Abstract:
The seismic amplifier includes an amplifying circuit which amplifies an analog signal to provide an amplified analog signal. The amplified analog signal is converted to digital signals. A controlled network connected to the amplifier circuit controls the gain of the amplifier circuit in a manner so that the gain for a current seismic operation, except for an initial seismic operation, is determined during the next previous vibration seismic operation.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to the seismic apparatus and methods in general and, more particularly, to seismic amplifying means and methods. 
     SUMMARY OF THE INVENTION 
     The seismic amplifier includes an amplifying circuit which amplifies an analog signal to provide an amplified analog signal. The amplified analog signal is converted to digital signals. A controlled network connected to the amplifier circuit controls the gain of the amplifier circuit in a manner so that the gain for a current seismic operation, except for an initial seismic operation, is determined during the next previous vibration seismic operation. 
     The objects and advantages of the invention will appear more fully hereinafter from a consideration of the detailed description which follows, taken together with the accompanying drawing wherein one embodiment of the invention is illustrated by way of an example. It is to be expressly understood, however, that the drawing is for illustration purposes only and is not to be construed as defining the limits of the invention. 
    
    
     DESCRIPTION OF THE DRAWING 
     The FIGURE is a partial simplified block diagram and a partial schematic of a seismic amplifier constructed in accordance with the present invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     The present invention relates to an improvement to vibration type seismic evaluations of earth formations. The present invention reduces the cost of apparatus for such operations by eliminating the need for a floating point amplifier and noise reduction apparatus as well as reduced bit requirements. It is well suited for telemetry operations involving very large numbers of recording channels. In that application the present invention would be located physically near the geophones. 
     Conventionally, a vibration seismic operation involves the use of a truck which induces vibrations into an earth formation by a pad which is pushed against the earth in accordance with a predetermined program. The frequency of the vibrations is swept through a range of frequencies typically from 10 Hz up to 100 Hz. The time duration for each sweep being approximately 10 to 20 seconds. At each shot point, the sweep may be repeated several times. The resulting seismic records are algebraically added. However, the vibration type seismic activity generates a weak energy signal. The present invention amplifies that signal in an economic manner. 
     The vibration truck itself and the control system are involved in the recording of the data in a conventional manner. The vibration operations are part of the prior art and are not part of the present invention but will be referred to as needed. 
     For a more convenient understanding of the present invention, the words, &#34;seismic shot&#34; hereinafter will refer to a plurality of individual vibration operations which are algebraically summed. A seismic vibration truck may be at one location for all the vibration of a seismic shot or the truck may be moved a predetermined distance from the site of the previous vibration during a seismic shot as is sometimes done in conventional seismic vibration usage. A seismic recording truck contains the seismic recording system. 
     Referring to FIG. 1, there is shown a channel amplifier which may be used with each geophone array. The amplifiers may be located in the field with the geophone arrays or located in the seismic recording truck with the other seismic processing equipment. The signals for one channel are provided to a preamplifier 4 having a high and low cut filter means 7 in a feedback loop and another high and low cut filter means 10 connecting the input of preamplifier 4 to ground 12. Preamplifier 4 provides an output signal to another amplifier 16 by way of a resistor 18. The output of amplifier 16 is provided to a sample/hold and analog to digital converter 24 (hereafter referred to as converter 24) which provides a plurality of digital signals, representative of the magnitude of the analog signal provided to it, including a most significant bit signal and a least significant bit signal shown. Converter 24 also provides a sign representative of the polarity of the analog signal at the time of conversion. The signals from converter 24 are recorded in a conventional manner by apparatus not shown. The gain of pre-amp 16, controlled as hereinafter explained, by an up-down counter 60 is also recorded by that same apparatus. 
     The signals from converter 24 are in the TWOs complement form. The sign bit and the most significant bit signals are provided to an Exclusive OR gate 26 which in turn provides an enabling signal whenever the output of converter 24 is over one-half of full scale of converter 24. 
     The gain of amplifier 16 is controlled by the relationship of one of a plurality of feedback resistors 28, 30, 32, and 34 and input resistor 18. The feedback resistor is selected by operation of a switch of a plurality of switches 38, 40, 42, and 44 which are controlled by control signals hereinafter explained. One set of gain values may be 8, 4, 2 and 1 respectively. Where a gain of 1 equals to 6 db, and a gain of 8 equals to 48 db. 
     Converter 24 receives clock pulses from a clock means 50. Clock means 50 also provides the clock pulses to an AND gate 54 connected to Exclusive OR gate 26 and passes the clock pulses when converter 24 provides an enabling signal. It should be noted that Exclusive OR gate 26 is necessary because converter 24 is providing its output in the TWOs complement format. If converter 24 was providing its output in the ONEs complement format, Exclusive OR gate 26 is not used. The most significant bit signal from converter 24 is then provided directly to AND gate 54. Counter means 58 counts the pulses provided by AND gate 54 and provides control signals to the up and down inputs of up/down counter 60 to control the counting direction of counter 60. Counter means 58 includes logic that causes it to provide an enabling `up` signal or an enabling `down` signal or no enabling signal to counter 60. 
     A pulse E1 is provided once after each shot point by the seismic system in the recording truck to the clock input of counter 60 and to the one shot multivibrator 65 which acts as a time delay. One shot 65, after the time delay, in turn provides a pulse to the clear input of counter means 58 thereby clearing counter means 58. 
     In operation, by way of example, the signal from the geophone channel is amplified via amplifier 16 and digitized by the A/D converter 24. If the amplitude of the signal exceeds one half of the full amplitude range of the A/D converter 24, the most significant bit and the sign bit of converter 24 will enable the AND gate 54 via the exclusive OR gate 26. Counter means 58 counts up every time AND gate 54 is enabled by the exclusive OR gate 26 and clock means 50 throughout the duration of the shot point. Thus counter means 58 counts the number of times the amplified geophone signal exceeded one half of the full scale of the A/D converter 24 during a shot. Clock means 50 provides one clock for each sample taken. At the end of a shot the seismic system provides a pulse E1 to the clock input of counter 60, and a clear pulse to counter means 58 via the one shot 65. Counter means 58 has internal logic which determines whether counter 60 will count up, count down or not change for each E1 pulse. The decision for that count depends on whether the gain of amplifier 16 was sufficient during the previous shot point, or a change in gain is desired. 
     For the first shot point, counter 60 is set to its lowest gain so that switch 44 is energized. If the amplified signal level during the previous shot point was not sufficient, counter means 58 will enable counter 60 to count up, when E1 pulse is provided. Counter 60 will then enable switch 42, and amplifier 16 will amplify the geophone signal during the next shot point to a higher level. If the amplified signal level during the previous shot point was too high, counter means 58 will enable counter 60 to count down, when E1 pulse is provided. When the amplified geophone signal was neither too high nor too low, no change in counter 60 will occur, and the gain of amplifier 16 will not change. The maximum gain level is 15 when all switches are energized and the least gain level is 1 when only switch 34 is energized. 
     It should be noticed that a sudden burst in the amplitude of the geophone signal level, which is considered as noise, will be clipped by the A/D converter (or even by the amplifier). This is considered as the noise reduction method. 
     The present invention provides a low cost amplification alternative to the floating point amplifier in seismic vibration or similar operations. The present invention may be used on one-for-one basis for each geophone channel, i.e. the number of channel amplifiers equals the number of geophone channels. Variations on this basic approach involve the use of a multiplexer which could be inserted at the input to the A-D converter 24 or at input to the preamplifier. This would permit an increase in the number of channels with relatively little hardware.