Turtle Geophone

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Item Turtle Geophone
Item code Seismic, Acquisition, Detector
Manufacturer Geophysical Research Corporation (GRC), Superior Oil Company, and Others
Circa 1930
Donor Al Knox
Location NHC
Serial number

The Geophysical Society of Houston museum collection has an example of a turtle geophone. The instrument is described by Laurence G. Cowles who designed an amplifier for it:

The Superior Oil Company “Turtle Geophone” is a variable-reluctance, moving magnet type that was first developed by Henry Salvatori, I think. It is described in “Geophysics”, vol. IV, No.1, pages 1-7, and 53-68. 

An earlier model of the Superior geophone was about 6: in diameter and 10” high. 

The variable-reluctance geophone had a resonant-like frequency response that was about ideal for record character, provided the amplifier had an approximately flat frequency response. This made difficult the design of a satisfactory AGC amplifier. 

Superior’s first AGC amplifier was designed in 1946 by Laurence G. Cowles. The need for a nearly flat frequency response forced the designer to use an uncomplicated circuit with a limited control characteristic. That amplifier used only one tube type and 1N34 diodes. 

The Superior amplifier was redesigned when transistors and FETs became available with the FETs used as the variable-gain elements. This is believed to be the first use of FETs in the industry. 

By Laurence G. Cowles 

May 1981

Turtle geophone 1 assembled.JPG Turtle geophone 2 unassembled.JPG
Pictures show the instrument assembled (ready for use) and unassembled to expose the internal mechanism.

References : Daniel Silverman (1939). ”THE FREQUENCY RESPONSE OF ELECTROMAGNETICALLY DAMPED DYNAMIC AND RELUCTANCE TYPE SEISMOMETERS.” GEOPHYSICS, 4(1), 53-68. https://doi.org/10.1190/1.1440481

Neil R. Sparks and Paul F. Hawley (1939). ”MAXIMUM ELECTROMAGNETIC DAMPING OF A RELUCTANCE SEISMOMETER.” GEOPHYSICS, 4(1), 1-7. https://doi.org/10.1190/1.1440478