ASBOG - Association of State Boards of Geology Geophysics and Seismology Questions and Answers 2 — Questions and Answers
Question 1: In seismic reflection profiling, what does a 'two-way travel time' (TWT) measurement represent?
- The time for a P-wave to travel from the source to a seismic station on the surface
- The total time for a seismic wave to travel from the source down to a reflector and back to the surface receiver (Correct answer)
- The duration of shaking at a seismic recording station during an earthquake
- The time between successive seismic shots in a survey
Correct answer: The total time for a seismic wave to travel from the source down to a reflector and back to the surface receiver
Two-way travel time is the elapsed time for a seismic wave to travel from the source to a subsurface reflector and return to the surface, used to calculate the depth to that reflector using seismic velocity.
In reflection seismology, seismic energy generated at the surface travels downward, reflects off impedance contrasts (changes in velocity × density) at subsurface interfaces, and returns to surface receivers (geophones or hydrophones). The TWT is measured from the shot moment to arrival of the reflected pulse. Depth to the reflector is calculated as depth = (TWT × velocity) / 2. Velocity analysis (NMO correction) is used to determine the appropriate interval velocities for converting TWT profiles to depth sections.
Question 2: What is the moment magnitude (Mw) scale, and why is it preferred over the Richter scale for large earthquakes?
- Mw measures the frequency of seismic waves; preferred because it saturates at lower values than Richter
- Mw is proportional to the seismic moment (fault area × displacement × rigidity); it does not saturate for large earthquakes and provides a physically meaningful measure of energy release (Correct answer)
- Mw measures the acceleration of ground shaking; preferred for engineering applications
- Mw is a logarithmic scale of felt intensity reports; preferred because it uses instrumental data
Correct answer: Mw is proportional to the seismic moment (fault area × displacement × rigidity); it does not saturate for large earthquakes and provides a physically meaningful measure of energy release
The moment magnitude scale is based on the seismic moment (M0 = μAD) and does not saturate like the Richter scale for large earthquakes, making it the standard for characterizing all earthquake sizes globally.
The moment magnitude (Mw) scale, developed by Hanks and Kanamori (1979), is calculated from the seismic moment M0 = μ × A × D, where μ is the shear modulus, A is the fault rupture area, and D is the average slip. The relationship is Mw = (2/3)log(M0) − 6.07. The Richter (local) magnitude ML saturates above about Mw 6.5–7 because it is based on peak wave amplitudes at a fixed distance and period. Mw correctly characterizes all earthquakes from microseisms to the largest megathrust events (Mw 9+) without saturation.
Question 3: In gravity surveys, what correction accounts for the elevation difference between survey stations and a reference datum?
- Free-air correction (Correct answer)
- Bouguer correction
- Terrain correction
- Latitude correction
Correct answer: Free-air correction
The free-air correction adjusts gravity measurements for the decrease in gravitational acceleration with elevation above the reference datum (approximately 0.3086 mGal/m), without accounting for the mass of rock between the station and datum.
Gravity data reduction involves several corrections to isolate the effect of subsurface density variations. The free-air correction (FAC) accounts for the reduction in gravity with height above sea level due to increased distance from Earth's center: FAC = +0.3086h mGal (where h is height in meters). The Bouguer correction additionally removes the gravitational effect of the rock mass between the station and sea level (assuming an infinite slab of average density 2.67 g/cm3). Together they produce the Bouguer anomaly used to interpret subsurface geology.
Question 4: What physical property is measured by electrical resistivity tomography (ERT) surveys in subsurface investigations?
- Magnetic susceptibility of soil and rock minerals
- P-wave velocity of compressional seismic waves
- Electrical resistance of subsurface materials to current flow (Correct answer)
- Gamma radiation emitted by radioactive minerals
Correct answer: Electrical resistance of subsurface materials to current flow
ERT measures the electrical resistivity (in ohm-meters) of subsurface materials by injecting current into the ground and measuring voltage differences, sensitive to variations in water content, porosity, clay content, and pore fluid chemistry.
Electrical resistivity tomography (ERT) is a geophysical technique that uses arrays of electrodes to inject current into the ground and measure the resulting potential differences. The apparent resistivity is calculated and inverted to produce 2D or 3D resistivity models of the subsurface. Resistivity varies widely: dry sand/gravel (>1000 Ω·m), saturated sand (50–500 Ω·m), clay (<50 Ω·m), saltwater (<1 Ω·m), fresh water (10–100 Ω·m). ERT is widely used for mapping aquifers, contaminant plumes, faults, and lithologic contacts.
Question 5: In a seismic refraction survey, what is the 'critical distance' (crossover distance)?
- The maximum distance at which refracted arrivals from the lower layer are recorded before the signal attenuates
- The offset distance at which the refracted wave arrives at the same time as the direct surface wave, beyond which refractions arrive first (Correct answer)
- The depth to the refracting interface calculated from the refraction travel-time curve
- The distance between adjacent geophones in the receiver array
Correct answer: The offset distance at which the refracted wave arrives at the same time as the direct surface wave, beyond which refractions arrive first
The crossover distance is the source-receiver offset where direct wave and head wave (refracted) travel times are equal; beyond this distance, the refracted wave traveling through the faster lower layer arrives before the direct wave.
In seismic refraction, the travel-time versus offset plot shows two distinct segments: the direct wave (slope = 1/V1) and the refracted head wave (slope = 1/V2, where V2 > V1). The crossover distance Xcross = 2z√((V2+V1)/(V2−V1)), where z is the depth to the refractor. At distances beyond the crossover, the head wave arrives before the direct wave. The intercept time and slope of the refraction segment allow calculation of refractor depth and velocity, fundamental for engineering site investigations and groundwater exploration.
Question 6: What is the primary difference between body waves and surface waves in seismology?
- Body waves travel along Earth's surface; surface waves travel through Earth's interior
- Body waves (P and S waves) travel through Earth's interior; surface waves (Rayleigh and Love waves) travel along the surface and cause most ground damage in earthquakes (Correct answer)
- Body waves are longitudinal; surface waves are transverse, both traveling at the same velocity
- Body waves are only recorded near the earthquake epicenter; surface waves travel globally
Correct answer: Body waves (P and S waves) travel through Earth's interior; surface waves (Rayleigh and Love waves) travel along the surface and cause most ground damage in earthquakes
Body waves (P and S) propagate through Earth's volume, while surface waves (Rayleigh and Love) travel along the Earth's surface or along interfaces and are responsible for most of the damage and shaking felt in large earthquakes due to their larger amplitude and slower decay with distance.
Seismic waves generated by earthquakes propagate as body waves (P: compressional, fastest; S: shear, ~1.7× slower than P) that travel through Earth's interior. Surface waves, generated by interference of body waves near the free surface, include Rayleigh waves (elliptical particle motion, analogous to ocean waves) and Love waves (horizontal shear motion). Surface waves have larger amplitudes and travel more slowly than body waves but are dispersive (different frequencies travel at different speeds), causing prolonged ground shaking that is the primary cause of structural damage in distant earthquakes.
ASBOG - Association of State Boards of Geology Geophysics and Seismology Questions and Answers 2
In seismic reflection profiling, what does a 'two-way travel time' (TWT) measurement represent?