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

The time for a P-wave to travel from the source to a seismic station on the surface

Answer

The duration of shaking at a seismic recording station during an earthquake

Answer

The time between successive seismic shots in a survey

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

Mw measures the frequency of seismic waves; preferred because it saturates at lower values than Richter

Answer

Mw measures the acceleration of ground shaking; preferred for engineering applications

Answer

Mw is a logarithmic scale of felt intensity reports; preferred because it uses instrumental data

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

Bouguer correction

Answer

Terrain correction

Answer

Latitude correction

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

Magnetic susceptibility of soil and rock minerals

Answer

P-wave velocity of compressional seismic waves

Answer

Gamma radiation emitted by radioactive minerals

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 maximum distance at which refracted arrivals from the lower layer are recorded before the signal attenuates

Answer

The depth to the refracting interface calculated from the refraction travel-time curve

Answer

The distance between adjacent geophones in the receiver array

Question 6

What is the primary difference between body waves and surface waves in seismology?

Accepted Answer

Body waves (P and S waves) travel through Earth's interior; surface waves (Rayleigh and Love waves) travel along the surface and cause most ground damage in earthquakes

Answer

Body waves travel along Earth's surface; surface waves travel through Earth's interior

Answer

Body waves are longitudinal; surface waves are transverse, both traveling at the same velocity

Answer

Body waves are only recorded near the earthquake epicenter; surface waves travel globally