Bulletin of the Seismological Society of America. Vol. 66, No. 6, pp. 2039-2047. December 1976 A GEOPHYSICAL WIRE STRAINMETER BY GEOFFREY KING AND ROGER BILHAM* ABSTRACT A self-zeroing geophysical strainmeter design is described which has been used extensively by Cambridge University in the past 4 years. The performance of the buried 10-m instrument is adequate to study geophysical signals in the Earth at periods from 1 sec to DC and with magnitudes from 10-10 to more than 10-5 strain. Low current consumption enables the instrument to operate remotely from batteries for up to 1 year. Better than 0.1 per cent linearity allows its use in the study of nonlinear geophysical signals, and relative calibration to 0.2 per cent allows clusters of instruments to be used in arrays. The absolute calibration of such an array is 2 per cent. INTRODUCTION There are two basic problems in earth strain measurement technique. First, it is not known whether strain signals from distant sources (and with large wavelengths) are coherent in strain over large distances. This has been called the site homogeneity prob- lem. Second, it is not known whether observed noise and drifts which are presumed to be due to climatic or "tectonic" effects are localized noise or truly represent coherent phenomena affecting large volumes of the Earth's crust. To gain insight into these problems, there is no substitute for using a large number of strainmeters. Low cost, ease of installation, reliability, good calibration and linearity and an ability to run unattended for periods between a month and a year are of primary importance to instruments for this purpose. It is perhaps of less importance that each instrument should be inherently low noise. The sensitivity needs to be adequate to provide tidal information and the stability sufficient to exclude the instrument or mounts as the major cause of long-term drift. The instrument described fulfills these criteria, although we have little insight into the cause of some of the long-term drifts recorded by our instruments and at least some drift is due to the instrument and mounts. More than 80 of these instruments have been operat- ing in areas of geophysical interest. PRINCIPLE OF OPERATION A wire strainmeter is shown schematically in Figure 1. It consists of an invar wire held in tension by a frictionless balance. Strain in the ground appears as rotations of the balance which are detected using a displacement transducer. Instruments using this basic design have been described previously by Sydenham (1969), Bilham and King (1970), and Gerard (1971). The instrument which we have developed has two systems to perform automatic rezeroing. One of these incrementally shifts an electronic level to the chart recorder by up to 16 times. This is fast in response and adequate under most circumstances. However, *Present Address: New York 10964. Lamont-Doherty Geological Observatory of Columbia University, Palisades, 2039