Abstract:
A method and apparatus for construction of a seismic vibrator control signal which comprises manually selecting sweep parameter data values for starting frequency, ending frequency, sweep time and taper time and inputting to an addressable storage medium. Thereafter, determining for each of a plurality of sample points throughout the sweep time, the sweep rate of change per sample point and the accumulated frequency value per sample point, and outputting in real time the digital sweep values for each successive sample point, and subsequently converting and smoothing said successive digital sweep values to an analog control signal of the selected frequency, relative amplitude and duration of sweep length.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to control signal generators and, more particularly, but not by way of limitation, it relates to a digital signal generator for use in controlling a seismic vibrator. 
     2. Description of the Prior Art 
     The prior art includes various forms of analog signal generator which have been utilized in controlling frequency, duration and amplitude of seismic vibrators. In general, the prior equipment has taken the form of analog generation devices for generating the prescribed replica or control signal. Such prior types of generator have not been capable of providing the frequencies necessary to resolve thin layering in geologic events, nor have they been able to provide the requisite sweep linearity to minimize ghosting or correlation background. One known prior teaching that is directed to digital construction of a prescribed control signal is the subject of U.S. Pat. No. 3,460,648 in the name of Waters et al. This patent describes a digital system for providing accurate sweep signals, but it has proven impractical due to the very large drum requirements when utilizing such computational digital equipment. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a sweep generation system utilizing digital microprocessor technology that achieves the sweep parameters necessary to meet the requirements of not only the commonly used sweep spectrums, but also the requirements for the high-resolution seismic surveying now being practiced. 
     The sweep generation system consists of a microprocessor including program memory and random access memory which functions under a real-time interval timer to construct a desired seismic vibrator control signal in accordance with manually input parameters. Input parameters are initially input to the system by thumb wheel switches to select such as starting frequency, ending frequency, sweep time and taper time, and the central processing unit of the microprocessor orders the incrementing of output sweep values through a digital to analog converter and smoothing filter to the seismic vibrator system. Control output from the microprocessor through an external device control logic also allows selected transmit control logic as well as a pseudo-random code output for synchronization of more than a single seismic vibrator. 
     Therefore, it is an object of the present invention to provide a seismic vibrator control signal generator capable of increased frequency range and having starting and ending frequencies that are selectable in 1 hertz increments. 
     It is also an object of the present invention to provide a vibrator control signal that is capable of producing sweep lengths of greater duration and sweep taper times of greater length. 
     It is a further object of the present invention to provide a control signal generator for a seismic vibrator which enables a great reduction in correlation background in the measurable frequency ranges. 
     Finally, it is an object of the present invention to provide a control signal generator which is also capable of encoding and transmitting a pseudo-random code that can be decoded at selected sites for activating one or more sweep vibrators. 
     Other objects and advantages of the invention will be evident from the following detailed description when read in conjunction with the accompanying drawing which illustrate the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a system block diagram of the control signal generator; 
     FIG. 2 is a schematic diagram of the sweep start logic and central processing unit of the present invention; 
     FIG. 3 is a schematic diagram of the read only memory and random access memory in bus interconnection with the input/output stages of the present invention; 
     FIG. 4 is a schematic diagram of the parameter input logic of the present invention; and 
     FIG. 5 is a schematic diagram of the sweep output stages of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The central control of the present system is a microprocessor which of necessity utilizes relatively slower processor speeds; therefore, the system utilizes an algorithm allowing for realtime output of the control signal sweep as it is developed. The particular algorithm as based on the classical vibrational sweep generation formula is: ##EQU1## where, f 1  =starting frequency 
     f 2  =ending frequency 
     Δt=sample time increment 
     T=total sweep time 
     n=number of current sample. 
     Letting K represent accumulated frequency for any sample point, the accumulated frequency will then be equal to the starting frequency plus the sum of the rate of change per unit time from sample n=0 to the current sample. As, ##EQU2## where, ##EQU3## rate of change per unit time. 
     If DF equals the change in frequency per unit time and K is the accumulated frequency at a given sample, the next sample will be 
     
