Geophysical methods

Introduction

Geophysical methods are a set of techniques used to explore the subsurface of the Earth for various applications such as mineral exploration, groundwater assessment, environmental studies, and geological mapping. These methods use different physical properties of the Earth’s subsurface to obtain information about its structure and composition.

Geophysical methods can be broadly classified into three categories:

Seismic Methods

Seismic methods use sound waves to investigate the subsurface of the Earth. The waves are generated using different sources, such as dynamite, vibroseis trucks, or air guns, and their propagation and reflection through different rock layers are recorded using geophones or seismometers. Some examples of seismic methods include:

• Reflection Seismology: This method is used to map the subsurface structure of sedimentary basins and identify oil and gas reserves. The seismic waves are reflected back to the surface by rock interfaces, and the time taken for the waves to travel back is used to calculate the depth and thickness of the layers.
• Surface Wave Analysis: This method is used to assess the engineering properties of the subsurface materials. The surface waves are generated by a hammer or a shaker, and their velocity and attenuation are measured to estimate the shear wave velocity, which is related to the stiffness of the soil or rock.

Electromagnetic Methods

Electromagnetic methods use the variations in electrical and magnetic properties of the subsurface materials to obtain information about their distribution and conductivity. Some examples of electromagnetic methods include:

• Frequency Domain Electromagnetics (FDEM): This method involves the use of a primary electromagnetic field to induce currents in the subsurface. The secondary field created by these currents is measured to determine the electrical conductivity of the materials.

• Time Domain Electromagnetics (TDEM): This method uses a transient electromagnetic field, which is created by a sudden change in current or voltage. The secondary field generated by the induced currents is measured over time to determine the electrical conductivity of the materials.

• Induced Polarization: This method is used to detect and map metallic minerals in the subsurface. The method involves the measurement of the induced chargeability of the minerals in response to an applied electrical field.

Potential Field Methods

Potential field methods use the variations in gravity and magnetic fields to obtain information about the subsurface materials. Some examples of potential field methods include:

• Gravimetry: This method is used to map the distribution of density variations in the subsurface. The method involves the measurement of variations in the Earth’s gravity field using a gravimeter.

• Magnetic Survey: This method is used to map the distribution of magnetic minerals in the subsurface. The method involves the measurement of variations in the Earth’s magnetic field using a magnetometer.