ASBOG Geophysics and Seismology 5 — Questions and Answers
Question 1: The 'shadow zone' in global seismology (approximately 103°–143° epicentral distance) results from:
- Surface wave attenuation at long distances
- Refraction and deflection of P-waves at the core-mantle boundary (Correct answer)
- Absorption of seismic energy by the asthenosphere
- Constructive interference of multiple surface reflections
Correct answer: Refraction and deflection of P-waves at the core-mantle boundary
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?
- 2D marine streamer
- Wide-azimuth 3D land survey (Correct answer)
- Single-fold refraction profile
- Shallow MASW survey
Correct answer: Wide-azimuth 3D land survey
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?
- Depth to the water table
- Chargeability — the ability of rocks to store electrical charge (Correct answer)
- Seismic velocity contrasts
- Magnetic susceptibility variations
Correct answer: Chargeability — the ability of rocks to store electrical charge
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:
- There are 10 times more earthquakes for each unit decrease in magnitude (Correct answer)
- Magnitude and frequency are linearly (not logarithmically) related
- Large earthquakes are as frequent as small ones
- Seismicity decreases by 10× for each additional fault segment
Correct answer: There are 10 times more earthquakes for each unit decrease in magnitude
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?
- Seismic interval velocity
- Thermal conductivity of core samples under in-situ pressure and saturation (Correct answer)
- Formation water salinity
- Radioactive heat production of basement rocks
Correct answer: Thermal conductivity of core samples under in-situ pressure and saturation
Thermal conductivity changes with pressure, temperature, and fluid saturation, so core measurements must replicate in-situ conditions to yield accurate heat flow values.
Question 6: In airborne gravity gradiometry, a gravity gradiometer measures which quantity directly?
- Absolute gravitational acceleration
- Spatial derivatives of the gravitational acceleration tensor (Correct answer)
- Magnetic susceptibility of the underlying rocks
- Elevation above the geoid
Correct answer: Spatial derivatives of the gravitational acceleration tensor
Gravity gradiometers measure the rate of change of gravitational acceleration in space (second derivatives of gravitational potential), providing better resolution of shallow density contrasts.
Question 7: During an earthquake, liquefaction of saturated sandy soils is most likely when:
- Cohesive clays dominate the soil profile
- Strong ground shaking causes pore water pressure to exceed effective stress in loose sands (Correct answer)
- The water table is more than 10 m below the surface
- Bedrock is exposed at the surface
Correct answer: Strong ground shaking causes pore water pressure to exceed effective stress in loose sands
Liquefaction occurs when cyclic seismic loading builds up pore pressure in loose saturated sands until effective stress drops to zero and the soil behaves as a fluid.
ASBOG Geophysics and Seismology 5
The 'shadow zone' in global seismology (approximately 103°–143° epicentral distance) results from: