METHOD OF HIGH-RESOLUTION SEA BOTTOM PROSPECTING AND TUNED ARRAY OF PARABOLOIDAL, ELECTROACOUSTIC TRANSDUCERS TO CARRY OUT SUCH METHOD

Abstract: A method of and an apparatus for the high-resolution sea bottom prospecting, in which an electroacoustic paraboloidal transducer (P1-P10) is placed into water, the electrodes of which are near the focus of the paraboloid and are connected both to a capacitor bank (C1-C10) the capacitance of which may be varied at will and to a supply circuit (G) 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 (C1-C10) connected each to an electroacoustic paraboloidal transducer (P1-P10) dipped in water, the electrodes of which are supplied so as to generate the underwater prospecting pulse.

Description: METHOD OF HIGH-RESOLUTION SEA BOTTOM PROSPECTING AND TU¬ NED ARRAY OF PARABOLOIDAL, ELECTROACOUSTIC TRANSDUCERS TO CARRY OUT SUCH METHOD. This invention relates to a method of sea bottom pros¬ pecting through the generation of submarine acoustic pul¬ ses having time characteristic and spectrum adapted to a high-resolution prospecting, and a tuned array of paraDo- loidal electroacoustic transducers to carry out such method. The described apparatus can be advantageously used for the prospecting of the sea bottom structure and the sub¬ surface of the sea bottom both in civil engineering and archaeological research and in geophysical applications as well. Furthermore, it can be used for the prospecting of lakes or in any natural sheet of water. Generally the apparatus of tnis invention can be advantageously used in all of applications of submarine acoustics in which a high acoustic power with high resolution is needed in the low and medium frequency range (0,1 to 15 kHz) not co¬ vered by the traditional acoustic devices like SONAR. Most of the traditional sources for the sea prospecting have only a few of tne necessary requirements to generate high-resolution prospecting pulses because the frequency band thereof is rather limited and the high frequency harmonic content is poor. Moreover the cavitation pulse associated to the primary pulse further reduces the fea- sibility of a good performance of the sources. The cri¬ teria for ju 2inj of the quality of a seismic sea-source are essentially based upon tne duration of the acoustic pulse, tne frequency content and the repeatability thereof, and tne pri ary/cavitation pulse ratio (see G. ParKes and L. Hatton, The Harime Seismic Source, D. Reidel edition 1985, Dordrecht, Holland). As for the first, two characteristics, i.e. duration of the acoustic pulse and frequency content, the ideal sig- nal would be a socalle "spike" or pulsed signal which can oe represented oy a Dirac delta function, i.e. a very shorr pulse in wnich all frequencies are contained (see . B. Dobrin, Introduction to Geophysical Prospecting, Mc Graw-Hill, 1960, New York). The cavitation pulse (see A. Prosperetti, Physics of Acoustic Cavitation, Rendiconti Societa Italiana Fisica, 1984, Varenna), is a drawback always present in seismic underwater prospecting which, however, can be eliminated in a digital signal process¬ ing, for example, by deconvolution techniques (see E. A. Robinson and S. Treitel, Geopnysical Signal Analysis, Prentice-Hall, Inc., 1980, Englewood Cliffs). Neverthe¬ less a prospecting pulse is desired which is similar as far as possible to the ideal signal oefore any digital processing of the seismic signal. This is the necessary condition to obtain hign-resolution "acoustic images", thus minimizing cos ; , anc waste of time during the signal nroc ssin.:. Metnoo ana aevice cf tne present, invention have some im¬ portant, features allowing a proper approacn to the above mentioned problem. They are based upon some exDerimental examinations carriec out oy tne inventors in unαerwater acoustic tests usin; an already .tnown paraQoloidal elec- :roαcoustic transducer v/hich in th;- ooject of a previous patent of the sarri" inventors (see . ___ . Cannelli, J. O'Ot- tavi and S. Santoboni, Electroacoustic Pulse Source For Hig.n Resolution Seismic Prospecting, Italian Patenr No. 49127-A/84 filed on 23.10.84). The inventors have preferred this type of transducer with suitable modifications as a basic component of the appa- ratus of the present invention because it is provided witn sucn a versatility as to allow various experiments to be carried out under different conditions. Therefore, the electrical and ecnanical parameters can be easily modified in such transducer in order to provide acoustic waves having different characteristics which cannot be provided by other known devices of sea bottom prospecting (see G.B. Cannelli, E. D'Ottavi, ft. Gasperini, First Re¬ sults of Sea Bottom