There are many kinds of artificial seismic sources by piezoelectric method: explosives, rammers (rock drills), seismic source guns, etc. Explosives are widely used in the literature of the former Soviet Union, light rock drills are commonly used in Canada, and seismic cannons are also used in domestic experiments besides explosives. The selection of seismic source mainly considers the energy size, frequency spectrum range and the safety of the work site of the excited elastic wave.

The advantage of using explosives is that the energy can be large, so the exploration depth can be improved. The efficiency of explosion (the intensity of piezoelectric signal) will inevitably decrease with the decrease of wave velocity of surrounding rock. Therefore, the selection of explosive quantity must be based on geological conditions. For example, in areas with loose sediments on the ground, the excitation conditions are very unfavorable, not only with a large amount of explosives, but also deeply buried. In addition, the maximum pressure generated by explosion mainly depends on the air content adsorbed in the soil. Especially in unsaturated soil and water, a slight increase in air content will lead to a rapid decrease in explosion pressure with the increase of distance from the source. So, be sure to water it. Explosives need waterproof packaging. In the literature (Bodapov OA, 1992), the relationship between "the effective radius of explosion" (referring to the maximum possible distance between the source that can excite piezoelectric signals and the piezoelectric body) and "the effective receiving radius of piezoelectric oscillation" (referring to the maximum possible distance between the piezoelectric body and the measuring point that may receive piezoelectric signals) is discussed. This determines the amount of explosives and the choice of piezoelectric observation system. The maximum charge used in piezoelectric method in the former Soviet Union is 5 ~ 10 kg (soft rock, VP < 1000 m/s) to1~ 5 kg (VP > 3000 m/s); When working underground (VP > 3000 m/s), the explosive quantity is usually 0. 1 ~ 2 kg. Explosives are high explosives, such as trinitrotoluene, ammonium, trimethylammonium trinitramine, etc. Of course, the choice of explosive quantity is not only related to seismic geological conditions, but also depends on the required exploration depth. In order to solve a series of general survey and detailed investigation tasks within a certain exploration depth, the former Soviet Union formulated a set of Q-value table of standard explosive quantity. The disadvantages of explosives as seismic source are: firstly, the safety of workplace is greatly limited; Second, the electromagnetic radiation generated by the explosion itself is very disturbing. This electromagnetic interference signal is related to the composition of explosives and the breaking of rocks near the earthquake source, and it is concentrated in the initial stage of the expansion and diffusion of gaseous gun smoke dust (charged particles).

Figure 4-5-5 Field Strength and Buried Depth H of Piezoelectric Rod

Using rammer (hammer) as seismic source, the excitation energy is generally small, but the signal-to-noise ratio can also be improved by multiple stacking methods in seismic exploration. 500 ~ 1000 times tamping superposition is commonly used abroad. In addition, the general rammer is bulky and driven by diesel compressor, so it needs to be far away from the work site to reduce the interference of electromagnetic signals.

The source gun is a common source in shallow seismic exploration in China. Its characteristics are good directivity, narrow and high frequency band, strong energy and little interference. The source gun uses the high-pressure gas generated by gunpowder to push the warhead to move and hit the medium, which directly produces a kind of mechanical work (explosive explosion is to convert chemical energy into mechanical energy to do work). The pressure at the warhead can be 25,000 ~ 29,000 N/cm2. It is also possible to tailor bullets with different energy types according to requirements. Because the bullet explodes after penetrating a certain distance in the soil or rock stratum, it will not cause great damage to the ground or the pit wall, so the work site is very safe, and the gun weighs about 14 kg, which is convenient to carry.

(2) Measuring instruments

YYD- 1 field piezoelectric instrument has been developed by Institute of Mineral Deposits, Chinese Academy of Geological Sciences. It consists of analog amplification and digital recording. The analog amplification part includes: eight independent amplification and filtering units, which can suppress interference and amplify weak piezoelectric and seismic signals to meet the needs of analog-to-digital converters by properly selecting filter parameters and amplifier magnification; The hammering or explosion signal identification circuit amplifies and shapes the hammering or explosion signal and outputs a trigger pulse to the recording part; The delay circuit can delay 0 ~ 999 ms (per order 1 ms) and then output the trigger pulse to the recording part. The digital recording part consists of 12-bit 8-channel analog-to-digital conversion unit and portable microcomputer. Eight channels can be preset at will. When only one channel is used, the highest acquisition rate is 100 kHz. When multi-channel works, actual acquisition rate = preset acquisition rate/number of channels. The data buffer is 256 KB, and 128 KB data can be collected at one time (each data occupies two bytes). The data is stored on the disk, and the piezoelectric or seismic waveform curves of all or one channel can be displayed on the screen at 5 12 points per page, and the hard copy of the waveform can be obtained on the printer.

Geophone is used for elastic wave receiving sensor. Piezoelectric signal can be received by a pair of grounding electrodes (long antenna is also used abroad) or by tuning magnetic coil (inductive magnetometer is also used abroad).

