Annals 47, 1, 2004, 01/07def 93 ANNALS OF GEOPHYSICS, VOL. 47, N. 1, February 2004 Mailing address: Dr. Anatoly N. Kamshilin, Schmidt United Institute of Physics of the Earth, Russian Academy of Sciences, ul. Bolshaya Gruzinskaya 10, D-242, GSP-5, 123995 Moscow, Russia; e-mail: kamshilin@ifz.ru Key words self-oscillations – mechanoelectric self-oscillator – transformations – rocks 1. Introduction It is known that a geological medium which has large resources of energy is a complicated multiphase system in which transformations and interactions of geophysical fields of different ori- gin proceed. There are experimentally detected parametrical, resonance, non-linear properties of rocks. A strong response to weak impacts was observed (Nikolaev and Vereschagina, 1991; Ta- rasov and Tarasova, 1995). The recently detected activation of seismic regime in seismoactive zones after powerful artificial electromagnetic pulses and geomagnetic storms (Tarasov et al., 1999; Tarasov and Tarasova, 2002) should be es- pecially noticed. The investigation of relations between geo- physical fields and the investigation of proper- ties of the lithosphere, as a converter of energy, as well as the possibility to use this knowledge in practice have aroused increasing interest. This paper is dedicated to these problems. 2. Conditions of experiment Figure 1 shows the skeleton diagram of a usual laboratory device for the investigation of mechanoelectric phenomena in rocks. The elec- tric generator (1) is connected to the piezoelec- tric transducer (2), which generates acoustic sig- nal in the sample (3). The energy of the acoustic oscillations is transformed in the sample into the energy of the electric oscillations. Electrodes (4) receive electric signals, which are amplified by the amplifier (5) and directed to the recorder (6). The authors designed the device, the skele- ton diagram of which is shown in fig. 2. The signal from the electrodes (3) is directed to the special electronic unit (4). From the output of the electronic unit the signal is directed to the input of the piezoelectric transducer (1). The set-up of the components of the elec- tronic unit allows for a positive feedback in a closed circuit and provides the conditions of stimulation of the self-oscillations. The self-oscillations arise because of the mechanoelectric transformations in rock sam- ples and the positive feedback and not because of an external forcing oscillator. The authors Self-oscillations in rocks, results of laboratory experiments Anatoly N. Kamshilin (1), Elena N. Volkova (1), Oleg R. Kuzichkin (2) and Mikhail A. Sokolnikov (2) (1) Schmidt United Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, Russia (2) Muromsky Institute, Vladimir Region, Russia Abstract The method of generation of self-oscillations in rocks is developed here. Self-oscillations arise as a result of di- rect and inverse mechanoelectric transformations without an external generator. Laboratory experiments were executed with different samples. A relation between self-oscillation parameters from samples’ humidity and di- rect electrical field applied to samples was detected. 94 Anatoly N. Kamshilin, Elena N. Volkova, Oleg R. Kuzichkin and Mikhail A. Sokolnikov Fig. 1. The skeleton diagram of the laboratory device for the investigation of mechanoelectric phenomena in rocks: 1 – electric generator; 2 – piezoelectric transduc- er; 3 – sample; 4 – electrodes; 5 – amplifier; 6 – recorder. Fig. 2. The skeleton diagram of the laboratory device for the investigation of self-oscillations: 1 – piezoelec- tric transducer; 2 – sample; 3 – electrodes; 4 – elec- tronic unit; 5 – recorder. Fig. 3. The scheme of the self-oscillator (MSO) of the first type (S-E): 1 – sample (box with sand); 2 – meas- uring transducer; 3 – emitting piezoelectric transducer; 4 – electronic unit; 5 – recording device; 6 – additional electrodes; 7 – measuring electrodes; 8 – source of constant voltage. 95 Self-oscillations in rocks, results of laboratory experiments have termed the device the Mechanoelectric Self-Oscillator (MSO). Various materials were used as samples in the experiments: sand, clay, loam, concrete, bricks. In this paper, some results of the exper- iments with sand samples are presented. The sample was made in the form of a plas- tic box (450 × 140 × 180 mm) filled with 19 kg of sand. A piezoelectric transducer was located on the sand surface. The resonant frequency of the transducer was 30 kHz. The system of elec- trodes was located inside the sample’s body. Two types of mechanoelectric self-oscillators were investigated. In the self-oscillator of the first type (fig. 3) a transformation of acoustic oscillations (S) into electric oscillations (E) takes place. In the self-oscillator of the second type (fig. 4) a transformation of electric oscilla- tions into acoustic oscillations takes place. The piezoelectric transducer (3) in the first device functions as an emitter. Measuring electrodes (7) are connected to the input of the electronic unit (4). The signal from the measuring elec- trodes is amplified by the electronic unit and is then directed to the input of the piezoelectric transducer. Additional electrodes (6) are con- nected to the constant voltage source (8). The recording device (5) records two sig- nals: the signal UE ∗ K from the output of the preamplifier of the electronic unit (4) and the signal US from the output of the measuring piezoelectric transducer (2). UE denotes a poten- tial difference between the measuring electrodes (7), K denotes the amplification factor of the preamplifier. US corresponds to the displace- ment of the sample surface in the relative units. The analog-digital converter L-761 of the «L-Card» Russian corporation (14 bits, signal processor AD-2184, Analog Devices Inc.) was utilized as the recording device. Fig. 4. The scheme of the self-oscillator (MSO) of the second type (E-S): 1 – sample (box with sand); 2 – measuring piezoelectric transducer; 3 – piezoelectric transducer-receiver; 4 – electronic unit; 5 – recording de- vice; 6 – emitting electrodes. 