MASW stands for Multichannel Analysis of Surface Waves. It is used to determine the shear-wave velocity (Vs) profile of the subsurface. The principle of MASW is based on the analysis of surface waves generated by seismic sources and recorded by a linear array of geophones.
Seismic refraction is used to investigate subsurface structures, typically for mapping the depth and velocity of subsurface layers. The principle of seismic refraction is based on the fact that seismic waves change their direction and velocity as they encounter different geological materials with varying elastic properties.
In a seismic downhole survey, a specialized geophone sensor is lowered into a borehole drilled into the Earth's crust. Controlled seismic waves are then generated at the surface, and the resulting reflections and refractions are detected by the downhole geophone. This process allows for highly detailed subsurface imaging, capturing nuances that might be missed by surface-based seismic surveys.
The principle of Ground-Penetrating Radar (GPR) involves emitting electromagnetic waves into the subsurface. These waves interact with subsurface materials, causing reflections based on differences in dielectric properties. The reflected signals are then detected, recorded, and analyzed. The travel times and amplitudes of these signals are used to create images of subsurface features.
EM surveys operate on the fundamental principles of electromagnetic induction. In these surveys, a transmitter coil generates a primary electromagnetic field that induces secondary currents in conductive subsurface materials. Receivers measure the secondary electromagnetic responses, allowing for the creation of detailed conductivity maps.
The principle of Electrical Resistivity Tomography (ERT) involves measuring electrical resistivity variations in the subsurface using electrodes. By injecting current and measuring voltages, ERT creates images that depict changes in resistivity, helping identify subsurface features like water, rocks, or voids.
In a VES, a series of electrodes are arranged in a linear array, with a current electrode and potential electrodes set at varying distances. A controlled electrical current is introduced into the ground, and the resulting potential differences are measured. By analyzing the relationship between current and potential, valuable data on subsurface resistivity distributions are obtained.
In Magnetotelluric surveys, natural electromagnetic fields generated by solar wind interactions with the Earth are measured at the surface. Variations in the frequency-dependent electrical conductivity of subsurface materials induce variations in the measured electric and magnetic fields. Through careful analysis, valuable information about the electrical resistivity distribution beneath the Earth's surface can be extracted.
The principle of magnetometry involves measuring variations in the Earth's magnetic field using magnetometers. Subsurface materials with different magnetic properties create anomalies. By mapping these anomalies, magnetometry helps identify features like archaeological structures or ore bodies.
Gravitational surveys operate on the principle that variations in subsurface density lead to corresponding changes in gravitational attraction. Precise measurements of gravitational acceleration are taken at the Earth's surface, providing a nuanced dataset. This data is then processed and analyzed to create detailed gravity anomaly maps, offering insights into subsurface geological structures.
