BRANTAX » Seismic Prospecting

Seismic Prospecting

by seo

Seismic prospecting 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.

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.

 

SEISMIC PROSPECTING METHODS

 

* APPLICATIONS OF GEOPHYSICAL INVESTIGATIONS:

* Other methods used in geophysical investigations:

 

Seismic Refraction

Refraction 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.

 

Applications of seismic refraction:

 

  • Detection and mapping of geological layers
  • Bedrock depth
  • Identification of fractures in the bedrock
  • Mapping of sand and gravel deposits
  • Properties of soil and rock
  • Stratigraphic profiling or mapping
  • Mapping groundwater depth
  • Deposits filling
  • Determination of the sliding plane

 

Seismic Reflection

The reflection method has developed very much because it is the main method for surveying the oil and gas retaining rock layers. In geophysics engineering  this method is used for studding the bedrock proprieties, detecting possible sinkholes and discontinuities in the earth’s consistency, this being very useful in big construction projects.

Applications of seismic reflection:

 

  • Detection and mapping layers of clay
  • Bedrock depth
  • Depth profiling
  • Identification of fractures in the bedrock
  • The thickness of the layers
  • Mapping of sand and gravel deposits
  • Stratigraphic profiling or mapping
  • Mapping and detection of cave
  • Deposits filling

 

Surface Waves Analysis (SWA)

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.

 

Multi-Channel Analysis of Surface Waves (MASW)

The MASW method allows profiling of shear wave velocity Vs, which is a direct indicator of soil resistance (stiffness) and is commonly used to obtain the bearing capacity.

It is particularly useful where the signal / noise ratio is unfavorable investigations using conventional seismic (reflection and refraction), in urban areas with heavy traffic.

MASW seismic prospecting method allows to determine, besides velocity profile, certain geotechnical parameters such as Young’s modulus, shear modulus, endometrial module, compression module, etc.

 

Down-hole (DH) and Cross-hole (SH)

Down-hole (DH) and cross-hole (SH) seismic methods involve measuring the time required for P and S waves to travel from seismic source, located at the ground surface or in drilling, to the receptors placed in another drill.

These seismic methods assume the existence of one or more wells for the investigations. These seismic investigations may well highlight stratification characteristics and the layers traversed by drilling.

Once processed seismic data can be obtained: the average density, depending on the speed and depth, Poisson’s ratio, modulus of compressibility endometrium, Young’s modulus and modulus of compressibility volume.

 

Horizontal-to-Vertical Spectral Ratio (HVSR)

HVSR (Horizontal-to-Vertical Spectral Ratio) consists in recording the vibrations produced by ambient noise in three directions for a few minutes to determine the fundamental resonance frequency. To apply this method is used a single geophone especially designed for this purpose.

Estimating sediment thickness deposits and bedrock geometry is an essential component in numerous geological and hydrogeological studies. Some studies do not require or do not help pay for the acquisition of high resolution so would benefit from using HVSR method, the low cost with a solid estimate of the thickness of sediments and bedrock depth at a point.

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