(3) Observation system

Observation system (or device arrangement) refers to the mutual position of seismic source and sensor that receive electromagnetic wave field and elastic wave field. Reasonable selection of observation system should consider many factors: working purpose; Geological-geophysical and morphological characteristics and burial conditions of piezoelectric bodies; Focus energy and instrument sensitivity; Surface topography and distribution of tunnels and boreholes. It can be classified according to the combination relationship of explosion profile line, sensor installation line and receiving electrode MN connection line. It can be roughly divided into the following ten categories.

(1) line profile. That is, the receiving electrodes are arranged on the explosion profile, and adjacent Mns can be connected or overlapped (Figure 4-5-6(a)).

Figure 4-5-6 Piezoelectric Observation System

(2) Longitudinal sections with different elevations (tunnels or holes can be used).

(3) The cross section of the straight line, that is, the connecting line of MN is perpendicular to the explosion cross section (Figure 4-5-6(b)).

(4) Parallel longitudinal section. That is, it explodes on one section and is received on one or two adjacent sections, and MN can be adjacent or overlap (Figure 4-5-6(c)).

(5) Parallel cross section. Same as above, but the connecting line of MN is perpendicular to the explosion profile (Figure 4-5-6(d)).

(6) The combination segment of a straight line, that is, the combination of 1 and 2.

(7) Parallel combination profile, namely combination 3 and combination 4.

(8) Inclined longitudinal or transverse section (Figure 4-5-6(e)).

(9) Circular (or arc) longitudinal section, that is, the explosion point or receiving point is fixed in the center, while the receiving point or explosion point moves in the direction (Figure 4-5-6(f)).

(10) integrated observation system. That is to say, the explosion point and the receiving point are alternately arranged between the borehole, the tunnel and the ground.

(4) Data collation of observation results

In the work of piezoelectric method, the piezoelectric instrument simultaneously records the time-varying curves of electric (or magnetic) and elastic wave oscillation at the measuring point. The arrangement of elastic wave records is completely similar to that of seismic methods, and can be arranged into various time-distance curves to obtain wave velocity points (YYD- 1 piezoelectric instruments can be used as seismic traces of shallow seismographs). The time-distance curves of piezoelectric signals at the same measuring point relative to different seismic sources can also be made to obtain the wave velocity.

The oscillation curve of electricity is usually called piezoelectric waveform diagram or piezoelectric diagram. First of all, we should sort out the first arrival time of useful signals (all kinds of industrial interference and piezoelectric oscillation of surrounding rock are background signals) and the arrival time of continuous useful signals; Secondly, we should pay attention to the shape, amplitude, frequency, complexity and duration of the recorded piezoelectric signal. At the same time, in order to compare the piezoelectric signal intensity of each measuring point, it is necessary to sort out the piezoelectric spectrum into a piezoelectric intensity profile or a piezoelectric profile. Take the ratio of the maximum amplitude (volt or millivolt) of piezoelectric signal to the seismic wave intensity (v or mV) of the same measuring point (or fixed point) and draw a graph along the measuring line. Because of the complexity of seismic wave, the intensity of seismic wave usually takes the amplitude of first break wave.

(5) data interpretation and examples

1. Identification of Abnormal Piezoelectric Signals

The principle of piezoelectric signal identification is as follows: ① Because piezoelectric effect is a linear response in seismoelectric effect, its frequency spectrum should be roughly the same as that of elastic wave generated by seismic source. ② The waveform of piezoelectric signal should be similar to sine wave attenuation oscillation with a certain duration. ③ For a single piezoelectric body, the electromagnetic signals from different measuring points must arrive at the same time, and the arrival time will lag with the increase of the distance from the source to the piezoelectric body. In addition, in field observation, when repeated explosions occur under the same conditions, the repeatability of piezoelectric signals is good, which is also the main method to identify anomalies in the field (but when the source is close to the outcrop of smaller piezoelectric bodies, the piezoelectric bodies are destroyed due to repeated explosions, which will also cause the comparability of piezoelectric signals to become worse).

The types of interference to piezoelectric signals are as follows: ① Electromagnetic radiation generated by the explosion itself. Its characteristic is that it appears earlier on the piezoelectric waveform diagram, and often appears as a sharp pulse waveform with high intensity, high frequency and short duration. When the explosion is very close to the piezoelectric body, this interference signal is superimposed on the piezoelectric signal, which often makes the piezoelectric waveform difficult to identify. According to the domestic work experience, the use of the source gun can greatly reduce this interference. ② Various industrial interferences. 50Hz industrial electrical interference can be easily identified and eliminated by digital filtering. Because of its high frequency, the interference signal emitted by the radio station has been confined in the instrument filter circuit, and it has no effect unless the transmitting station is close to the working area. In tunnel work, sometimes the interference of irregular and strong electromagnetic signals caused by various electrical equipment will make the piezoelectric method unable to work. ③ Magnetotelluric interference. Generally, the low-frequency magnetotelluric field does not interfere, but the piezoelectric method cannot work when encountering a magnetic storm. (4) The grounding electrode is disturbed by the change of electrode polarization caused by seismic wave vibration. It can be identified by repeatedly observing or changing the grounding state of the electrode. ⑤ Interference of seismoelectric E effect. Because it is also a seismoelectric response, it is difficult to distinguish. Generally speaking, its signal is weak, but when the seismoelectric body is in the shallow part, it can be strong. As shown in Figure 4-5-7, the first strong signal with a time difference of 4 ~ 15 ms after hammering (20 times superposition) is caused by the electrokinetic E effect at the interface between weathered layer and bedrock with underground 1 ~ 3 m dip angle.

