Seismic parameter measurement method and apparatus

• US 4,455,631 A
• Filed: 07/20/1981
• Issued: 06/19/1984
• Est. Priority Date: 07/20/1981
• Status: Expired due to Fees

• Current Assignee: United States Of America As Represented By The Secretary Of The Army (Government of the United States of America)
• Original Assignee: The United States Of America As Represented By The Secretary Of The Army
• Inventors: San Giovanni, Ronald D.
• Primary Examiner(s): Moskowitz, Nelson
• Assistant Examiner(s): Lobo, Ian
• Application Number:
  US06/284,429
• Time in Patent Office: 1,065 Days
• Field of Search: 367/93, 367/136, 367/94, 367/189, 367/44, 367/135, 340/566, 340/541, 340/565, 340/554, 340/546, 340/563, 340/651, 330/146
• US Class Current: 367/136
• CPC Class Codes: G01V 1/16 Receiving elements for seismic signals; Arrangements or adaptations of receiving elements

Abstract

The apparatus comprises a geophone connected to a circuitry case containing battery, a voltage regulator, a solid state preamplifier and amplifier tuned to respond to seismic signals between 2 and 25 Hz. The amplified seismic signals are rectified and applied to a microammeter connected to efficiently respond to pulsations at the step frequency of walking or crawling personnel. The apparatus can be used to measure seismic noise and to determine soil conditions prior to the emplacement of an intrusion detection system.

Claims

1. Seismic parameter measurement apparatus, comprising;

an electrodynamic geophone comprising a coil and a sharpened portion adapted to be driven into the ground in an area to be tested for the seismic parameters thereof, said coil being connected to an electronic circuitry case by means of an electric cable, said case including a microammeter, an on-off switch and a gain control knob mounted on the top surface thereof, said cable connecting within said case to the input of a solid state bandpass preamplifier comprising three cascaded transistor amplifiers adapted to preamplify seismic signals generated in said coil of sad geophone, said preamplifier comprising a high pass filter, the output of said preamplifier being connected to an emitter-follower circuit adapted to function as a buffer, the output of said emitter follower being applied to an integrated circuit amplifier through a low-pass filter, said integrated circuit amplifier comprising a negative feedback circuit, a diode rectifier bridge circuit connected to the output of said integrated circuit amplifier, said microammeter and a capacitor being connected across the output of said bridge, the ground return circuit of said diode bridge being arranged to provide operating bias voltage for said integrated circuit amplifier, and a battery within said case arranged to provide B+ operating voltage for said preamplifier and integrated circuit amplifier.

2. The apparatus of claim 1 wherein said low pass filter has a cutoff frequency of approximately 25 Hz and wherein said high pass filter has a cutoff frequency of approximately 2 Hz and wherein said geophone is undamped and has a resonant frequency of approximately 12 Hz.

3. A seismic signal discriminator circuit, comprising;

an electrodynamic geophone adapted to be driven into the ground in an area where it is desired to measure the seismic parameters of the ground, said geophone comprising a coil, means to connect said coil to a solid state preamplifier designed to amplify seismic signals generated in said coil, means to connect the output of said preamplifier to the input of an integrated circuit which is connected as an amplifier with negative feedback, said preamplifier and amplifier comprising filter means having a passband of approximately 2 Hz to 25 Hz, said geophone being undamped and having a resonant frequency of approximately 12 Hz, a battery power supply for said preamplifier and amplifier and a zener diode voltage regulator connected to said battery, seismic signals at the output of said amplifier being applied to a bridge type rectifier comprising four diodes, and a microammeter connected to the output of said rectifier.

4. Apparatus designed to measure the seismic parameters of the ground in a given area, comprising;

an electrodynamic geophone implanted in the ground in said area, the coil of said geophone being connected to a solid state preamplifier and amplifier connected in cascade and tuned to amplify seismic signals from 2 to 25 Hz, said geophone being undamped and having a resonant frequency between 2 and 25 Hz, the output of said amplifier being rectified by means of a bridge circuit comprising four diodes, and the rectified seismic signals being applied to a microammeter, the ground return circuit of said microammeter including a capacitor which functions as part of an integrator in conjunction with the forward resistance of said diodes, the time constant of said integrator being chosen to permit said microammeter to respond to step frequency seismic signals without appreciable integration and to integrate higher frequency seismic signals.

5. A method of determining the optimum placement of an array of geophones to surround an area to be protected from intruders on foot comprising the steps of:

implanting an electrodynamic test geophone in the ground in said area to be protected, said geophone being undamped and the coil of said geophone being connected to a solid state preamplifier and amplifier connected in cascade;

amplifying the seismic signals;

rectifying the signals by means of a bridge circuit comprising four diodes;

displaying the seismic signals on a microammeter, the ground return circuit of which includes a capacitor which functions as part of an integrator in conjunction with the forward resistance of said diodes, the time constant of said integrator being chosen to permit said microammeter to respond to step frequency signals without appreciable integration, to determine the presence of seismic noise in the area;

if said seismic noise is below a predetermined value, determining the proper spacing of said geophones in said array of geophones by generating seismic signals at various distances from said test geophone; and

, coordinating the readings on said microammeter with the positions of the signal generator.

6. The method of determining placement of an array of geophones of claim 5 further comprising the steps of;

measuring the amount of seismic noise in the area to be protected by said array of geophones before implanting same;

implanting said geophones; and

, then measuring the soil attenuation and geophone-to-soil coupling.

7. The method as in claim 5 of determining the proper spacing of an array of geophones intended to pick up seismic waves produced by intruders on foot, comprising the further steps of;

generating a signal outward from said geophone;

determining at what distance from said geophone the seismic signals fall below a threshold value below which they are not detectable; and

spacing said geophones in an array of geophones at twice and determined distance.