ASBOG Geophysics and Seismology 4 — Questions and Answers
Question 1: Vertical electrical sounding (VES) using a Schlumberger array measures subsurface resistivity by varying which survey parameter?
- Electrode separation (current electrode spacing AB/2) (Correct answer)
- The frequency of injected current
- The depth of electrode burial
- The azimuth of the array
Correct 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.
Question 2: Seismic attenuation is commonly described by the quality factor Q. A low Q value indicates:
- High seismic velocity
- High attenuation and energy loss per cycle (Correct answer)
- Low attenuation — the medium is nearly elastic
- Increased frequency content of the wave
Correct 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.
Question 3: The geothermal gradient in a stable continental craton is typically closest to which value?
- 5–10°C/km
- 25–35°C/km (Correct answer)
- 60–80°C/km
- 100–120°C/km
Correct answer: 25–35°C/km
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:
- Dim spot
- Flat spot
- Bright spot
- Polarity anomaly (Correct answer)
Correct 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.
Question 5: In magnetic surveying, the International Geomagnetic Reference Field (IGRF) is subtracted from total-field measurements to produce:
- The free-air anomaly
- The residual magnetic anomaly (Correct answer)
- The Bouguer anomaly
- The diurnal correction
Correct 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.
Question 6: Which seismic acquisition parameter most directly controls the maximum unaliased frequency that can be recorded?
- Geophone spacing
- Record length
- Sample interval (Δt) (Correct answer)
- Shot interval
Correct 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.
Question 7: Paleomagnetic studies use the concept of 'virtual geomagnetic pole' (VGP). VGP is the position of the geomagnetic pole calculated from:
- The present-day magnetic declination at a single site
- The inclination and declination of the remanent magnetization measured at a single sampling locality (Correct answer)
- The global average of observatory data for a given year
- The tectonic reconstruction of a plate back in time
Correct 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.
ASBOG Geophysics and Seismology 4
Vertical electrical sounding (VES) using a Schlumberger array measures subsurface resistivity by varying which survey parameter?