ASBOG Geophysics and Seismology 4

Question 1

Vertical electrical sounding (VES) using a Schlumberger array measures subsurface resistivity by varying which survey parameter?

Accepted Answer

Electrode separation (current electrode spacing AB/2)

Answer

In VES, the current electrodes (A and B) are progressively spread apart while potential electrodes remain near center, increasing investigation depth with larger AB/2.

Question 2

Seismic attenuation is commonly described by the quality factor Q. A low Q value indicates:

Accepted Answer

High attenuation and energy loss per cycle

Answer

Q is inversely related to attenuation; low Q means energy is absorbed rapidly, reducing amplitude and high-frequency content over distance.

Question 3

The geothermal gradient in a stable continental craton is typically closest to which value?

Accepted Answer

25–35°C/km

Answer

Stable cratons generally exhibit geothermal gradients of about 20–35°C/km, much lower than tectonically active regions.

Question 4

A geophysicist observes that a seismic reflection has reversed polarity compared to the seafloor reflection. In hydrocarbon exploration, this 'phase reversal' is called a:

Accepted Answer

Polarity anomaly

Answer

A phase reversal (polarity anomaly) occurs when acoustic impedance decreases downward — as at a gas-sand interface — reversing the reflection coefficient sign.

Question 5

In magnetic surveying, the International Geomagnetic Reference Field (IGRF) is subtracted from total-field measurements to produce:

Accepted Answer

The residual magnetic anomaly

Answer

Subtracting the IGRF (the regional main field) from observed total-field data isolates short-wavelength anomalies caused by crustal susceptibility contrasts.

ASBOG - Association of State Boards of Geology Practice Test

ASBOG - Association of State Boards of Geology Practice Test

ASBOG Geophysics and Seismology 4