ASBOG - Association of State Boards of Geology Geophysics and Seismology Questions and Answers 2

Question 1

In seismic reflection profiling, what does a 'two-way travel time' (TWT) measurement represent?

Accepted Answer

The total time for a seismic wave to travel from the source down to a reflector and back to the surface receiver

Answer

Two-way travel time is the elapsed time for a seismic wave to travel from the source to a subsurface reflector and return to the surface, used to calculate the depth to that reflector using seismic velocity.

Question 2

What is the moment magnitude (Mw) scale, and why is it preferred over the Richter scale for large earthquakes?

Accepted Answer

Mw is proportional to the seismic moment (fault area × displacement × rigidity); it does not saturate for large earthquakes and provides a physically meaningful measure of energy release

Answer

The moment magnitude scale is based on the seismic moment (M0 = μAD) and does not saturate like the Richter scale for large earthquakes, making it the standard for characterizing all earthquake sizes globally.

Question 3

In gravity surveys, what correction accounts for the elevation difference between survey stations and a reference datum?

Accepted Answer

Free-air correction

Answer

The free-air correction adjusts gravity measurements for the decrease in gravitational acceleration with elevation above the reference datum (approximately 0.3086 mGal/m), without accounting for the mass of rock between the station and datum.

Question 4

What physical property is measured by electrical resistivity tomography (ERT) surveys in subsurface investigations?

Accepted Answer

Electrical resistance of subsurface materials to current flow

Answer

ERT measures the electrical resistivity (in ohm-meters) of subsurface materials by injecting current into the ground and measuring voltage differences, sensitive to variations in water content, porosity, clay content, and pore fluid chemistry.

Question 5

In a seismic refraction survey, what is the 'critical distance' (crossover distance)?

Accepted Answer

The offset distance at which the refracted wave arrives at the same time as the direct surface wave, beyond which refractions arrive first

Answer

The crossover distance is the source-receiver offset where direct wave and head wave (refracted) travel times are equal; beyond this distance, the refracted wave traveling through the faster lower layer arrives before the direct wave.

ASBOG - Association of State Boards of Geology Practice Test

ASBOG - Association of State Boards of Geology Practice Test

ASBOG - Association of State Boards of Geology Geophysics and Seismology Questions and Answers 2