In situ calibration of and algorithm for strain monitoring using four-gauge borehole strainmeters (FGBS)

• Published in: Journal of geophysical research. Solid earth 2013-04, Vol.118 (4), p.1609-1618
• Main Authors: Qiu, Zehua, Tang, Lei, Zhang, Baohong, Guo, Yanping
• Format: Article
• Language: English
• Subjects: absolute in situ calibration; algorithm; Algorithms; Boreholes; Calibration; data credibility; Gages; Gauges; Geophysics; Mathematical analysis; Mathematical models; relative in situ calibration; self-consistency; Strain
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1002/jgrb.50112

Borehole strainmeters have proved very useful in geodynamic research. Because the sensors are imbedded in rock, their in situ calibration is of crucial importance. The four‐gauge borehole strainmeter (FGBS) is a Chinese invention to monitor the temporal variation in horizontal strain. The four gauges in the FGBS are arranged at 45° intervals to bring about a simple self‐consistency equation, which serves as a means of checking that the measurements obtained from the FGBS are correct. The instruments currently used in China are usually placed at depths of several tens of meters to avoid disturbances at the surface, while still being sufficiently near the surface for the vertical stress to be regarded as zero—the premise on which the theoretical model of this observation is based. In this paper, an index of data credibility is established, based on the self‐consistency equation, to allow evaluation of the observations. A relative in situ calibration has been developed to calculate a relative correction factor for each gauge's sensitivity, termed the gauge weight, and this has proven effective in enhancing data credibility. Parameters for deriving strain from readings are determined by a concise absolute in situ calibration with the aid of the theoretical Earth tide. Instead of averaging four groups of solutions, a simpler comprehensive algorithm is developed to transform readings into strain. Data from 24 Chinese sites of YRY‐4‐type FGBS are processed and evaluated to be fairly good. Key Points Relative in situ calibration is significant to improve FGBS data credibility.Parameters for deriving strains from readings are determined by absolute in situ calibration.A simple symmetrical algorithm is made to derive strains from the readings.