1. Utilization of the first kind of directional characteristics
1) single-component detector and three-component detector
As we all know, when the effective wave passes through the low-speed zone from underground to the ground, the ray direction is almost perpendicular to the ground. Therefore, for longitudinal waves, the direction of particle vibration is almost perpendicular to the surface. At this time, the maximum sensitivity can be obtained by recording the vertical component of ground displacement with vertical geophone. Similarly, using horizontal geophone to record the horizontal component of ground displacement has the greatest sensitivity. Because these two kinds of single-component geophones have different directional characteristics, only vertical geophones are used in longitudinal wave exploration and only horizontal geophones are used in transverse wave exploration. Assuming that the maximum sensitivity of the directional characteristics of vertical geophone is W0 1, the effective sensitivity of waves vibrating in any direction is w 0 1:
W 1=W0 1cosβ (3-6-4)
Where β is the included angle between the particle vibration direction (wave propagation direction) and the ground normal. According to equation (3-6-4), the maximum sensitivity of the vertical geophone is wo 1 in the vertical direction and zero in the horizontal direction. If expressed in polar coordinates, you can get the graph as shown in Figure 3-6-2. For horizontal geophones, there is a similar relationship:
Seismic wave field and seismic exploration
Where W02 is the maximum sensitivity of the horizontal geophone. Direction characteristic diagram as shown in figure 3-6-3.
It can be seen that when longitudinal and shear waves are jointly used to explore or record vibrations in any direction in three-dimensional space, three-component detectors should be used. This geophone has electromechanical conversion devices in X, Y and Z directions.
2) Azimuth observation
There is another form of observation that uses the first directional feature. That is, at an observation point on the ground, three or more vertical geophones (up to 23 at most) are arranged in different directions along the conical surface at the same inclination angle, as shown in Figure 3-6-4, forming a so-called azimuth device. This device records seismic waves in different directions. Its amplitude is related to the displacement direction of ground particles and the axial direction of geophone. Let the included angle (inclination angle) between the geophone axis and the ground be φ, and the included angle between the vibration direction of seismic wave and the ground normal be β. Polar coordinates are still used to express the relationship between recorded amplitude and geophone inclination, which is called azimuth observation pattern, as shown in Figure 3-6-5.
Figure 3-6-2 Direction characteristics of vertical geophone
Figure 3-6-3 Directional Characteristics of Horizontal Geophone
Figure 3-6-4 Seismic observation azimuth device
As can be seen from the figure:
A. When φ = 0, the directionality is illustrated as two tangent circles, which have the greatest sensitivity to horizontal vibration and the same directional characteristics as the horizontal geophone.
B when 0 < φ < β, the pattern becomes asymmetric, with a small circle on the left and a heart-shaped line on the right. In azimuth seismic records, the amplitude and phase change, and the phase change is 180.
Figure 3-6-5 Directional Observation Pattern and Earthquake Records
C. When β = φ, the directivity pattern is asymmetric, with a point on the left and a centerline on the right. The amplitude of only one azimuth in azimuth seismic records is zero; The phase is almost constant.
D when β