What Is Seismic Refraction?

In urban active fault exploration, for areas where surface wave interference is large and seismic reflection wave method is difficult to work, you can try to use the seismic refraction method to detect, and use the time term and difference time interval curve for the original records detected by the refraction method. And finite difference imaging and other methods are used for comprehensive calculation and analysis to explore the application effect of refracted wave method in urban active fault exploration.

Seismic refraction method

Right!

In urban active fault exploration, for areas where surface wave interference is large and seismic reflection wave method is difficult to work, you can try to use the seismic refraction method to detect, and use the time term and difference time interval curve for the original records detected by the refraction method. And finite difference imaging and other methods are used for comprehensive calculation and analysis to explore the application effect of refracted wave method in urban active fault exploration.

Chinese name

Seismic refraction method

Subject

Geography

Applied discipline

geology

Application area

Seismology

Measurement method for deep ocean structure detection using seismic refraction waves. It is one of the most important methods in the study of oceanic crust.

In this type of method, the physical model of seismic wave propagation is shown in Figure 1. When the seismic wave excited from the source is projected onto the refraction interface to a critical angle i, a first wave gliding along the interface R is generated. The condition for the existence of the critical angle i is: i = arc sin (0 / 1). Where 0 and 1 are the propagation speed of the seismic wave in the medium above and below the interface, respectively. Therefore, a refractive interface exists only if 0 <1 is satisfied. The first wave causes the upper medium particle to vibrate and returns to the ground when it glides along the refraction interface R. This wave is called a seismic refraction wave. Received by the detectors (s0, s1, s2, ...). The relationship between the travel time of the refracted wave and the observation distance is called the refracted wave time-distance curve t (x). In the case of a horizontal interface, it is a straight line starting from s0. Seismic refraction waves are not observed between the source O and s0, which is called the blind zone. Seismic refraction waves can only be observed outside the dead zone. Because 2> 1, when the refraction wave at a deeper level leaves the source at a certain distance, it will exceed the shallow refraction wave and reach the detector first. The refracted wave will now appear on the disturbing background, which is called its continuation zone s1s2. Therefore, in the refracted wave method, the time-distance curves of refracted waves at different horizons are straight lines that intersect each other.

In the early days when the refracted wave method was applied, observations were mainly made in the first arrival zone, which is called the refracted wave first arrival method. Later, the technique of applying the reflected wave method was also used to observe refracted waves in the continuation area, which is called the refracted wave contrast method. The refracted wave contrast method is used for deep crustal research. While observing refracted waves, it also uses wide-angle reflected waves (incident angle is greater than the critical angle) for observation, which is called deep seismic sounding. It is an important method for studying the crust and obtaining deep seismic data in the marine area in modern times.

Due to the existence of blind spots, two ships are usually used when conducting refraction wave observations at sea, one boat excites seismic waves and one boat receives seismic waves. A large-capacity combined air gun can be used as the source, but in deep seismic soundings that study the deep structure of the crust and obtain data from the Moho surface, the energy of the combined air gun is often insufficient and explosive sources are still used. For the reception of refracted waves, multiple observations are sometimes made with floating combination receivers, but seismic acoustic buoys are more widely used. This buoy mainly includes a seismic hydrophone, a preamplifier, and an antenna device that amplifies and modulates the output signal and then transmits it. During the operation, the seismic acoustic buoy is dropped to a certain depth under water according to the design requirements, in order to minimize the wave disturbance and the absorption of seismic waves by seawater. After launching, the observation ship (actually the source ship) was driven along the survey line with powerful air guns or explosives at equal intervals in the seawater. Acoustic buoys receive deep refracted waves and wide-angle reflected waves, including the Moho interface, and send them to the observation ship for recording by radio (Figure 2). The operation method of moving the source ship by fixing the buoy position is called the explosive moving point method.

When working on two ships, there are two ways:

One ship is used as the source and one ship is used as the receiver. Generally, the fixed explosion point method is adopted, that is, the mobile observation point (receiving ship);

The two ships are the source ship and the receiving ship, and they run opposite to each other at the same time, and work according to the pre-designed observation system. The greater the penetration depth of the seismic wave, the lower the frequency of the recorded seismic wave, and the lower its resolving power. The deep crustal data collected by deep seismic sounding not only have low-frequency characteristics, but also refracted waves appear in the form of wave groups. For this reason, only wave group comparison can be performed, and the velocity value can be obtained according to the slope of the time-distance curve to judge the nature of the refractive horizon. It gives the interface velocity. In addition to studying the crustal structure, it can also provide a layered basis for the reflection wave method data of the shelf and the slope area and calculate the effective velocity of the reflection wave.

The investigation using seismic refraction method has basically found that the crust of the Ocean Basin in the world has a three-layer structure: layer 1 is a loose sedimentary layer, the seismic wave propagation speed is about 2 km / s, and the average thickness is 0.5 km; 0.63 km / s, with an average thickness of 1.71 ± 0.75 km; Layer 3 speed is 6.69 ± 0.26 km / s, and thickness is 4.86 ± 1.42 km. Below layer 3 is the upper mantle. The average velocity of seismic wave propagation is 8.13 ± 0.24 km / s (see Oceanic crust). Deep seismic sounding is the main method of acquiring deep crustal data in marine areas, and studying deep crustal structure is of great significance for understanding the formation and evolution of sedimentary basins and for oil and gas exploration.