GEOLOGY FINALS PART 5

Geophysical Method

the study of the Earth’s subsurface using physical forces based on the following like magnetism, electricity, or sound waves. These kinds of techniques are fast and non-invasive, and also covering large areas efficiently. However, the results are indirect and require drilling for confirmation.

Geophysical Method

Magnetic Survey

Electromagnetic (EM) Survey

Electrical Resistivity

Induced Polarisation (IP)

Seismic Survey

Radiometric Survey

Magnetic Survey

• It usually detects naturally magnetic minerals like magnetite and pyrrhotite.

• It helps to identify such metal deposits (e.g., copper, lead, zinc).

• It was conducted using a magnetometer, either handheld or attached to aircraft.

• Used in archaeology and detecting buried metal structures.

Electromagnetic (EM) Survey

• This uses a transmitter and receiver to send and detect to electromagnetic fields.

• it measures on how the ground conducts electricity — also helps to find metallic minerals, groundwater, or buried objects.

• Moreover, useful for detecting minerals like cobalt, and used in renewable energy batteries.

Electrical Resistivity

• it measures how much the ground tries to resists electric current.

• Conductive materials = low resistivity.

• Used to map subsurface layers, find ore bodies, groundwater, and contaminated zones.

• It can also create 2D or 3D images of the underground.

Induced Polarisation (IP)

• Measures how long rocks hold an electric charge after the current is turned off.

• Often done together with resistivity surveys.

• Detects sulphide minerals like (gold, silver, copper) and clay zones.

• Also useful in our environmental studies, like mapping contamination

Seismic Survey

• it usually works like an ultrasound for the Earth.

• also creates seismic waves using vibrations or controlled impacts.

• it measures how the waves reflect or refract underground to map the rock layers and its structures.

• Hence, it is commonly used in oil, gas, and mineral exploration.

• Safe for both land and marine environments

Radiometric Survey

• it’s the one who measures the natural radiation from elements like uranium, thorium, and potassium.

• Usually done by on how low-flying aircraft to map large regions.

• Also helps to identify rock types and altered zones from its metal-rich fluids.

• Therefore its completely safe since the instruments only detect, not emit, radiation.

Seismic and Electrical Methods

are two important geophysical techniques used to investigate subsurface conditions without drilling or excavation.

Seismic Method

measure how seismic wave (vibrations) travel through the ground. These waves are generated by a small explosion, hammer strike, or mechanical vibrator, and sensors called geophones record the time it takes for the waves to reach them.

Types

Seismic Refraction

Seismic Reflection

Seismic Refraction

determine layer depth by measuring the travel time of waves refracted along layer boundaries.

Seismic Reflection

records waves that bounce back from subsurface layers.

ELECTRICAL METHOD

measures the resistivity of the ground by passing an electric current into the earth through electrodes and measuring the resulting potential difference. Different materials have different resistivity

COMMON TECHNIQUES

Vertical Electrical Sounding (VES)

Electrical Resistivity Tomography (ERT)

Vertical Electrical Sounding (VES)

determines vertical variation of resistivity with depth.

Electrical Resistivity Tomography (ERT)

creates 2D or 3D images of subsurface resistivity.