         K.sub.i =K.sub.i-1 +DF                                     (4) 
    
     In order to determine the amplitude of a sample, a table of values corresponding to π radians or one-half cycle of a sine function is first calculated. These values are then spaced at 256 equal increments along the half cycle. The accumulated frequency K which is in radians is represented somewhere in the table of values by a corresponding amplitude. Thus, the correct amplitude can be located in the table by setting J as a number that starts at zero and accumulates the amount of phase advanced from sample to sample. Since the table of values is also disposed in equal radian increments, J will also be the address of the desired amplitude value. Therefore, incrementing of J will adhere to 
     
         J.sub.o =0, 
    
     
         J.sub.i =J.sub.i-1 +K.sub.i                                (5). 
    
     Since the table is only π radians in length and J is an eight-bit binary number representing 256 addresses, a means of determining when π radians (a polarity sign change) has been exceeded in necessary. Thus, as the new K is summed into J, an overflow will occur when π radians have been reached, and the sum of this overflow will be an indicator of sine; even numbers will signify a positive polarity and odd numbers will signify a negative polarity requiring the complement of the table value. 
     The taper function utilized in this particular algorithm is linear, although non-linear sweeps and/or non-linear tapers can be generated with minor changes to the program, if desired. As described herein, the taper rate of change per sample point, i.e., ΔT, is a binary fraction which is the reciprocal of the taper length TL in seconds times the number of samples per second, NSPS. The taper factor TF is then a binary fraction which is a summation of successive ΔT values as shown below. ##EQU4## 
     
         TF.sub.o =0, sweep start, and                              (7) 
    
     
         TF.sub.i =TF.sub.i-1 +ΔT, beginning taper            (8). 
    
     The taper factor TF is applied to all sweep values in succession until the taper lenth has been reached. At this point, TF is maximum allowing full amplitude values to be output. When time is reached for the end taper, the reverse takes place and the ΔT values are subtracted as shown below. 
     
         TF.sub.i =TF.sub.i-1 -ΔT, end taper,                 (9) and 
    
     
         TF.sub.n =0, sweep end.                                    (10) 
    
