- 标准号：ASTM D5777-2000(2011)e1
- 发布日期：2000
- 英文题名：Standard Guide for Using the Seismic Refraction Method for Subsurface Investigation
- 英文主题词：Archeological investigations ; Field testing--soil ; Geological investigations ; Geophysical investigations/geophysics ; Hydrogeologic models/investigations ; Mineral exploration ; Petroleum exploration ; Refraction ; Seismic refraction ; Subsurface investigation--soil/rock ; Surface analysis--soil/rock/related materials
- 标准状态：现行
- 发布单位：US-ASTM
- 国际标准分类号：13.080.01
- 载体形态：14P.;A4
- 中国标准分类号：D04
- 国别：美国材料与试验协会
Concepts:
This guide summarizes the equipment, field procedures, and interpretation methods used for the determination of the depth, thickness and the seismic velocity of subsurface soil and rock or engineered materials, using the seismic refraction method.
Measurement of subsurface conditions by the seismic refraction method requires a seismic energy source, trigger cable (or radio link), geophones, geophone cable, and a seismograph (see Fig. 1).
The geophone(s) and the seismic source must be placed in firm contact with the soil or rock. The geophones are usually located in a line, sometimes referred to as a geophone spread. The seismic source may be a sledge hammer, a mechanical device that strikes the ground, or some other type of impulse source. Explosives are used for deeper refractors or special conditions that require greater energy. Geophones convert the ground vibrations into an electrical signal. This electrical signal is recorded and processed by the seismograph. The travel time of the seismic wave (from the source to the geophone) is determined from the seismic wave form. Fig. 2 shows a seismograph record using a single geophone. Fig. 3 shows a seismograph record using twelve geophones.
The seismic energy source generates elastic waves that travel through the soil or rock from the source. When the seismic wave reaches the interface between two materials of different seismic velocities, the waves are refracted according to Snell''s Law (4, 8). When the angle of incidence equals the critical angle at the interface, the refracted wave moves along the interface between two materials, transmitting energy back to the surface (Fig. 1). This interface is referred to as a refractor.
A number of elastic waves are produced by a seismic energy source. Because the compressional P-wave has the highest seismic velocity, it is the first wave to arrive at each geophone (see Fig. 2 and Fig. 3).
The P-wave velocity Vp is dependent upon the bulk modulus, the shear modulus and the density in the following manner (4):
| where: | ||
|---|---|---|
| = | compressional wave velocity, | |
| = | bulk modulus, | |
| = | shear modulus, and | |
| = | density. | |
The arrival of energy from the seismic source at each geophone is recorded by the seismograph (Fig. 3). The travel time (the time it takes for the seismic P-wave to travel from the seismic energy source to the geophone(s)) is determined from each waveform. The unit of time is usually milliseconds (1 ms = 0.001 s).
The travel times are plotted against the distance between the source and the geophone to make a time distance plot. Fig. 4 shows the source and geophone layout and the resulting idealized time distance plot for a horizontal two-layered earth.
The travel time of the seismic wave between the seismic energy source and a geophone(s) is a function of the distance between them, the depth to the refractor and the seismic velocities........