Seismic
The seismic method is by far the most important geophysical technique in terms of expenditures and number of geophysicists involved. Its predominance is due to high accuracy high resolution, and great penetration. Seismic methods are important in groundwater searches and in civil engineering, especially to measure the depth to bedrock in connection with the construction of large buildings, dams, highways, and harbor surveys. Seismic techniques have found little application in direct exploration for minerals where interfaces between different rock types are highly irregular. However, they are useful in locating features, such as buried channels, in which heavy minerals may be accumulated.
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Despite the indirectness of the method – most seismic work results in the mapping of geological structure. Likewise, engineering surveys, mapping of water resources, and other studies requiring accurate knowledge of subsurface structure derive valuable information from seismic data.
Generically the methods of investigations are divided into three categories:
This was the first method used in seismic prospecting. It has a major applicability in shallow investigation, being frequently used in geology and hydrogeology investigations.
The method is based on tracking and analyzing the seismic waves which are refracted on a surface that separates two different mediums with different proprieties of wave velocity. Generating the signal for the measurements does not pose particular problems for small depths, like in case of engineering projects.
This method has developed very much because it is the main method for surveying the oil and gas retaining rock layers. In engineering geophysics this method is used for studding the bedrock properties, detecting possible sinkholes and discontinuities in the earth’s consistency, this being very useful in big construction projects.
The method is based on tracking the waves that are reflected from the limit between two layers with different elastic properties.
In an infinite homogeneous isotropic medium, only P and S waves exist. However, when the medium does not extend to infinity in all directions, other types of waves can be generated. These waves are called surface waves because they are confined to the vicinity of one of the surfaces that bound the medium.
In exploration seismology, the main type of surface wave of importance is the Rayleigh wave, often called ground roll. This wave travels along the surface of the earth and involves a combination of longitudinal and transverse motion with a definite phase relation to each other. The amplitude of this wave motion decreases exponentially with depth. The particle motion is confined to the vertical plane, which includes the direction of propagation of the wave. During the passage of the wave, a particle traverses an elliptical path and the major axis of the ellipse is vertical (near the surface). The direction of particle motion around the ellipse is called retrograde because it is opposite to the more familiar direction of motion of particles in waves on the surface of water.
The surface waves are of big importance in investigating the homogeneity of hard surfaces (concrete platforms, asphalt layers, etc.) because they have a higher energy than the other type of waves.
| Advantages and limitations of seismic-refraction energy source: | ||||||
| Type | Source | Energy into ground | Field portability | Cost | Relative dipth investigation [m] | Aplications |
| Impact | Sledge hammer | Small | Excellent | Low | < 30 | Engineering, environmental |
| Weight drop | Small-medium | Poor | High | 30 - 60 | ||
| Impulsive | Shotgun | Medium | Fair | High | 100 | Engineering |
| Explosives | Small-large | Excellent | Low | No limit | Land, hidrocarbon | |
| Vibratory | Vibroseis | Small-large | Fair | Medium | >1 000 | Land, hidrocarbon |
Estimate Seismic Velociti:
| Earth Materials | Velociti - longitudinal waves [m/s] | |
| Minim | Maxim | |
| Weathered surface material | 305 | 610 |
| Sand and gravel | 468 | 915 |
| Sand (wet) | 610 | 1,830 |
| Clay deposits | 915 | 2,750 |
| Water (depending on temperature and salt content) | 1,430 | 1,680 |
| Sea water | 1,460 | 1,530 |
| Sandstone | 1,830 | 3,970 |
| Shale | 2,750 | 4,270 |
| Chalk | 1,830 | 3,970 |
| Limestone | 2,140 | 6,100 |
| Salt | 4,270 | 5,190 |
| Granite | 4,580 | 5,800 |
| Metamorphic rocks | 3,050 | 7,020 |
| Glacier ice | 12,050 | 12,050 |
| Earth Materials | P wave Velocity (m/s) | S wave Velocity (m/s) |
| Air | 332 | |
| Water | 1400-1500 | |
| Petroleum | 1300-1400 | |
| Steel | 6100 | 3500 |
| Concrete | 3600 | 2000 |
| Granite | 5500-5900 | 2800-3000 |
| Basalt | 6400 | 3200 |
| Sandstone | 1400-4300 | 700-2800 |
| Limestone | 5900-6100 | 2800-3000 |
| Sand (Unsaturated) | 200-1000 | 80-400 |
| Sand (Saturated) | 800-2200 | 320-880 |
| Clay | 1000-2500 | 400-1000 |
| Glacial Till (Saturated) | 1500-2500 | 600-1000 |