     The number of sweep values is determined by multiplying the sweep length by 2048. The number 2048 is the number of samples per second and is fixed at this value because it is the minimum number of samples to adequately describe a desirable upper frequency limit of 500 hertz; however, a higher sampling rate may be used thereby to extend the upper frequency limit. The number of sweep values to be effected by the taper factor is also determined by multiplying the taper time by 2048. Implementation of the algorithm is made by the use of integer arithmetic rather than floating point arithmetic in order to achieve minimum calculation, and all input values, counters, and the parameters K, DF, J, TF are represented as integers. 
     FIG. 1 shows the total system block digram, the specific portions of the system shown in FIGS. 2-5 to be discussed in addition. The sweep generation system 10 consists of a microprocessor 12 and peripheral elements. Microprocessor 12 functions under control of interval timer 14 and sweep-start logic 16 as basic clock frequency is input from system clock and reset logic 18 via lead 20. Data access from microprocessor 12 is carried out through address bus 22, data bus 24, and control bus 26. The microprocessor 12 is constituted of chip circuitry, Intel 8080A, to be further described, and the requisite program, amplitude table and pseudo-random code was converted to the Intel Microprocessor Language and burned into erasable program read only memory (EPROM) modules 28. Once programmed, the EPROM modules 28 cannot be changed without erasing and totally reprogramming the system. A selected amount of random access memory 30 (RAM) is included for use as scratchpad storage for parameters, sweep length counter, and taper length counters. Both memory modules, EPROM 28 and RAM 30, are controlled and addressed by the microprocessor 12 over address bus 22 and control bus 24. Input and output logic is also controlled by microprocessor 12 so that any data input or output is synchronized with the basic microprocessor timing. 
     Parameter input switches 32, decimal thumb wheel switches, provide selected binary coded decimal inputs of the sweep parameters, i.e., f 1 , f 2 , sweep time and taper time, via line 34 to parameter input logic 36. An encode switch 38 is also available if the desired operation is to transmit the pseudo-random code prior to outputting the sweep. The external device control logic 42 directed by the data bus 24 and controlled by the control bus 40 provides CLEAR and ADVANCE inputs to the parameter input logic 36 to correctly enter the parameter switch values through data input 44 to data bus 24. The external device control logic 42 also provides an output enabling the transmitter control logic 50 so the pseudo-random code can be transmitted while a SWEEP END output on line 52 is returned to sweep start logic 16. 
     Final data output is available from data bus 24 via line 54 to data output 56 under control of control bus line 58. Output from data output in stage 56 is then applied to a digital to analog converter 50 and the final analog signal on lead 62 is applied through a smoothing filter 64 for output as control signal on the SWEEP OUT line 66. 
     FIG. 2 illustrates in greater detail the microprocessor 12 and attendant bus interconnections along with interval timer 14 and the sweep start logic 16. The microprocessor 12 is of the Intel type utilizing a type 8080A central processing unit 70 in connection with a type 8224 clock generator and driver 72 and a type 8228 bi-directional bus driver and system control 74. The clock generator and driver 72 provide basic system timing as output on line 76, i.e., 2.048 mgHz, and as input to interval timer 14. The interval timer 14 consists of series divider circuits 78, 80, and 82, each type 7490, which provide the basic interval timing pulse output on line 84 at 2048 Hz. The timing signal on lead 84 is then applied to input of gate 86 for input to a flip-flop 88, IC type 7474, when the gate is enabled. 
     Start up of the system is effected either through manual start 90 or remote start 92 through gate 94 as input to a first flip-flop 96, type 7474. Flip-flop 96 then provides output via line 98 enabling gate 86 to pass interval timer pulses to actuate the second flip-flop 88 thereby to provide interrupt pulse output on line 100 to the central processing unit 70. The first flip-flop 96 output on the gate lead 98 is also applied to each of the divider circuits 78, 80 and 82. A SWEEP END input on line 52 is applied through an inverter 102 to first flip-flop 96, such SWED signal being derived from the external device control logic 42. SWEEP TRUE signal as derived from external device control logic 42 is also applied on line 104 through an inverter network to provide SWEEP TRUE validity signal indication. RESET of the system is effected by means of a grounded push-button switch 106 in control of clock generator and driver 72 which provides reset of the control inputs to the central processing unit 70. 