96 Anatoly N. Kamshilin, Elena N. Volkova, Oleg R. Kuzichkin and Mikhail A. Sokolnikov Fig. 5a,b. MSO of the first type (E = 0): a) electric (UE) and acoustic (US) signals; b) spectra of the signals UE and US. The distinction between the first and the sec- ond devices (fig. 4) is that the piezoelectric trans- ducer functions as the receiver of acoustic sig- nals. The electric output of the piezoelectric transducer (3) is connected to the input of the electronic unit (4). The output of the electronic unit is connected to the emitting electrodes (6). UE denotes the potential difference between the emitting electrodes (6). 3. Results of the experiments 1) The experimental results for the MSO of the first type (fig. 3, transformation S-E) are shown in figs. 5a,b, 6a,b, 7a,b and 8a,b. Elec- tric and acoustic signals, generated in the sam- ple during self-oscillations, are shown in figs. 5a, 6a, 7a and 8a. The corresponding spectra are shown in figs. 5b, 6b, 7b and 8b. Figures 5a,b and 6a,b illustrate the relation between the parameters of self-oscillations and the value of the external direct electric field E. Figure 5a,b: E = 0, fig. 6a,b: E = 9 v/cm. Change in the phase shift between the sig- nals US and UE by almost 180° and change in the self-oscillations frequency by 680 Hz is caused by the influence of E. It is possible to observe a small decrease of the oscillations amplitude and the Q factor, comparing figs. 5b and 6b. Figures 7a,b and 8a,b show a relation be- tween the parameters of self-oscillations and the sample humidity. Figure 7a,b corresponds to the room-dry sample; fig. 8a,b corresponds to the sample, whose humidity is increased by 3%. The phase shift between UE and US is changed by several tens of degrees. The frequency of self-oscillations is changed by 290 Hz. The amplitude of oscillations is de- creased to half the value of the room dry sample. 2) Results, obtained with the use of the MSO of the second type (fig. 4, transforma- tion E-S) are shown in fig. 9a-d. A self-oscillations mode was generated in the dry sample (fig. 9a). After that 100 ml of water was poured onto the surface of the sam- a b 97 Self-oscillations in rocks, results of laboratory experiments Fig. 6a,b. MSO of the first type (E = 9 v/cm): a) electric (UE) and acoustic (US) signals; b) spectra of the sig- nals UE and US. Fig. 7a,b. MSO of the first type, dry sample: a) electric (UE) and acoustic (US) signals; b) spectra of the sig- nals UE and US. a b a b 98 Anatoly N. Kamshilin, Elena N. Volkova, Oleg R. Kuzichkin and Mikhail A. Sokolnikov Fig. 9a. MSO of the second type. Change in self- oscillations after sample damping. Measurements were executed with 30-40 s intervals: dry sample, electric (UE) and acoustic (US) signals. Fig. 8a,b. MSO of the first type, sample’s humidity is increased by 3%: a) electric (UE) and acoustic (US) sig- nals; b) spectra of the signals UE and US. ple and three more measurements were taken (fig. 9b-d). All measurements were executed with 30-40 s intervals. Comparison of the presented oscillograms traces the process disturbing the self-oscilla- tions mode as a result of sample damping. Os- cillations become unstable, and at the end of the experiment, the amplitudes of oscillations in both channels have decreased by approximate- ly one order of magnitude. 4. Discussion Only part of the obtained results are pre- sented in the paper, other results are now being processed. The presented material allows the following conclusions. 1) Self-oscillations based on direct and in- verse mechanoelectric transformations could be created in rocks. The sample of rock is the fre- quency defining element; frequency of self-os- cillations, Q factor and amplitude are deter- mined by the properties of the sample (object). a b b c d 99 Self-oscillations in rocks, results of laboratory experiments External influence (direct electric field) also changes the parameters of self-oscillations. This result is indirectly confirmed by a relation between seismoelectric transformations and di- rect electric field, as was detected earlier (Cher- niak, 1987). 2) A change in condition of the sample causes a change in phase shift between electric and acoustic signals in the sample. 3) Preliminary results show that mechano- electric self-oscillators of the first and second type have different sensitivity to changes in condition of the samples. 5. Conclusions Investigation of the self-oscillation processes in rocks in general, and processes based on mech- anoelectric transformations in particular, are in- teresting from the point of view of both funda- mental and applied science. There are some pos- sible directions: 1) Development of new methods of rock investigation as mechanoelectrical transformer, as well as development of methods of investi- gation of different physical properties of rocks (material composition, humidity, fluid quality, porosity, anisotropy, etc.). 2) Development of a phenomenological model of the mechanism of earthquake initia- tion under impacts of different origin. 3) Natural modeling will open ways of development of new methods of control con- dition and purposeful influence on rocks and other objects. Acknowledgements The authors are sincerely grateful to Prof. S.D. Vinogradov, Dr. C. Anastasiadis, Dr. A.B. Fig. 9b-d. MSO of the second type. Change in self- oscillations after sample damping. 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