ASBOG Geophysics and Seismology 3

Question 1

The seismic moment M₀ is defined as the product of the shear modulus, the fault rupture area, and which third quantity?

Accepted Answer

Average slip on the fault

Answer

M₀ = μ × A × D, where μ is the shear modulus, A is rupture area, and D is the average displacement (slip) on the fault.

Question 2

Telluric current surveys measure naturally occurring electric currents in the Earth. These currents are primarily induced by:

Accepted Answer

Variations in the geomagnetic field driven by solar activity

Answer

Telluric currents are induced in conductive Earth materials by time-varying external geomagnetic fields caused by ionospheric and magnetospheric disturbances.

Question 3

In well-log analysis, the neutron porosity tool primarily responds to which subsurface property?

Accepted Answer

Hydrogen content (porosity/fluid)

Answer

The neutron log measures hydrogen index; because hydrogen is concentrated in pore fluids, it is a direct indicator of porosity.

Question 4

Isostasy can be modeled using either the Airy or Pratt hypothesis. Which statement correctly distinguishes the two?

Accepted Answer

Airy assumes uniform crustal density with variable crustal thickness; Pratt assumes constant crustal thickness with variable density

Answer

Airy isostasy compensates topography with variable crustal roots at uniform density; Pratt compensates with lateral density variations at a fixed compensation depth.

Question 5

During seismic data processing, NMO (Normal Moveout) correction is applied to convert:

Accepted Answer

Offset travel times to zero-offset travel times

Answer

NMO correction flattens reflection hyperbolas on common midpoint gathers by removing the offset-dependent travel time increase, producing approximate zero-offset traces.

ASBOG - Association of State Boards of Geology Practice Test

ASBOG - Association of State Boards of Geology Practice Test

ASBOG Geophysics and Seismology 3