Prospecting With Paraboloidal Acous¬ tic Sources, VI National Meeting About Eartn Geophysics, 198b, CNR Rome). In tne above mentioned paraboloidal transducer the acous¬ tic wave is generated by an electrical high energy di¬ scharge which is provided between two electrodes which ar» placed near the focus of the paraboloidal reflector filled with a suitable liquid and are connected to a ca¬ pacitor bank supplying the electrostatic energy, rfnen the potential difference of the discnarge between the elec¬ trodes is determined, a suitable device allows tne capa¬ citanc e vaiues to o" modified, thus providing tne elec- trostatic energy for t " electrical discnarge and ds er- :r.inin» accordin£jy tne energy of tht- emitted acoustic • energy and the characteristic of the relative frequency spectrum, fiotwitnstandin; its versatility tne aoove men¬ tioned paraboloidal transducer provides some problems which are common to all underwater acoustic sources, i.e. 5 cavitation pulse and limited frequency spectrum. In order to avoid the above mentioned problems the inven¬ tors have devised a metnod and a relative device which are the object of the present invention and utilize the above paraboloidal transducer connected, however, to a 0 suitable circuit and tuned on different values of elec¬ trical capacitance as explained afterwards. The method and the relative device have been conceived on the basis of researches of the inventors who have esta¬ blished that: 5 a) the delay time between the primary and the cavitation pulse is a function of the electrostatic energy. That is to say, once determined the potential difference between the electrodes of the paraboloidal transducer, the delay time increases as the capacitance raises according to a 0 function ^ = f ( C), where ~ t is the delay time and C is the electrical capacitance; b) the dominant frequency of the spectrum of the acoustic pulse (i.e. the harmonic component with the highest am¬ plitude) is also a function of the capacitance according iL~ to a function of the type V = f(C), where " V is the m m dominant frequency and C is the electrical capacitance. In particular, the dominant frequency increases as the electrical capacitance decreases. Taκin2 advantage of the synergic action of the two elec¬ troacoustic effects the inventors have devised and pro¬ vided a system of 10 paraboloidal transducers associated to one another and connected according to a suitaσle electrical circuit so as to De tun ': on different values of selected electrical capacitances. Thus the primary acoustic pulses which can be syncnronizeα with grea" pre- cisioπ are summed constructively to disadvantage of tne cavitation pulse which are properly shifted as the elec¬ trical capacitance varies. Furthermore the primary pulses contribute to the frequency spectrum witn different domi¬ nant frequency, thus causing a wider spectrum than that corresponding to a single transducer. The advantages ensuing from the apparatus of tne present invention are considerable. In fact at any "shot" the apparatus acts so that the resulting acoustic pulse is considerably improved with respect to the pulse of the single transducer above all as far as the following fea¬ tures are concerned: - amplification of the primary pulse; - broadening of the frequency band; - increasing of tne primary/cavi ation signal ratio. Thus the most important requirements are fulfilled to obtain an optimum high resolution sea source. Further features and advantages of the method and the apparatus of the present invention will be oetter ex¬ plained in the following description of a non-limitative preferre embodiment with reference to the annexed drawing, in which: Fig. 1 shows a sequence of the cavitation phenomenon as the electrical capacitance used - - generate the acoustic prospecting wave oy means of a single paraboloidal trans- ducer varies ; Fig. 2 snows quantitatively the electroacoustic phenome¬ non of Fig. 1 by a diagram in which the delay time between the primary pulse and the cavitation pulse is the ordinate and the electrical capacitance is the abscissa; Fig. 3 shows a diagram of the dominant frequency of the prospecting pulse as a function of the electrical capa- citance ; Fig. 4 shows the principle diagram of the supply circuit of th«» 10 praboloidal transducers of the present inven¬ tion; Fig. 5 is an electrical circuit diagram showing the elec¬ trical connection between the various components of the apparatus of the present invention; and Figs. 6a and 6b show a comparison in the time and frequency domain, between the signal generated by a traditional spark array (Fig. 6a) and that of the apparatus of the present invention (Fig. 6b) under the same