms the long term stability of the GTSM system

 

Both instruments show the influence of the CIC water level to different degrees.  The GTSM data includes all major earthquake steps (Landers, HectorMine{channels 2 and 3}) which are not included in the LSM data because of loss of line lock during the seismic wave arrivals.

 

A calibration method that incorporates cross coupling of remote areal/shear strains into instrument areal/shear strains has brought strain tides measured by the Pinon Flat GTSM (borehole diameter 200mm) into very good agreement with strain tides independently measured by the co-located LSM (dimensions ~1km). The cross coupled calibration has also yielded good agreement between the GTSM observations of the 1992 Landers earthquake and geodesy-based modeling, and co-located EDM measurements (Hart,Gladwin,Gwyther,Agnew and Wyatt 1996).

The short baseline GTSM instrument shows relatively higher short period noise than the LSM throughout the record, as is to be expected.

 

One of the three channels (gauge 2) in the Pinon Flat BTSM instrument has suffered significant degradation in gain from 1998 onwards, due to component failure in the downhole preamp (after 15 years of continuous operation and several lightning strikes) . The internal gain compensation system ran out of dynamic range in mid-2000. In December 2000 an uphole gain compensating circuit was installed on channel 2 in an attempt to continue normal operation to extend the 18 year dataset . September 1, 2001 produced another massive electrical storm after which all instruments at PFO were disabled. Again repairs were attempted during the December 2001 field trip, and again in late May 2002, but degradation has continued. It is considered that this site is now irrecoverable.

 

Review of the field repair notes for the life of this instrument reveals that by December 1997, damage to the downhole system was suspected. Though failure of the gain compensation system occurred mid 1998, all channels post this date are suspect, because these pre-1980 instruments passively share bridge drive systems across channels downhole. Though normal tidal data is still evident on channels 1 and 3, long term stability is compromised.

 

Continued degradation of the last channel occurred through 2003 and 2004.  The system was shutdown in December 2005 after 22 years of operation.

 

A site was established at Coldbrook in the Sierra Madre mountains in 1996.  This was to be part of a composite array of strain meters for the transverse ranges complementing the Punchbowl dilatometer.       The array was only partially completed.  The CLT site is still in operation and includes a tilt monitoring system.