     FIG. 3 illustrates in greater detail the memory and input/output stages of signal generator system 10. Thus, the erasable programmable read only memory 28 includes two eproms 110 and 112 as connected in parallel to address bus 22 and data bus 24. The eprom 110 is a type 8708 while eprom 112 is a type 8704, each receiving supply voltage at pins 24, 19 and 21 with pins 2 and 18 at ground. Chip select input at pin 20 of eprom 110 is controlled by the output of NAND gate 118. The output of NAND 118 is determined by the two inputs; one of which is provided from the address bus 22 through a type 8205 8-to-1 decoder 114 and inverter 120 while the second is provided by the control bus 26 through inverter 120. Chip select input for eprom 112 is then controlled by the address bus 22 through the 8-to-1 decoder 114. 
     The random access memory 30 consists of RAM chips 122 and 124, each type 2112-2, as connected in parallel between address bus 22 and data bus 24. Chip enable input via line 126 is obtained from address bus 22 through 8-to-1 decoder 114, and memory write enable from control bus 26 is applied via line 128 to each of RAMS 122 and 124. Each of RAMS 122 and 124 is energized by +5 volt supply at pin 16 with pin 8 grounded. 
     Data output stage 56 is a type 8212 input/output stage consisting of an 8-bit latch with tri-state output buffers. Thus, input from data bus 24 is strobed out by data output 56 as 8-bit binary data B1-B8 to the digital/analog input 130 (FIG. 5) as will be further described. From control bus 26 I/O write signal is applied to the Device Select (DSI) input. A 1  of the address bus 22 is applied to DS2. 
     The external device control logic 42 is also a type 8212 input/output stage and it is connected similar to the data output stage 56 with DSI input being connected to I/O write of the control bus 22 and A o  of the address bus 22 applied to DS2. Binary input to the input/output stage 42 is then output as specific command pulses for SWEEP TRUE, SWEEP END, TRANSMITTER CONTROL, SWITCH CLEAR and SWITCH ADVANCE. The pseudo-random code is also output from this stage. 
     The data input stage 44 is made up of a third type 8212 input/output stage having binary connection to data bus 24 with input from encode switch 132 as well as binary coded decimal input 134 from parameter input switches 32 (FIG. 4), as will be further described. The input/output stage or data input 44 is connected with the Device Select 1 input connected to the control bus 26 I/O READ line, and the Device Select 2 input is connected to the A 1  line of address bus 22 and central processing unit 70. 
     FIG. 4 illustrates the parameter input switches 32 and parameter input logic 36 in greater detail. The parameter input switches 32 consist of eleven decimal thumb wheel switches that are coded so that a decimal number input is converted into a binary coded decimal output to the parameter input logic 36. Thus, the START frequency f 1  is dialed in three digits by decimal/BCD switches 140, 142 and 144 and the respective BCD output is input to a respective one of the 16-to-1 decoders, i.e., the 2 0  decoder 146, 2 1  decoder 148, 2 2  decoder 150 and 2 3  decoder 152. The BCD outputs from the respective switches 140-144 is via lead groups 34-1, 34-2, and 34-3, with each of the respective leads of the lead group applied to a respective input of the decoders 146-152 as designated. That is, the 1 output of lead group 34-1 is applied to the S1-1 input or pin 0 of decoder 146, the 2 output of lead group 34-1 is applied to the S1-2 input or pin 0 of decoder 148, etc. 
     In like manner, all of the switch inputs are applied in four conductor BCD form to the respective decoders 146-152. The END frequency or f 2  is dialed for input in three decimal digits by thumb wheel switches, 154, 156 and 158 as the BCD outputs on lead groups 34-4, 34-5 and 34-6 are applied to the respective decoders 146-152. The SWEEP LENGTH is dialed in by thumb wheel switches 160, 162 and 164 with BCD output on lead groups 34-7, 34-8 and 34-9; and the TAPER TIME, a two digit number, is dialed in by selection of switches 166 and 168 with BCD output on respective lead groups 34-10 and 34-11. All binary coded switch inputs from lead groups 34-1 through 34-11 are applied to the respective binary decoders 146-152 as designated. The 16-to-1  decoders 146-152 are each IC Type 74150. 
     The decoders 146-152 are each controlled in sequence by a Binary Counter integrated circuit 154, IC Type 7493, in response to switch ADVANCE input on lead 156 and switch CLEAR input on lead 158. The switch ADVANCE and CLEAR inputs are conducted from the external device control logic 42 (FIGS. 1 and 3). BCD output from the decoders 146-152 is then derived from the pin 10 connection to constitute lead group 134 as applied to switch input terminals of the data input circuit 44 (FIG. 3). 
     The output from data output 56 (FIG. 3) is in the form of eight-bit binary, i.e., lead B1-B8 or input group 130 of FIG. 5. The binary input of lead group 130 is then applied to a digital to analog converter 60, a Burr-Brown Type DAC 90 with output present at junction 160. The converter output at junction 160 is then applied through smoothing filters 64 which consist of series active filters 162 and 164. Each of the active filters is a National Type AF 100-2CJ op-amp filter, and sweep output is available on lead 66 for application in energization of the particular vibrator system. 
