Geophysics and Seismology Flashcards
7 cards from real ASBOG practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Geophysics and Seismology flashcards as text
Vertical electrical sounding (VES) using a Schlumberger array measures subsurface resistivity by varying which survey parameter?
Answer: Electrode separation (current electrode spacing AB/2)
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.
Seismic attenuation is commonly described by the quality factor Q. A low Q value indicates:
Answer: High attenuation and energy loss per cycle
Q is inversely related to attenuation; low Q means energy is absorbed rapidly, reducing amplitude and high-frequency content over distance.
The geothermal gradient in a stable continental craton is typically closest to which value?
Answer: 25–35°C/km
Stable cratons generally exhibit geothermal gradients of about 20–35°C/km, much lower than tectonically active regions.
A geophysicist observes that a seismic reflection has reversed polarity compared to the seafloor reflection. In hydrocarbon exploration, this 'phase reversal' is called a:
Answer: Polarity anomaly
A phase reversal (polarity anomaly) occurs when acoustic impedance decreases downward — as at a gas-sand interface — reversing the reflection coefficient sign.
In magnetic surveying, the International Geomagnetic Reference Field (IGRF) is subtracted from total-field measurements to produce:
Answer: The residual magnetic anomaly
Subtracting the IGRF (the regional main field) from observed total-field data isolates short-wavelength anomalies caused by crustal susceptibility contrasts.
Which seismic acquisition parameter most directly controls the maximum unaliased frequency that can be recorded?
Answer: Sample interval (Δt)
The Nyquist frequency (fN = 1/2Δt) is set by the time sampling interval; alias-free recording requires the sample rate to be at least twice the highest signal frequency.
Paleomagnetic studies use the concept of 'virtual geomagnetic pole' (VGP). VGP is the position of the geomagnetic pole calculated from:
Answer: The inclination and declination of the remanent magnetization measured at a single sampling locality
A VGP is computed from site-level paleomagnetic direction (inclination and declination) assuming a geocentric axial dipole, locating the ancient pole position.