The explosive is detonated by the detonator, and the time from the input current to the explosion of the explosive is very short. At most, 2 ms takes the disconnection of the mine pipeline as the explosion timing signal, indicating that the seismic wave has been excited and started to spread.
The amount of explosive, the lithology of explosive medium, the shape of explosive charge and its coupling with explosive medium have important effects on the shape, amplitude and frequency of seismic wave. There is little research in this field. Experience shows that the seismic vibration excited by explosion is a quasi-sinusoidal pulse with rapid attenuation, steep pulse front and high concentration of energy. When exploding in a homogeneous medium, the central symmetric expansion vibration is formed, mainly producing longitudinal waves.
The experimental results show that the amount of explosives affects the characteristics of seismic pulses. Assuming that the charge of spherical explosive is q, the relationship between the duration t of seismic pulse and the charge is as follows:
Seismic wave field and seismic exploration
The relationship between pulse amplitude a and explosive quantity q follows the following formula:
Seismic wave field and seismic exploration
Where k 1 is a variable coefficient. When the explosive amount is small, k 1 accessible1~1.5; K 1 can be reduced to 0.5 ~ 0.2 when the amount of explosives is large. This is because the amount of explosives is small, the damage to rocks is small, and most of the explosion energy is converted into elastic waves; When the amount of explosives increases, most of the energy is lost in the surrounding rock, and the proportion of energy allocated to elastic waves decreases. The relationship between the amplitude and the amount of explosives varies with the earthquake and geological conditions, and should be obtained through experiments in practical work. When exploding in seawater, the average value of k 1 is 0.65.
The relationship between apparent period or main frequency of seismic pulse wave and explosive quantity is as follows:
Seismic wave field and seismic exploration
It can be seen that the larger the dose, the longer the apparent period of the wave and the lower the main frequency.
The shape of explosive charge will also affect the characteristics of excited vibration. Generally speaking, spherical explosives have the best charging effect; The effect of long cylindrical charge is worse, which often produces stronger horizontal transmission interference. In field construction, explosives are usually sealed in cylindrical plastic bags and then detonated in wells several meters to tens of meters deep. In order to concentrate the explosion energy downwards, increase the energy of exciting seismic waves and facilitate construction, people have developed various shaped charges, earth rockets, blasting cables and so on. This greatly improves the effect of exciting seismic waves.
The properties of explosive media have great influence on the excited seismic pulse wave. Experiments show that when excited in loose rock with low speed, energy is absorbed in a large amount, and no good effect can be obtained, resulting in low vibration frequency and weak energy. When excited in hard rock, the vibration frequency is high. The excitation frequency in cement, mudstone or groundwater level is moderate.
The experiment also shows that the coupling relationship between explosion energy and medium affects wave energy, and there are two coupling relationships between explosion energy and medium: geometric coupling and impedance coupling. For cylindrical charge, geometric coupling is the ratio of charge diameter to blast hole diameter multiplied by 100. When the explosive charge diameter is equal to the explosion well diameter, the geometric coupling is 100%. The ratio of the characteristic impedance of explosive (explosive density × explosive initiation speed) to the characteristic impedance of medium (rock density × longitudinal wave velocity in rock) is called impedance coupling. When the ratio is equal to 1, the excited seismic wave energy is the maximum.
In view of the above situation, in order to select suitable lithology for excitation in the low-speed zone and control the frequency range and energy of effective waves, borehole explosion excitation is often used for seismic exploration on land. Especially where the surface wave interference is strong, if the explosion depth is greater than the surface wave wavelength, the surface wave interference can be weakened. In-well explosion has great freedom in choosing lithology and depth. In order to improve the explosion effect, the hole must be filled with water, otherwise the relative intensity of effective wave will be greatly weakened, the acoustic interference will increase and the hole will collapse. In areas where drilling is impossible or difficult, such as desert, paddy field, loess or gravel layer, pit or air explosion is often used. Although the explosion in the pit or in the air can be excited quickly and conveniently, it will produce strong sound waves and surface waves, so certain measures must be taken to reduce the interference in field work methods. Underwater explosion is used for the exploration of rivers, lakes and oceans. Experience has proved that good records can be obtained by excitation in the depth range of about 3 ~ 4 m in water, but attention should be paid not to excitation in underwater silt, because silt absorbs energy very strongly. When the excitation depth is large, the interference wave of repeated impact may be generated, so it is necessary to increase the charge or reduce the charge depth.
In order to improve the effective energy, sometimes the method of combined explosion is adopted, that is, a large charge is divided into several small charges and exploded at the same time. It is generally believed that such a combined source can improve the recording quality of effective waves and produce the following beneficial effects: ① some interference waves can be suppressed by directional effect; (2) due to statistical effect, increase the amplitude ratio of effective wave to random interference caused by explosion; ③ Increase the amplitude ratio of effective wave to random interference background unrelated to explosion, that is, earthquake effect; ④ Increase the vibration frequency. The first few functions are the same as the combined detection which will be discussed later.
In addition to simultaneous explosion, combined explosion also has delayed explosion technology. In this technology, either the small explosive charges placed at different depths explode at different times, or the seismic waves excited by each single gun are recorded on the magnetic tape separately, and then superimposed after applying different delay times to form a plane wavefront with a certain inclination. As a result, the reflections of those interfaces in front of the parallel Apollo are enhanced, while other reflected waves are weakened. This is essentially a directional explosion.