experimental conditions ( at 5 m from the source and witn electrostatic energy E = 1000 Joule). The principle upon which the present invention is based is illustrated in Figs. 1 to 3 showing the diagrams re¬ sulting from the experimental researches conducted by the inventors in a series of tests using as starting point tne paraooloidal transducer conveniently modified for the underwater prospecting. The modification of the preceding prototype relates to tne striking of the electrical αi- scnarge in the transducer. In the present invention the electrical discharge is provided directly in the water, which can be salt-water or soft-water according to the aπ licatioπ field in whicn tne paraboloidal transducer is dipped, and is triggered off by an -air spark generator acting as switcn as better explained with reference to Fig. 5. In tne aDove mentioned preceding typ*» tne strik¬ ing was provider. Dy • * . third electrode oetweeπ the main 5 electrodes in order to ionize tne insulating liquid con¬ tained in the paraboloidal transducer and neld oy :■ neo- prene diaphragm closing the base. With reference to tne single paradoloidal transducer sui¬ tably modified as mentioned above the acoustic pulses 0 shown in Fig. 1 as a function of the time nave been pro¬ vided. Each signal corresponds to a single "shot" effec¬ ted with tne paraboloidal transducer at a predetermined capacitance value. In Fig. 1 four "shots" are shown in a sequence by way of example, said shots corresponding to 5 the capacitance values: 40 juF, 120 juF, 200 ΛF and 280/uF. The delay time between the primary pulse and the cavita¬ tion pulse is a function of the energy or of only the capacitance once determined the potenti.al difference between electrodes. This phenomenon is better illustrated C, in a quantitative way in the diagram of Fig. 2 snowing as the delay time increases when the capacitance increases. This diagram has oeen used to select the most suitable values of capacitance to be assigned .to the single com¬ ponents of tne apparatus which is the object of the pre- = sent -Invention in order tnat the cavitation pulses are suitably shifted so that th«=> contrioution thereof is n»- •2li"iole with respect to the resulting primary sisnal whicr. is instead provided ~ y the constructive contri¬ bution of tne singi-÷ primary pulses whicr. can o '., sync ronized. i:«. i snows an electroacousti effe t si:.ιul taneous to 'that shown in Figs. 1 and 2. The dominant frequency of the spectrum of the primary pulse, i.e. tne harmonic component having the highest amplitude value is also a function of the capacitance and increases as the capa¬ citance decreases. This means that the acoustic pulse resulting from the simultaneous "shot" of the 10 para¬ boloidal transducers forming the apparatus of the in¬ vention will have a much wider frequency spectrum than that corresponding to the single transducers. The use of both electroacoustic effects to provide an optimum acoustic pulse for the underwater prospecting is illustrated in the diagram of Fig. 4 showing the supply¬ ing of ten paraboloidal transducers indicated at P to P_ belonging to the apparatus of the present invention. 10 Generator G supplies an alternate voltage of 1.773 V rms which allows all capacitors C - C to be charged at the peak voltage of 2,5 kV through diode D and resistors - R . The stored electrostatic energy and then the elec¬ trical capacitances C - C have been selected so as to suitably shift the cavitation pulses from one another ac¬ cording to the characteristic of the diagram of Fig. 2. The high voltase resistors R -R are calculated so as 1 10 to provide the same time constant ( '" T " -T 200 ms ) for all RC-pairs in order to assure a perfect synchronism among the ten paraboloidal elements fo the apparatus which will operate simultaneously when switch I is closed. The al¬ lowed rate of repetition of tne "shots" nas a maximum value cf a pulse per second if a potential difference of 2,5 kV is applied. Such rate can be increased by using a lower potential difference. In such a case the time shift, b w en the primary pulse and the cavitation oulse will • be reduced accordingly, even if it remains different for any paraooloidal transducer. Switch τ operates when i- is closed an air spark generator S which in turn triggers off tne electrical discnarge between the elecrodes of = the single paraboloidal transducers P - P . 