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U.S. Geological Survey Open-File Report 77-224

Factors Limiting the Sensitivity and Dynamic Range of a Seismic System Employing Analog Magnetic Tape Recording and a Seismic Amplifier with Adjustable Gain Settings and Several Output Levels

By J.P. Eaton and J.R. Van Schaack

Introduction

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In the course of modernizing the low-speed-tape-recorder portable seismic systems and considering the possibilities for the design of a cassette-tape-recorder seismic refraction system, the factors that limit the sensitivity and dynamic range of such systems have been reviewed. These factors will first be stated briefly, and then their influence on systems such as the new 5-day-tape seismic system will be examined in more detail. To fix ideas, we shall assume that the system consists of the following elements:

1. A seismic sensor: usually a moving coil inertial seismometer with a period of about 1 second, a coil resistance of about 5000 ohms, and an effective motor constant of 1.0 V/cm/sec (across a 10K load terminating the seismometer sensitivity-and-damping-adjustment resistive network).

2. A seismic amplifier/voltage controlled oscillator unit made up of the following components:

   a) A fixed gain preamplifier with an input resistance of 10K and an internal noise level of 0.5 µVpp referred to the preamp input (0.1 Hz ≤ freq. ≤ 30 hz).

   b) An adjustable gain (0 to 42 db in 6 db steps) intermediate amplifier

   c) One or more fixed gain output amplifiers.

   d) Two sections of 6 db/octave bandpass filter serving to couple the 3 amplifier stages together.

   e) Voltage controlled oscillators for each output amplifier to produce modulated FM carriers for recording on separate tape tracks or modulated FM subcarriers for subsequent multiplexing and direct recording on tape in the California Network format.

3. An analog magnetic tape recorder: e.g. the PI 5100 (15/80 ips recording in the FM mode or in the direct mode with the “broad-band” variant-of the Cal Net multiplex system, or 15/16 ips recording in the direct mode with the standard Cal Net multiplex system), or the Sony TC-126 cassette recorder operating in the direct record mode with the standard Cal Net multiplex system.

4. Appropriate magnetic tape playback equipment: e.g., the Bell and Howell 3700-B for the PI-5100 or the Sony TC 126 for its own tapes.

5. Appropriate discriminators (employing subtractive compensation, at least for the multiplexed systems) to restore the data signals to their original forms.

6. An A/D convertor to digitize the seismic signals for computer processing and/or a strip chart recorder (e.g., the Siemens Oscillomink) for playout of the data.

Last modified October 28, 2010
First released in paper form 1977

  • This report is available only on the Web.

For additional information:
Contact Information, Earthquake Science Center, Menlo Park Science Center
U.S. Geological Survey
345 Middlefield Road, MS 977
Menlo Park, California 94025
http://earthquake.usgs.gov/

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Suggested citation:

Eaton, J.P., and Van Schaack, J.R., 1977, Factors limiting the sensitivity and dynamic range of a seismic system employing analog magnetic tape recording and a seismic amplifier with adjustable gain settings and several output levels: U.S. Geological Survey Open-File Report 77-224, 14 p.



Contents

I. Introduction

II. Fundamental Limits of the Amplifier And Tape Recorder

III. Digital Recording Vs Analog Recording

IV. Electronic Amplification Required for Optimum Sensitivity

V. 3-Level Amplifier Design and Characteristics

VI. Signal Detection Thresholds with Moving Coil Seismometers and Quiet Sitebackground Noise


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