ASBOG Geophysics and Seismology 3 — Questions and Answers
Question 1: The seismic moment M₀ is defined as the product of the shear modulus, the fault rupture area, and which third quantity?
- Hypocenter depth
- Average slip on the fault (Correct answer)
- Epicentral distance
- Peak ground acceleration
Correct answer: Average slip on the fault
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:
- Radioactive decay in the crust
- Variations in the geomagnetic field driven by solar activity (Correct answer)
- Groundwater flow potentials
- Ocean tidal loading
Correct answer: Variations in the geomagnetic field driven by solar activity
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?
- Density of matrix minerals
- Hydrogen content (porosity/fluid) (Correct answer)
- Electrical resistivity
- Acoustic impedance
Correct answer: Hydrogen content (porosity/fluid)
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?
- Airy assumes uniform crustal density with variable crustal thickness; Pratt assumes constant crustal thickness with variable density (Correct answer)
- Airy uses a floating iceberg analogy for lithospheric flexure; Pratt uses elastic plate theory
- Pratt applies only to oceanic crust; Airy applies only to continental crust
- Both models produce identical free-air gravity anomalies
Correct answer: Airy assumes uniform crustal density with variable crustal thickness; Pratt assumes constant crustal thickness with variable density
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:
- Offset travel times to zero-offset travel times (Correct answer)
- Two-way travel time to depth
- Amplitude to reflectivity
- Frequency spectra to time-domain signals
Correct answer: Offset travel times to zero-offset travel times
NMO correction flattens reflection hyperbolas on common midpoint gathers by removing the offset-dependent travel time increase, producing approximate zero-offset traces.
Question 6: Which geophysical method is most appropriate for mapping lateral variations in near-surface soil conductivity for agricultural or environmental applications?
- Deep seismic reflection profiling
- Airborne time-domain EM (TDEM)
- Ground-penetrating radar (GPR)
- Frequency-domain EM terrain conductivity (e.g., EM38) (Correct answer)
Correct answer: Frequency-domain EM terrain conductivity (e.g., EM38)
Instruments like the Geonics EM38 use frequency-domain EM to measure apparent conductivity of the upper 1–2 m of soil rapidly across fields.
Question 7: The focal mechanism (beach ball) of a pure thrust fault is characterized by which pattern?
- Two white quadrants at the top and bottom, two dark quadrants on the sides
- Two dark quadrants at the top and bottom, two white quadrants on the sides (Correct answer)
- A single dark hemisphere and a single white hemisphere divided vertically
- Alternating quadrants rotated 45° compared to a strike-slip fault
Correct answer: Two dark quadrants at the top and bottom, two white quadrants on the sides
Thrust faults show compressional (dark) P-axis quadrants at the top and bottom of the focal sphere and tensional (white) T-axis quadrants on the sides.
ASBOG Geophysics and Seismology 3
The seismic moment M₀ is defined as the product of the shear modulus, the fault rupture area, and which third quantity?