A new seismometer equipped for electromagnetic damping and electromagnetic and optical magnification (theory, general design, and preliminary results)

Abstract

A procedure which may be followed in the development of a general design for a seismometer, to give approximately a specified performance, is illustrated by a concrete example. In this design the steady mass is about 500 g; the arrangement is such that the period may be determined and the damping adjusted from the recording station, which may be at a distance from the seismometer; and the magnification and its variation with the period of earth displacements, in the range from 2.5 to 60 seconds, is substantially the same as is given by a seismometer of the ordinary type having a magnification for shortperiod displacements of 1,250, a period of 12.5 seconds, and critical damping.Preliminary results obtained with an experimental seismometer constructed substantially in accordance with this design are given, and a photograph of the instrument is shown.

Earthquake seismograph development: A modern history—Part 1

“…This arrangement makes the response constant to ground dis placement within ±3 dB from 2 to 6000 s. When capacitor and resistor are removed, response to velocity is constant over the same range. However, today's negative attitudes about galvanometers have tended to close this avenue, even though it was investigated by Wenner [1929] and reinvesti gated by the National Bureau of Standards (NBS) about 1960, in a study by D. P. Johnson and H. Matheson of the Bureau. * In addition to their theoretical studies, Johnson and Math eson, with the aid of master machinist Henry A. Schmidt, also of NBS, designed a short-period vertical component seis mometer in which the permanent magnet was used for the mass, while the electrical coil was mounted in the frame.…”

“…Seismologists followed this earliest documented work by de- vising various models for different purposes. Among better known names within this country are those of Wenner [1929], Benioff [1932], and the W. F. Sprengnether Instrument Com pany, Inc., of St. Louis, Missouri, which built seismometers for various seismologists. In general, the electromagnetic seismograph in use until 1950 employed this photographic registration on film or paper wound around a slowly rotating drum.…”

Some properties of Heavily Damped Electromagnetic Seismographs

“…The theory of the electromagnetic seismograph was developed in stages by Galitxin (1914), by Wenner (1929) and by Grenet & Coulomb (1935). Following on the lines indicated by these authors, we imagine the seismometer and galvanometer to be connected through a general 4-terminal network, which we replace by a "T" network of 3 resistors as shown in Figure the arm AB and the seismometer coil is called R, the resistance of BC and the galvanometer coil is called r, and the shunt resistance is called S. Currents I and i flow through the seismometer and galvanometer coils respectively.…”

Mathematical problems in seismology

“…In galvanometrlc instruments the fact that the galvanometer is not tightly coupled to the pendu lum means that the number of degrees of freedom is Increased and the order of the differential equation of motion is higher than that (two) of the mechanical instrument. The theory of the galvanometrlc seismograph has been studied by Galitzin [68], Wenner [69], Sohon [8, part II], Benioff [70], Blake [71], Grenet and Coulomb [72], Scherbatskoy and Neufeld [73], and others, but many interesting problems are yet to be discussed.…”