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TOP GEOPHYSICAL METHODS USED IN THE INVESTIGATION OF HYDROPOWER PROJECTS

 Electricity Resistivity Tomography (ERT)

Resistivity contrast occurs in the subsurface between, for example, dry and water-bearing sediments, differing rock lithology, and differing weathering histories. Using an electrical apparatus with two current electrodes, one a source and the other a sink, and two potential electrodes, a depth electrical profile can be measured when the electrode spacing is progressively expanded. The field between the electrodes is distributed only near the surface when the spacing of the electrodes is close but the electrical flux flows deeper when the electrodes are further apart. The flux will crowd into the more conductive layers and will rarefy in the more resistive layers. The potential at the surface will reflect these path differences and will provide a data set for which an electrical profile model of the subsurface can be calculated.


Seismic Refraction Tomography (SRT)

Seismic refraction maps contrast in seismic velocity – the speed at which seismic energy travels through soil and rock. This parameter typically correlates well with rock hardness and density, which in turn tend to correlate with changes in lithology, degree of fracturing, water content, and weathering. There are two basic approaches to seismic refraction data analysis: layer-cake and tomographic inversion. The former is the more traditional approach, although tomography has become more popular as faster computers have made it much more feasible than in the past. Especially in the near-surface, it is not always the case that seismic velocities are divided into high contrast, discrete layers. Nor is it the case that velocities are constant horizontally. Conventional layer-cake inversion techniques, such as the delay-time method, assume both and require the geophysicist to provide layer assignments before the data inversion can be completed. Tomography is less constrained in this sense; it does not “think”  in terms of layers,  and it better accommodates horizontal velocity variations. If discrete layering is not apparent in the raw data, the tomographic approach is generally more appropriate. As such, Geometrics’ SeisImager Refraction Analysis software offers both options.

 Multichannel Analysis of Surface Waves (MASW)

 Multichannel Analysis of Surface Waves (MASW) is widely used in geotechnical engineering for the measurement of shear wave velocity, identification of the material properties, martial boundaries and spatial variations of ground, etc. In the MASW method, high-frequency surface waves (Rayleigh waves) are inverted to estimate the S-wave velocity in its propagation path. In a non-uniform, heterogeneous medium, the propagation velocity of a Rayleigh wave is dependent on the wavelength (or frequency) of that wave. The Rayleigh waves with short wavelengths (or high frequencies) will be influenced by material closer to the surface than the Rayleigh waves with longer wavelengths ( or low frequencies), which reflect properties of the deeper material. This dependence of phase velocity on frequency is called dispersion. By generating a wide range of frequencies, MASW surveys use dispersion to produce velocity and frequency (or wavelength) correlations known as dispersion curves. These dispersion curves are then used to calculate shear wave velocities at different depths and thus to delineate different soil layers along with the given profile.


Microtremor Array Measurement  (MAM)

New procedures of passive seismic data acquisition and data processing have improved the accuracy and reliability of these methods. Passive seismic surface wave techniques such as Microtremor Array Measurement (MAM) are used to estimate the shear wave velocity of the subsurface. MAM is able to provide information from a deeper part of the subsurface than the active source method. The fundamental basics of MAM are that a high-frequency surface wave, which propagates with a short wavelength, only stresses material near the exposed surface and thus samples the properties of the shallow, near-surface material. A lower frequency surface wave, which has a longer wavelength stresses material to a greater depth and thus samples the properties of both shallower and deeper material


Microtremor Horizontal to Vertical Spectral Ratio  (HVSR)

Microtremor Horizontal to Vertical Spectral Ratio (HVSR) is a single station method. Most geophysicists consider HVSR to be the passive body wave method whereas others consider it to be a passive surface wave method. Theoretical basics for HVSR is that horizontally traveling shear wave which has the same frequency as that of the fundamental frequency of the site gets trapped and amplified. So, resonance frequencies measured by HVSR are considered to be the most reliable. The degree of amplification mostly depends on the contrast in the acoustic impedance of the seismic surface layer and the basement layer: the higher the contrast larger the amplification factor. Some damping could also take place in the presence of very loose layers within the seismic surface layer.



APPLICATIONS AND LIMITATIONS OF GEOPHYSICAL INVESTIGATION METHODS




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