Michelson interferometer as an earth strain sensor

Abstract

A Michelson interferometer with 2 metre optical arms has been used to measure a shear component of earth strain over a 12 month period, 140 feet below ground in the University of Queensland experimental mine. Satisfactory fringes are obtained with an ordinary mercury lamp and filter over any required path length and a continuous record of strain is produced by focussing the fringes onto a moving film. Diurnal temperature changes are satisfactorily eliminated but seasonal changes in temperature penetrate the walls of the mine causing an annual strain cycle of the amplitude 10−5 in the vicinity of the interferometer. Loading of the ground by heavy rainfall causes a pronounced transient strain, which reached 2 × 10−6 in one extreme case. Surface waves from shallow earthquakes appear clearly on the film. As a tectonic strain sensor the interferometer has the advantages of cheapness, simplicity and built-in calibration; the important disadvantage is that strain at different times can be compared only if a continuous record is obtained.