PRRE1003 Lecture 5: Exploration Geophysics

Vocabulary flashcards covering core geophysics concepts, physical properties of rocks, and engineering techniques for mineral, water, and hydrocarbon exploration.

Geophysics

A branch of natural science related to the study of the Earth’s composition and structure that applies the principles of physics to measure the geological makeup of the subsurface.

Density ($\rho$)

The amount of mass per unit of volume, measured in $\text{g/cm}^3$ or $\text{kg/m}^3$, which is used in gravity and seismic methods to evaluate subsurface composition.

Seismic Velocity ($v$)

The speed at which a sound wave propagates through a medium, typically measured in $\text{m/s}$, which varies based on stress, temperature, and rock porosity.

Magnetic Susceptibility

The degree to which a material can be magnetized in an external magnetic field; it is a key rock property for identifying minerals like Magnetite and Pyrrhotite.

Electrical Conductivity ($\sigma$)

A measure of how easily electric current flows through a medium, with SI units expressed in $\text{mS/m}$ (millisiemens per meter).

Gravity Method

A geophysical technique used to estimate subsurface density by measuring variations in the gravitational field over the Earth.

Magnetics

A geophysical method that involves recording the Earth’s magnetic field using a magnetometer to identify magnetically susceptible rock formations.

Electromagnetics (EM)

A technique used to estimate subsurface electrical conductivity by transmitting and measuring electromagnetic fields, often used to find water and conductive minerals.

Seismic Exploration

A primary geophysical method that measures subsurface density and velocity by transmitting sound waves and recording their reflections.

Acoustic Impedance ($Z$)

The physical property defined as the product of density and seismic velocity, represented by the formula $Z = \rho \times v$.

Magnetometer

A specialized instrument used to measure the intensity of the magnetic field, which can be attached to an airplane or carried on the ground.

Geophone

A seismic receiver typically used on land to listen to and record reflected sound waves from the subsurface.

Hydrophone

An active seismic receiver made of piezo polymer used to record reflected sound waves in water environments.

Snell’s Law

A physics principle describing the relationship between the path taken by a sound wave when crossing the boundary between two different media with different velocities.

Reflectivity ($R$)

A coefficient representing the amplitude of a reflected wave, determined by the contrast in acoustic impedances between layers: $R = \frac{Z_2 - Z_1}{Z_2 + Z_1}$.

Vibroseis Truck

A mobile seismic source equipment used in acquisition to generate controlled sound waves by vibrating the ground.

Distributed Acoustic Sensing (DAS)

A technology that utilizes optical fibers and laser pulses to record the acoustic field by measuring backscattered light.

Compressional Waves ($V_p$)

Also known as P-waves, these are longitudinal sound waves that propagate through the earth, typically traveling at higher velocities than shear waves.

Transient Electromagnetics

A process where a transient electromagnetic field is generated in a coil loop and its diffusion into the earth is measured to determine electrical conductivity.

Radiometrics

A geophysical method that uses a scintillator to measure radioactivity from the decay of isotopes such as $U238$, $Th232$, and $K40$.