2. Determination of piezoelectric position

Because the propagation speed of elastic wave in medium is completely negligible compared with electromagnetic wave. Therefore, from the multi-channel piezoelectric waveform diagram, the distance between the focus and the piezoelectric surface can be obtained by multiplying the arrival time of the first wave by the propagation speed of the elastic wave. The wave velocity can be obtained by seismic exploration. When the underground (or around the tunnel) is a uniform medium, the problem is simple and the distance is accurate. In the case of multilayer medium, when the piezoelectric body is buried in the first layer, the wave velocity of the first layer is needed; When the piezoelectric body is buried in the nth layer, the average velocity from 1 layer to the nth layer is needed, and the distance accuracy obtained is also very poor.

Figure 4-5-7 Seismic signal received by gravel pavement in Hani area

On the piezoelectric profile, when there is a single piezoelectric body underground, the position of the piezoelectric body can also be determined according to the abnormal peak value of the piezoelectric body, and its tendency can also be determined when the piezoelectric body is slowly inclined. When there are multiple piezoelectric bodies close to each other, it is difficult to determine their positions separately because the time difference between elastic waves and each piezoelectric body is very short, and the observed piezoelectric waveform is a comprehensive effect of superposition. In addition, when the first arrival of the piezoelectric signal is less than the first arrival of the seismic waveform obtained on the geophone at the measuring point, the corresponding piezoelectric intensity should be drawn at the focal point or the midpoint between the focal point and the measuring point according to this difference. When the electric first break is greater than the earthquake first break, the piezoelectric intensity should be drawn on the measuring point. Similar to the dipole device in electrical prospecting, when the source and measuring dipole move along the profile at the same time, there will be two peaks on the piezoelectric profile curve of a single piezoelectric body, which should be paid attention to in interpretation.

Figure 4-5-8 is the original record of piezoelectric testing work in a crystal (timely) mine in Donghai County, Jiangsu Province. Inspired by a source gun at work. Using longitudinal dipole arrangement, the first pair of ground dipoles is received by the magnetic sensor at the same time, and the detector is buried between the second and third dipoles. The crystalline minerals in this area occur in the Quaternary gravel layer in the shape of "chicken nest", with a thickness of 1 ~ 2 m and rich in timely gravel of 30% ~ 40%. The timely vein appears in the lower gneiss, showing many veinlets. On the first observed electrical signal, three anomalies can be clearly divided. YD 1 the small electric oscillation (f≈ 130 Hz) at the head immediately after the initial motion time is related to the shallow stress gravel layer below the source. YD2 anomaly (f≈75Hz) appeared late, but it was still earlier than the first arrival of seismic trace S2, and it should be located in the shallow part of the source, M 1N 1. The anomaly duration of YD 1 and YD2 is short (< < 10 ms), which indicates that the piezoelectric body is small. YD3 anomaly (f≈34 Hz) appears after the first arrival of seismic wave and lasts for a long time. Its source should be located at the lower part of the observation point or far away from the source horizontally. According to the circular arc method, it can be found that the piezoelectric body is about 40 m away from the source. Due to the wide distribution of gravel layers (crystals in some areas), piezoelectric signals are often complex waveforms. Fig. 4-5-9 is the waveform curve and power spectrum analysis result of another measuring point. As can be seen from the figure, its main frequency is about 97 Hz, which is consistent with the main frequency band of elastic waves excited by the source, so it can be determined that it is a piezoelectric signal. In addition, although some electrical signals are weak, they reach all measuring points (different channels) at the same time, which is obviously comparable and can also be judged as piezoelectric signals.

Figure 4-5-8 Original piezoelectric records of a crystal and quartz mine in Jiangsu.

Figure 4-5-9 Piezoelectric waveform curve and power spectrum of a crystal and quartz mine in Jiangsu.

In addition, because the distance between the source and the piezoelectric body is the smallest when the source is located directly above the piezoelectric body, the first arrival of the piezoelectric signal is earlier than when the source is located at other positions. Therefore, according to the position of the minimum value of the time-distance curve of piezoelectric signals with different focal points (the same measuring point), the position of the piezoelectric body on the cross section can also be determined.