abstract

An experimental shallow strain seismograph was installed at the Wichita Mountains Seismological Observatory in June, 1963. Short-period and long-period pendulums are operated in combination with strain seismometers to study the improvement in the discrimination of wave types and the improvement in signal-to-noise ratio for P waves. Four 20 m horizontal strain seismometers with velocity and displacement transducers are oriented at 45 deg intervals at an average depth of 3.6 m. The least detectable strain increment for Rayleigh waves with the velocity transducer is 4 × 10−13 at 1 Hz with short-period galvanometers, and 1.4 × 10−12 at 100 sec with long-period galvanometers, where Johnson noise is the limiting factor. The least detectable strain increment with the displacement transducer is 2.5 × 10−12 in the range 0.01–10 Hz. A preliminary evaluation of environmental effects indicates that high humidity in the vault requires sealing of the instruments. The vault must be sealed against wind pressure changes. Wind noise from air-to-ground coupling is equal to the amplitude of 0.5-sec microseisms (1.0 mμ peak-to-peak) at a wind velocity of about 14 km/h at a depth of 3.6 m. Seasonal drift rates as high as 30 μ per month, corresponding to a vault temperature change of 5°C, prevent unattended operation of the displacement transducers without drift compensation.

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