Apparatus for producing adaptive pilot signals

• US 3,895,343 A
• Filed: 06/17/1974
• Issued: 07/15/1975
• Est. Priority Date: 05/07/1973
• Status: Expired due to Term

• Current Assignee: John B. Farr
• Original Assignee: John B. Farr
• Inventors: Farr, John B.
• Primary Examiner(s): Wilbur, Maynard R.
• Assistant Examiner(s): Moskowitz, N.
• Application Number:
  US05/479,968
• Time in Patent Office: 393 Days
• Field of Search: 340/15.5TA,15.5TD,17 73/71.5,71.6 323/101,106 324/83FE
• US Class Current: 367/190
• CPC Class Codes: G01V 1/04 Details

Abstract

A seismic prospecting system where the frequency and duration of vibratory signals are automatically adjusted to compensate for deleterious variations in soil transmissivity which distort the equivalent reflection pulses. The actual signals injected into the earth are monitored by a near-field detector and compared with a predetermined standard. When the energy of one frequency of the injected signal equals the standard, a new frequency signal is computed and injected until it likewise equals the standard. In like manner, frequencies and durations of remaining segments in the vibratory signal are continuously generated until the spectrum of the equivalent reflection pulse closely approximates the standard spectrum chosen to best delineate the geological objective of the seismic survey.

Claims

1. In a seismic prospecting apparatus of the type having a controllable source which radiates a seismic signal into the earth in accordance with an electrical drive signal, a plurality of seismometers spaced along the surface of the earth at a distance from said source for the purpose of generating electrical signals corresponding to the seismic waves impinging on the surface from subterranean reflecting interfaces, and an apparatus for amplifying and reproducibly recording said electrical seismometer signals, the improvement which comprises:

a. an adjustable signal generation means connected to said controllable source for producing said electrical drive signal whose characteristics may be changed in response to an external electrical control signal;

b. a detecting means located near said source for producing a near-field electrical signal corresponding to said radiated seismic signal after it has been distorted by the earth material adjacent to said source;

c. means for producing a first value proportional to the energy of said near-field electrical signal which is electrically connected to said detecting means;

d. means for storing a predetermined second value proportional to the energy desired in said radiated seismic signal;

e. means electrically connected to said first energy value producing means and said second value storage means for the purpose of producing a comparison signal; and

f. means in circuit relationship with said comparison means and said adjustable signal generator means for producing said external electrical control signal for the purpose of modifying a characteristic of said electrical drive signal in accordance with said comparison signal.

2. A seismic prospecting apparatus as recited in claim 1 further comprising:

a. a filtering means electrically connected between said detecting means and said first energy value producing means for the purpose of attenuating harmonics and other extraneous noises produced by said source; and

b. means electrically connected to said filtering means for reproducibly recording said filtered near-field electrical signal.

3. A seismic prospecting apparatus as recited in claim 1 in which said detecting means comprises a transducer mounted on said source, said transducer being responsive to the motion of that portion of said source in direct contact with the earth.

4. A seismic prospecting apparatus as recited in claim 1 in which said detecting means comprises one of said plurality of seismometers spaced along the surface of the earth at a short distance from said source, said seismometer being positioned such that its electrical output signal essentially represents the radiated seismic signal after said near-surface distortions have occurred.

5. A seismic prospecting apparatus as recited in claim 1 in which said means for producing said first energy value comprises:

a. an analog multiplier circuit electrically connected to said near-field detector for producing an electrical signal proportional to the square of the output of said detector;

b. an analog digital converter circuit electrically connected to said analog multiplier circuit for the purpose of converting said squared electrical signal to equivalent electric signals in the form of digital numbers;

c. a summing register means connected to the output of said converter circuit for accumulating said digital numbers such that each successive digital number is added to the previously existing total of such numbers in the register;

d. a clock means connected to said analog digital converter and said summing regIster means for generating timing pulses to control the time intervals at which said near-field electrical signal is digitized and transferred to said summing register means, as well as the time intervals at which the squared digital number is added to the pre-existing total of said squared numbers; and

e. means connected to said analog digital converter and said summing register means for resetting said analog converter and summing register for the purpose of clearing any remaining digital numbers preparatory to beginning a new energy value computation.

6. A seismic prospecting apparatus as recited in claim 3 in which said means for producing said first energy value comprises:

a. a full wave rectifier circuit electrically connected to said near-field detector for producing an absolute value electrical signal proportional to the absolute value of said near-field electrical signal;

b. an analog-digital converter circuit electrically connected to said full wave rectifier circuit for converting said absolute value electrical signal to electric signals equivalent to the digitized value of said absolute value;

c. a summing register means connected to said analog digital converter for accumulating said digitized values such that each successive absolute value digital number is added to the previously existing total of such numbers in the register;

d. a clock means connected to said analog digital converter and said summing register means for generating timing pulses to control the intervals at which said absolute value electrical signal is digitized, transferred to said summing register means and added to the pre-existing total of said absolute value numbers; and

e. means connected to said analog digital converter and said summing register for resetting and clearing any remaining digital numbers preparatory to beginning a new energy value computation.

7. An improved seismic prospecting apparatus of the type having a controllable vibratory source which radiates a plurality of truncated sinusoids into the earth in response to electrical drive signals, wherein the improvement comprises:

a. means connected to said controllable vibratory source for generating said electric drive signals including circuitry for controlling the frequency, the beginning time and the ending time of each truncated sinusoid in response to external signals;

b. a frequency selection means connected to said signal generator means for establishing a current operating frequency;

c. a signal initiation means connected to said signal generator to supply a start pilot signal for starting said electrical drive signal at said current operating frequency;

d. a detecting means located near said controllable vibratory source for producing a near-field electrical signal corresponding to said radiated sinusoid after it has been distorted by the earth material adjacent to said source;

e. a tracking filter connected to said near-field detector for the purpose of attenuating harmonics and noise at frequencies other than said current operating frequency, said tracking filter also being connected to said signal generation means to receive said current operating frequency;

f. an analog multiplier circuit connected to said tracking filter for producing a signal proportional to the square of the output thereof;

g. an analog digital converter connected to said analog multiplier circuit for converting said squared near-field signal to a digitized electric signal proportional thereto;

h. an energy summing register connected to said analog digital converter to accumulate said squared digital values;

i. a first programmable storage unit for storing predetermined digital values as a function of frequency which represent a desired energy distribution;

j. a first logic unit connected to said first programmable storage unit and to said signal generation means for the purpose of determining an individual predetermined digital value corresponding to said current oPerating frequency;

k. a digital energy comparator connected to said first logic unit, said energy summing register, and said signal generation means for the purpose of generation a stop pilot signal when said cumulative squared digital value at least equals said predetermined digital value.