Dictionary:Fig A-11

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FIG. A-11. Seismic amplifier. (a) Schematic of a seismic amplifier. ‘‘Amp’’ is an amplification stage; there are usually several such stages. AGC control produces negative feedback. Filters may be located at various positions. Usually there are many channels in parallel. (b) Block diagram of an IFP digital recording system. Each channel has its own components prior to the multiplex switch. The line filter reduces radio-frequency static picked up by geophone cables. The preamplifier increases the signal level by a constant amount while providing impedance matching. The low-cut filter supplements geophone filtering by removing very low frequencies where ground roll is excessive. The high-cut filter prevents aliasing; its slope is typically 72 dB/octave. The notch filter reduces 50 or 60 Hz power-line pickup (or 16 2/3 Hz electric railroad pickup). The multiplexer connects each geophone sequentially to the quaternary-gain amplifier which automatically adjusts its gain in 4:1 steps until the amplitude falls within a prescribed range, after which a 3-bit word specifying the gain is sent to the formatter. The A-D converter measures the signal amplitude, one bit being output for polarity and 14 bits for magnitude. The formatter arranges the data for writing onto magnetic tape by the tape transport. Separate read heads read the magnetic tape immediately after the data have been written. The output is amplified in the digital AGC unit, converted to analog form in the D/A converter, and written by a camera to give a monitor paper record. (From Sheriff and Geldart, 1995, 233.)