Instrumentation and data processing

Two recent developments in seismometry allow the study of seismic phenomena over a broad period range. Block and Moore [1970] developed a quartz accelerometer with the required stability and sensitivity to record seismic waves as well as earth tides. The instrument, especially useful at periods greater than 80 sec, recorded free oscillations of the earth associated with an earthquake of magnitude 6.5 [Block et al, 1970]. Pomeroy et al. [1969] described three-component wide-band long-period seismographs that operate at maximum magnifications up to 500,000 in the period range of 30 to 60 sec. Relatively noise-free output at these magnifications was achieved by using rigid environmental control (i.e., an air-tight chamber in a deep mine). A displacement transducer output on the same instrumentation permits concurrent recording of the solid-earth tides. A symmetrical triaxial seismometer, in which the orthogonal directions of response form equal angles (approximately 55°) with the vertical, minimizes some problems of natural period variation and allows matched response characteristics [Melton and Kirkpatrick, 1970] . Farrell [1969] described a gyroscopic seismometer consisting of two counter-rotating pendulous gyroscopes that permits the separation of tilts f rom horizontal displacements. The Ocean Bottom Seismometer (OBS) off the coast of northern California produced a variety of geophysical data [Latham et al., 1967; Latham and Nowroozi, 1968; Nowroozi et al., 1968; Anderson and Latham, 1969]. Bradner [1970] described measurements with a free-floating midwater seismometer. A system for counting seismic events was designed by Decker [1968] . Mereu and Kovach [1970] described a portable seismic system for crustal studies. Response characteristics and seismograph design criteria were discussed by Rodgers [1968] , Russell et al [1968] , Bollinger [1969] , Mitchell and Landisman [1969] , Reasenberg [1969] , and Rodgers [1969] . Rihn [1969] presented data on the design of electromagnetic damping circuits. Research in instrumentation for the measurement of