3 1 10 Illustrated in Fig. 5 is the detailed electrical diagram showing the electrical connections among tne various com¬ ponents of the apparatus of the present invention. Trans¬ former T supplied with the line voltage at its input 0 terminals supplies at its output two voltages. The first voltage of 1.773 V suDolied at the output rms winding U is rectified by a diode bridge D formed for example of twenty elements 40HF160 and is then used to charge the ten capacitors C. - C. Λ associated to each 1 10 5 paraboloidal transducer P - P , respectively. Trans- 3 ^ 1 10 ducers P - P are formed each by a paraboloidal hollow body having an open base, in the focus of which two elec¬ trodes E - E are installed, respectively, the first one being connected to the respective associated capa- 0 citors C - C , the second one being connected to any second electrode, all of the latter being connected to air-spark generator S. The second voltage of 10 V supplied at the outout rms winding is fed through a second diode bridge and a 5 voltage regulator M to the electrical discharge control logic L. Connected between diode bridge D and regulator . * •' ar= an electrolytic capacitor 2„ for cnopping th* low frequency component and capacitor C in parallel for com- c pensating tne distriouted inductance of the electrolytic 0 capacitor C_ . The ' control can be effected manually by ooeninπ and closing switch I or automatical!'/ by -< sui- table outer programmed pulse source ES. When switcn I is manually closed or when an electrical pulse is supplied oy the outer source ES, control logic L supplies its output signal to power amplifier TR and to current amplifier TR , the output of which is supplied to diode SCR D through decoupling transformer T . Thus dio- de SCR D is conductive and causes capacitor C connec¬ ted to its output to discnarge through the primary of transformer T which generates in the secondary a high voltage charging output capacitor C in parallel with electrodes E and Ξ of air spark generator S which is connected, as mentioned above, to the second electrodes of the ten paraboloidal transducers P - p lπ ' n t ιe a r spark generator S, each time the voltage between the electrodes E and E connected to the secondary of transformer T overcomes a predetermined value, a spark is generated which ionizes the medium and allows genera¬ tor S to act as a closed switch, thus generating an elec¬ trical high energy discharge simultaneously in the ten paraboloidal transducers P - P . Capacitor C has the 1 10 12 function to increase the power of the sparks, while the voltage divider formed of the resistors R and R has 11 12 the function of dividing the voltage between the output of the diode bridge D and the output of spark generator S. Electrical capacitance C - C , the values of which are conveniently selected for example at intervals of 40 uF starting from 20 >uF up to 380 IF, are associated to ten paraboloidal transducers P - P ιn . The selection of the capacitances is such as to shift each other the cavita¬ tion pulses and supply a the same time ten different __ π _ values of dominant frequencies according to tne diagrams oz" Fi-j» 2 and Fi <-,. 3, respectively. The cor esponding resistances ?. - :. have oe β n calculatec so as to provide tne same time constant (for example = 20' ms for any RC-pairs in order to assure a perfect synchronism among the ten paraboloidal elements ? - P of the ,_opa- 1 10 ratus v/hicn v/ill provide simultaneously a pulse each time switch I is closed. The resulting acoustic pulse './ill oe considerably improved with respect to the pulse of the single paraboloidal transducer oecause an amplification of the primary pulse, a broadening of tne frequency oαn and an increasing of the primary/cavitation signal ratio are provided. The versatility of the present apparatus is such as to comply with different prospecting requirements by easy modifications of its electrical parameters. For example, for deep explorations lower frequencies and greater acoustic powers can be used. This involves the use of suitably higher capacitances. On the contrary, for shallow explorations higher frequencies and ooerat- powers provided by means of lower capacitances can be used . In order to illustrate the goodness of this invention, the results of two experiments performed by using the present apparatus and a commercial device are shown ir. Fig. βa and 6b, respectively. It noulc be noted mat the signal generated r.y th~ presen" apparatus (Fig. 6b) exhibi~s in tn*- time domain (lef cia.~raι.ι) a primary pulse Mucr. hi~n - than tnos'- du" to cavitation. ' n the contrary, tne corresponding signa-1 o:' tn- coπmerci- 1 s ar/. ar r~ y (Fi . or;} has a cavitat or. pulse large*" than tne primary one. Also the comparison in tne frequency domain (right diagrams) shows a remarkable difference between the two devices. Only the apparatus of the present invention exhibits suitable intense high frequency components to allow high resolution pros— pectings.

