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Gain Recovery

As the geophone sees it, a seismic signal decays with time ( Figure 1 ). This signal must be manipulated at leasttwice by the recording instruments. First, the system preamplifier boosts the incoming signal to ensure that the

smallest recoverable signal is above the level of any later system noise.

Figure 1

This is a fixed preamplifier gain it is recorded in the header bloc! of the tape and in the observer"s notes.

#ext, to ma!e the best use of a limited range in the a$d converter, the system %balances% the signal ( Figure & ).

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Figure 2

The gain is decreased and increased very 'uic!ly, to !eep the magnitude of the output within a narrow range.

any systems !eep the output within a 1& d range this is !nown as qua ternary gain-ranging. The re'uiredincrease or decrease in gain occurs fast enough (within a few microseconds) that modern systems are regardedas instantaneous. The record of gain changes needed to effect the balancing is stored in the gain code associatedwith each sample this is the characteristic of the amplitude. The balanced signal itself is recorded in the datawords as the mantissa. The first tas! before us, then, is the restoration of the signal.

*ain recovery is often done as part of the demultiplexing step. The tape format dictates the location and meaningof the gain codes, so we need to specify the format. For nonstandard formats that are not in the processingcenter catalogs, the processor must provide the re'uired information.

+emultiplexing and gain recovery must be done as soon as all the field tapes are available this allows us toevaluate the recording before too much field wor! is done.

At this stage, we are ready to examine our first 'c plots. ne set may be produced without any amplitudead-ustments other than gain recovery. These allow us to locate noise spi!es and noise bursts. For evaluation ofdata 'uality and event continuity, some amplitude ad-ustments may be re'uired.

Gain Recovery Example

e consider the case where the amplifier gain ranges in steps of /, and where seven such steps are provided.The amplifier gain can therefore range over /0, or approximately / d. There are gain levels, represented bygain codes2

Gain Code

1 0000

4 0010

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42 0100

43 0110

44 1000

45 1010

46 1100

47 1110

For the sa!e of illustration, we consider that the digiti3er compares the signal value to a reference value of 1 thisvalue of 1 then represents the largest allowable value, or %full scale.%

e consider the waveform ofFigure 4, and its samples a1, a&, a4.

Figure 3

e wish to measure these samples to a precision of 1/ bits. Then the reference value must be 1 to this sameprecision, or

1 000 000 000 000 00.

If a1 is very close to full scale, it might be1111 1111 1111 11.In this case the gain woul set itself automatically to 1, an so the sam!le woul be recore as"

antissa *ain code

a11111 1111 1111 11 5555

#ow if we suppose that the value of a& is one$'uarter of a1, the gain would set itself automatically to /, and thesample would be recorded with the same mantissa2

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a&1111 1111 1111 11 5515

The value of a4, as the waveform approaches a 3ero$crossing, is very small let us say it is 16/7of a&. For this

sample the gain automatically increases by a further /7, and the sample value is recorded as2

a41111 1111 1111 11 1115

hen we come to recover these sample values by %bac!ing out% the gain codes, we can assume for illustrationthat we have & bits available. Then, using the gain$recovery expression

sam!le # sign $ mantissa $ 2%gain coewe have"

a1# 1111 1111 1111 1100 0000 0000 0000

a&8 5511 1111 1111 1111 5555 5555 5555

a48 5555 5555 5555 5511 1111 1111 1111

&e notice that the full 14%bit !recision has been maintaine, even on the tiny values.