ASBOG Geophysics and Seismology 2 β Questions and Answers
Question 1: Which seismic wave type travels exclusively along the Earth's surface and produces horizontal ground motion perpendicular to the direction of propagation?
- Rayleigh wave
- Love wave (Correct answer)
- P-wave
- S-wave
Correct answer: Love wave
Love waves are surface waves that cause horizontal shear motion perpendicular to the propagation direction and cannot travel through water.
Question 2: The MohoroviΔiΔ discontinuity is defined by a seismic velocity increase to approximately what P-wave speed?
- 5.5β6.0 km/s
- 6.5β7.0 km/s
- 7.8β8.2 km/s (Correct answer)
- 10.0β11.0 km/s
Correct answer: 7.8β8.2 km/s
The Moho marks the crust-mantle boundary where P-wave velocities jump to approximately 7.8β8.2 km/s in the uppermost mantle.
Question 3: In gravity surveying, a Bouguer anomaly is calculated after applying the free-air correction plus a correction for which factor?
- Earth's rotation
- The mass of rock between the station and sea level (Correct answer)
- Tidal variations
- Instrument drift
Correct answer: The mass of rock between the station and sea level
The Bouguer correction removes the gravitational effect of the rock mass between the observation point and the datum (sea level).
Question 4: Which electromagnetic method measures the ratio of induced electric field to the applied magnetic field and is commonly used to map conductive ore bodies?
- Ground-penetrating radar (GPR)
- Magnetotellurics (MT)
- Frequency-domain EM (FDEM) (Correct answer)
- Induced polarization (IP)
Correct answer: Frequency-domain EM (FDEM)
Frequency-domain EM methods inject or induce an alternating magnetic field and measure both the primary and secondary (induced) fields to estimate conductivity.
Question 5: A geologist interprets a seismic reflection profile and notes an 'unconformity' as a truncation of reflectors below a surface. What does this indicate?
- Depositional continuity across time
- An erosional or non-depositional hiatus in the rock record (Correct answer)
- A fault plane parallel to bedding
- A velocity inversion in the subsurface
Correct answer: An erosional or non-depositional hiatus in the rock record
Reflector truncation below a surface on a seismic section indicates an unconformity β a gap in the stratigraphic record due to erosion or non-deposition.
Question 6: In seismic refraction surveys, the crossover distance is defined as the distance at which:
- The refracted wave arrives before the direct wave (Correct answer)
- The surface wave overtakes the body wave
- The S-wave arrival precedes the P-wave
- The geophone spacing equals the depth to the refractor
Correct answer: The refracted wave arrives before the direct wave
Beyond the crossover distance, the head wave (refracted along the faster layer) arrives before the direct wave through the slow upper layer.
Question 7: Which seismological parameter describes the number of times per second a seismograph records ground displacement exceeding a threshold, used to characterize earthquake swarms?
- b-value
- Seismic moment rate
- Recurrence interval
- Event rate (Correct answer)
Correct answer: Event rate
Event rate (events per unit time) is the basic catalog metric used to track the temporal density of seismicity in swarms or aftershock sequences.
ASBOG Geophysics and Seismology 2
Which seismic wave type travels exclusively along the Earth's surface and produces horizontal ground motion perpendicular to the direction of propagation?