Method of high-resolution sea bottom prospecting and tuned array of paraboloidal, electroacoustic transducers to carry out such method

• US 5,398,217 A
• Filed: 02/27/1992
• Issued: 03/14/1995
• Est. Priority Date: 09/15/1989
• Status: Expired due to Fees

• Current Assignee: Consiglio Nazionale Delle Ricerche (Repubblica Italiana)
• Original Assignee: Consiglio Nazionale Delle Ricerche (Repubblica Italiana)
• Inventors: D'Ottavi, Enrico, Cannelli, Giovanni B.
• Primary Examiner(s): ELDRED, JOHN W
• Application Number:
  US07/835,901
• Time in Patent Office: 1,111 Days
• Field of Search: 367/147, 367/15, 367/142, 181/110
• US Class Current: 367/147
• CPC Class Codes: G01V 1/006 generating single signals by using more than one generator, e.g. beam steering or focusing arrays G01V1/13, G01V1/3861 take precedence
• CPC Class Codes: G01V 1/157 using spark discharges; using exploding wires
• CPC Class Codes: G01V 1/3808 Seismic data acquisition, e.g. survey design

Abstract

A method of and an apparatus for the high-resolution sea bottom prospecting, in which an electroacoustic paraboloidal transducer is placed into water, the electrodes of which are near the focus of the paraboloid and are connected both to a capacitor bank the capacitance of which may be varied at will and to a supply circuit for the generation of an electroacoustic discharge between said electrodes. As the capacitance varies the delay time between the primary pulse or underwater prospecting pulse and the cavitation pulse is recorded at any discharge, and at the same time dominant frequency of the prospecting signal spectrum is determined. Afterwards capacitance values are set so as to provide both cavitation pulses the contribution of which to the resulting pulse is minimum and a dominant frequency spectrum as broad as possible. The preset capacitance values are assigned to a capacitor array connected each to an electroacoustic paraboloidal transducer dipped in water, the electrodes of which are supplied so as to generate the underwater prospecting pulse.

Claims

1. A method of high-resolution sea-bottom prospecting with electroacoustic paraboloidal transducers having first and second electrodes near their foci comprising the steps of:

a) providing in water an electroacoustic paraboloidal transducer and connecting its electrodes to a predetermined voltage generator and a capacitor bank having capacitances selectable to suitably shift cavitation pulses from one another according to the characteristics of a given experimental diagram;

b) adjusting a capacitance value of the capacitor bank;

c) generating an electric discharge between the electrodes of the transducer produced by electrostatic energy stored in the capacitor bank having said capacitances;

d) producing an acoustic wave pulse signal in the water from the electric discharge in the paraboloidal transducer;

e) supplying the pulse signal to a signal recording means and to a signal spectrum analyzer means;

f) determining a delay time between the pulse signal and a secondary cavitation pulse in the form of a time signal provided by said recording means;

g) determining a dominant frequency in the spectrum of the pulse signal;

h) repeating steps c) to g) for further adjusted capacitance values;

i) determining a mutual relation function of the delay times and the dominant frequencies with respect to the adjusted capacitance values and plotting diagrams reflecting delay time-capacitance and dominant frequency-capacitance;

j) selecting predetermined capacitance values for a predetermined number of paraboloidal transducers to provide the delay times of the secondary cavitation pulses and thereby reduce the contribution of the secondary cavitation pulses to a resulting signal and distribute the dominant frequencies to enlarge the spectrum of the resulting signal.

2. An apparatus for high-resolution sea-bottom prospecting comprising:

a generator for supplying a voltage;

an array of paraboloidal transducers each formed of a hollow open-base paraboloid in a liquid having first and second electrodes disposed near the focus of the paraboloid, the second electrodes of each transducer being connected to one another;

a capacitor bank having capacitors with different capacitances, each capacitance selectable to suitably shift cavitation pulses from one another according to the characteristics of a given experimental diagram, the capacitors each having first and second terminals, the first terminals being operably connected to the first electrodes;

an air-spark generator having a first arc electrode connected to the second electrodes of the transducers and a second arc electrode connected to the second terminals of the capacitors, the air-spark generator being adapted to respond to a change in capacitance value of the capacitor bank and strike an arc between the first and second arc electrodes from electrostatic energy stored in the capacitors to produce an electric discharge in the liquid around each paraboloid, the electric discharge generating an acoustic wave pulse signal;

a signal recorder coupled to each paraboloid for determining a delay time between the pulse signal and a secondary cavitation pulse;

a signal spectrum analyzer coupled to the signal recorder for determining a dominant frequency in the spectrum of the pulse signal; and

means, coupled to the spectrum analyzer, for determining a mutual relations function for the delay times and the dominant frequencies, the determining means being adapted to plot diagrams reflecting delay time-capacitance and dominant frequency-capacitance based on the mutual relation function and to select predetermined capacitance values for a predetermined number of the paraboloidal transducers to provide the delay times of the secondary cavitation pulses and thereby minimize the contribution of the secondary cavitation pulses and distribute the dominant frequencies so that the spectrum can be enlarged.

3. The apparatus of claim 2 further comprising a plurality of resistors, each resistor being connected to a capacitor in the capacitor bank to form an RC-pair having a time constant, each resistor having a resistance value selected so that the time constant of all of the RC-pairs is equal.

4. The apparatus of claim 3 wherein the time constant is 200 ms.

5. The apparatus of claim 2 wherein the array of paraboloidal transducers comprises ten transducers.

6. The apparatus of claim 2 wherein the capacitances of the capacitors have an increasing value from 20 μ

F to 380 μ

F by an arithmetical progression with a rate of 40 μ

F.

7. The apparatus of claim 2 further including a diode bridge connected to each RC-pair, the diode bridge being adapted to provide a potential difference that generates a pulse repetition frequency of one pulse per second.