The use of a laser extensometer to observe strain in a large ground sample

The design and construction are described of an experimental strainmeter. This observes changes in the number of waves of light standing in a one-kilometer gap between points on the earth’s surface. Strain is observed over a frequency range of zero to several hundred hertz, to an accuracy of an arbitrarily-small fraction of a wave-length of light. Frequency-dependent phase and amplitude distortion are absent. As the read-out is of the nature of a servo device, dynamic range limitations are not encountered. The prototype instrument is located in a region affected by the world rift system.