Digital recording of sonic log wavetrains

Claim 1

In a velocity logging system including a sonde and cable means for traversing said sonde through a borehole, said sonde including transmitter means for sequentially generating seismic waves in the fluid in said borehole, receiver means for detecting said seismic waves after travel by various paths through the surrounding fluid and earth media, and means to generate electrical signal counterparts of said detected seismic waves and to transmit said counterpart signals to the ground surface, the improvement comprising digitizing apparatus for said counterpart signals comprising (a) programmable-gain amplifier means for receiving and amplifying said counterpart signals in accordance with a digital gain control signal applied to said amplifier; (b) means to digitize a selected time sample of said amplified signal; (c) means comprising at least three digital storage areas to temporarily store a digital data word including said digitized sample and a digitized value of gain of said amplifier used in digitizing said sample; (d) means to determine from said data word a new digital gain value for said amplifier to be used on the corresponding sample of a subsequent trace and means to temporarily store said new digital value or gain; (e) means to read out of storage a digital value of gain and transmit that value to said amplifier; (f) means to read out of storage said data and to reproducibly record said data on magnetic tape; (g) clock means for generating a series of control impulses to time the repeated operation of said means (b) through (f) to produce a corresponding series of data words representing a seismic trace; and (h) switching means to use said areas in rotation for (1) the reading out of digital gain values and storing of data words, (2) the storing of digital gain values, and (3) the reading out of data words.