ASBOG Geophysics and Seismology 5

Question 1

The 'shadow zone' in global seismology (approximately 103°–143° epicentral distance) results from:

Accepted Answer

Refraction and deflection of P-waves at the core-mantle boundary

Answer

P-waves refract downward and away from the receiver zone when they enter the liquid outer core, creating a shadow zone with no direct P-wave arrivals.

Question 2

Which type of seismic survey layout is best suited for imaging steeply dipping fault planes in three dimensions?

Accepted Answer

Wide-azimuth 3D land survey

Answer

Wide-azimuth 3D surveys illuminate dipping structures from multiple azimuths, providing superior imaging of steeply dipping faults and complex geometries.

Question 3

Induced polarization (IP) surveys detect which subsurface characteristic that resistivity surveys alone cannot distinguish?

Accepted Answer

Chargeability — the ability of rocks to store electrical charge

Answer

IP measures chargeability, a property related to metallic minerals, clay content, or ionic diffusion in pore fluids that standard DC resistivity cannot differentiate.

Question 4

The Gutenberg-Richter relation (log N = a − bM) states that for most seismic regions the b-value is close to 1.0, meaning:

Accepted Answer

There are 10 times more earthquakes for each unit decrease in magnitude

Answer

A b-value of ~1 means the number of earthquakes increases by a factor of 10 for every one-unit decrease in magnitude.

Question 5

Heat flow (Q) is calculated as the product of thermal conductivity (k) and the vertical temperature gradient (dT/dz). In a borehole study, which measurement must be made in situ to accurately determine Q?

Accepted Answer

Thermal conductivity of core samples under in-situ pressure and saturation

Answer

Thermal conductivity changes with pressure, temperature, and fluid saturation, so core measurements must replicate in-situ conditions to yield accurate heat flow values.

ASBOG - Association of State Boards of Geology Practice Test

ASBOG - Association of State Boards of Geology Practice Test

ASBOG Geophysics and Seismology 5