Claims: CLAIMS 1. A method of high-resolution sea-botton prospecting oy electroacoustic paraboloidal transducers, characterized by the following steps: a) providing in water an electroacoustic paraboloidal transducer, the electrodes of which are connected to a predetermined voltage generator and a capacitor bank, the capacitance of which can be varied at will; b) adjusting a capacitance value of the capacitor bank; c) generating an electric discharge between the elec- trodes of tne transducer produced by the electrostatic energy stored in the capacitor bank having said preset capacitance ; d) producing an acoustic wave in the water generated by the electric discharge in the paraboloidal transducer; e) supplying the pulse relative to the generated acoustic wave to suitable signal recording means and to signal spectrum analyzer means; f) determining the delay time between the primary pulse or useful prospecting signal and the cavitation pulse by the seismogram provided by said recording means; g) determining the dominant frequency of the prospecting signal spectrum by the spectrum analyzer means; h) repeating steps c) to g) for further values of capa¬ citance preset in step b); i) determining the mutual relation functions of the delay times and the dominant frequencies itn respect to th*- preset capacitance values and plotting the corresponding diagrams; j) selecting the capacitance values so as both to provide " ) the delays of the cavitation pulses such as to reduce tneir contrioution to the resulting pulse to a minimum and to distribute the dominant frequencies so that the spectrum, can oe as broad as possible. 2 . An apparatus for the hign-resolution sea bottom pros- pecting for carrying out the method of claim 1, com¬ prising an array of paraboloidal transducers each formed of a hollow open-base paraboloid, in or near the focus of which two electrodes for the generation of an electro¬ acoustic pulse are placed, characterized in that the firs electrode of any paraboloidal transducer is con¬ nected to one terminal of a respective capacitor of the array, the capacitance of which is preset according to the method of claim 1 different from that of any other capacitor of the transducers' array, while the second electrodes of any paraboloidal transducer are connected to one another and to one electrode of air spark-gap means adapted to strike an arc between said one electrode and its second electrode which is connected . to the second terminal of any capacitor of the array having said preset capacitance. 3. The apparatus of claim 2, characterized in that any of said capacitors of preset capacitances is associated to a resistance, the value of which is selected so as to pro¬ vide the same time constant for any pair resistance—capa— citance of the paraboloidal transducers. 4. The apparatus of claim 3, characterized in that said time constant is 200 ms . 5. Tne apparatus of claim 2, characterized in that said paraDoloidal transducer array comprises ten elements. - 6. The apparatus of claim 2, 3 and 4, characterized in that said preset capacitances have an increasing value from 20 AIF to 380 JF by an arithmetical progression with a rate of 40 _tF. 7. The apparatus of claim 2, characterized in that the RC-pairs are supplied by a diode bridge of twenty ele¬ ments, five in each side, adapted to provide a potential difference such as to generate a pulse repetition fre¬ quency of one pulse per second. 8. Method of high-resolution sea bottom prospecting es¬ sentially as set forth in the present description and illustrated in the annexed drawing. 9. Apparatus for the high-resolution sea bottom pros¬ pecting essentially as set forth in the present de¬ scription and illustrated in the annexed drawing.

Inventors: CANNELLI GIOVANNI BOSCO, D OTTAVI ENRICO, CANNELLI, GIOVANNI, BOSCO, D'OTTAVI, ENRICO
Applicants: CANNELLI GIOVANNI BOSCO, OTTAVI ENRICO D, CANNELLI, GIOVANNI, BOSCO, D'OTTAVI, ENRICO
Application year: 1990
Main classification: G01V1
IPC codes: G01V1