     In operation, when power is turned on or the system reset is initiated the microprocessor 12 is reset back to the starting address. At this time the sweep start logic 16 is reset with a SWEEP END pulse on line 52. Next the sweep parameters f 1 , f 2 , sweep time and taper time (see FIG. 4) are scanned and input to the system as the decimal thumb wheel switches 140-168 provide their input through decoders 146-152 enabled by binary counter 154 for input on BCD lines 134. Each parameter is determined by setting the correct number into thumb wheel switches 140-168 and they are coded so that the decimal number is converted to a BCD output thereby reducing the number of output lines which must be applied to the 2 0  -2 3  decoders 146-152. The four-bit binary counter 154 then programs the decoders 146-152 to output the desired switch input as it is applied on input lines 134 to data input 44 (FIGS. 1 and 3). The counter 154 is reset and advanced by command of the microprocessor 12 through external control logic 42, i.e., flip-flops and output buffers (FIG. 3), and the decoder outputs 134 are then applied through input/output device 44 for gating onto data bus 24. 
     Thus, the data input 44 (FIG. 3) samples each switch 140-144 for f 1  in sequence, and the binary coded decimal values are buffered from the flip-flops onto the data bus 24 and then converted to a binary number and stored in RAM memory 30 (FIG. 1) until needed to calculate the parameter constants. In turn, the data input logic 44 samples the switches of f 2  (end frequency switches 154-158), sweep length or switches 160-164, and taper time switches 166 and 168. 
     With input of the operating parameters, the microprocessor 12 and associated memory circuitry undertakes to determine the sweep direction up or down, and the sweep rate DF is calculated. With input of the starting frequency f 1  and the calculated DF, the initial value of K is then calculated. J is then set equal to 0 and the taper factor is determined. Thereafter, the sweep length T is multiplied by the number of samples per second, i.e., 2048, to determine the total number of samples, and the taper time is also multiplied by 2048 to determine the number of values needed for the taper function at the start and end of the sweep. From these determined values, three counters are set to memorize beginning taper, end taper, and remaining sweep time. When all constants have been calculated and counter set, the processor goes into a halt state awaiting a sweep start signal to initiate the output of the sweep. 
     Referring to FIG. 2, when a sweep start pulse, either manual or remote, is received by the gate 94 of the start logic, a flip-flop 96 is set. This setting enables gate 86 to pass pulses from the interval timer 14 on lead 84 thereby to set flip-flop 88 which provides interrupt pulse to the central processing unit 70 of microprocessor 12. When the CPU 70 has recognized the interrupt, it resets flip-flop 88 to allow the next timer pulse to interrupt at the correct time interval. The interval timer 14 is controlled by a high frequency crystal and counted down to the desired frequency of 2048 Hertz by the divider circuit 78-82. 
     When the first interrupt occurs, the microprocessor 12 moves from its halt state and checks the status of the encode switch 132 (see FIG. 3). If the encode switch 132 is in encode position, the external transmitter is turned on and, after an appropriate delay, the pseudo-random code is output. The transmitter is then turned off. To allow the decoder at a remote site to recognize the code, another delay is taken before starting to output the first sweep value. After the first interrupt, the encode switch status is bypassed, and if the encode switch indication is false, the microprocessor 12 moves from its halt state and immediately begins to output a sweep value. The taper factor is applied to the sweep value addressed by the number J. This value is output through the output logic 56 as D/A input 130 to a digital-to-analog converter 60 (FIG. 5). 
     The output from D/A converter 60 is then applied from output junction 160 to a smooth filter or op-amp filter 162 to remove the voltage steps. Next, the appropriate counter of microprocessor 12 is decremented by one count and, (a) a new taper factor is formed by summing ΔT into the TF constant, (b) a new table address is formed by adding the previous J with K. K is then updated for the next value by adding K with DF. With the constants updated, the microprocessor 12 halts to wait for the next interval timer pulse, and this procedure is repeated 2048 times a second until all counters are decremented to 0. At this time, the flip-flop 96 (FIG. 2) is reset with a sweep end signal on lead 52 thereby disabling the interval timer 14 output. The microprocessor 12 is then reset to the starting address where it resamples all input parameters, recalculates all constants and counters, halts and waits for its next operating start. 
     As an example, one form of program that is suitable for carrying out the present invention may be programmed in standard Intel microprocessor language, as follows. ##SPC1## ##SPC2## 
     The pseudo-random code values and sweep table values constitute 128 and 256 addressable values, respectively, as addressed consecutively. In abreviated form, the program is as follows: ##SPC3##