US 3,947,806 AGranted

Automatic gain control for seismometer recorders

AssigneeThe Standard Oil Company

InventorsDavid P. Corkhill, Francis R. Freeman

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Abstract

This is a seismic digital recording system for use where the seismic amplifiers are placed near the geophone locations and their outputs transmitted by common cable to the recording truck or recorded on magnetic tape at the geophone location. In such a digital recording system, it is impractical to use a high-speed, high gain-ranging amplifier of 15 or 16 bits range. In our design the gain-ranging amplifier is high-speed but employs fewer bits total range. The extra range is made up in a preamplifier which has a variable range of gain, which can be automatically set, but at slower speed. The invention involves a control system which is responsive to the output of an A/D converter which is connected to the output of the gain-ranging amplifier. This control examines the bit output of the A/D converter throughout the whole record and, depending on whether the most significant bit is a 0 or 1, and whether the less significant bits are 0 or 1, it acts to control the gain of the preamplifier on the suceeding record by increasing the gain, decreasing the gain, or leaving the gain set at its instant value. The gain of the preamplifier can be adjusted by 6-db steps for each succeeding record until it reaches a value for which the operation of the gain-ranging amplifier is optimum.

Claims10

Dependent claims

In a digital seismic recording system having at least one recording channel responsive to a first geophone group and including therein a preamplifier, a gain-ranging amplifier, and an analog-to-digital converter, all connected in series, said preamplifier including switching means to vary the gain of said preamplifier in a plurality of steps, said gain-ranging amplifier including means to vary the gain in steps at a relatively high rate, over a selected plurality of steps, said analog-to-digital converter adapted to produce a digital word representing the instantaneous amplitude of the signal output of said gain-ranging amplifier at each digitizing interval, said digital word comprising at least

1. a most significant bit,
2. a second most significant bit, and
3. an intermediately significant bit, the improvement comprising:
a. a first latch and means responsive to said digital word output of said analog-to-digital converter to set said first latch when said most significant bit is a logical 1 at any time during the time of recording of a complete record;
b. a second latch and means responsive to said digital word output of said analog-to-digital converter to set said second latch when at least one intermediately significant bit is a logical 1 at any time during the time of recording of a complete record;
and
c. means responsive respectively to said first and to said second latches to modify the gain of said preamplifier.

4 more elements

Dependencies are inferred from the claim text.