ASBOG - Association of State Boards of Geology Geophysics and Seismology Questions and Answers — Questions and Answers
Question 1: An airborne magnetic survey is being used for regional geologic mapping. Which of the following geologic features is most likely to produce a strong, positive magnetic anomaly?
- A buried granite batholith intruding into sedimentary rocks. (Correct answer)
- A thick sequence of limestone and shale.
- A syncline composed of folded sandstone beds.
- A salt dome (diapir) that has pierced overlying strata.
Correct answer: A buried granite batholith intruding into sedimentary rocks.
Magnetic anomalies are caused by variations in the magnetic susceptibility of near-surface rocks. Felsic intrusive rocks like granite contain magnetic minerals (e.g., magnetite) and typically have a much higher magnetic susceptibility than sedimentary rocks. This contrast in magnetic properties creates a detectable positive anomaly. Salt is non-magnetic (diamagnetic) and would produce a negative anomaly relative to most surrounding rocks.
Question 2: The Mohorovičić discontinuity (Moho) is a major seismic boundary within the Earth. What does it represent?
- The boundary between the lithosphere and the asthenosphere.
- The boundary between the crust and the mantle. (Correct answer)
- The boundary between the mantle and the outer core.
- The boundary between the liquid outer core and the solid inner core.
Correct answer: The boundary between the crust and the mantle.
The Mohorovičić discontinuity, or Moho, is defined by a sharp increase in the velocity of seismic waves (both P-waves and S-waves). This velocity change marks the transition from the lower-density crustal rocks to the higher-density mantle rocks (peridotite).
Question 3: A seismologist is analyzing seismograms from three different stations (A, B, and C) to locate an earthquake's epicenter. The S-P time interval is smallest for station A, larger for station B, and largest for station C. What can be concluded about the earthquake's location?
- The earthquake occurred an equal distance from all three stations.
- The earthquake's epicenter is closest to station C.
- The earthquake's epicenter is closest to station A. (Correct answer)
- The S-P time interval is unrelated to the distance from the epicenter.
Correct answer: The earthquake's epicenter is closest to station A.
P-waves travel faster than S-waves. Therefore, the time difference between their arrivals (the S-P interval) increases with distance from the earthquake's epicenter. A smaller S-P interval indicates a shorter travel distance. Since station A has the smallest S-P time, it is the closest to the epicenter.
Question 4: Which of the following statements most accurately describes the key difference between primary (P) waves and secondary (S) waves?
- S-waves have a higher velocity than P-waves and are the first to arrive at a seismograph station.
- P-waves cause particle motion perpendicular to the direction of wave propagation, while S-waves cause particle motion parallel to it.
- S-waves are a type of surface wave, while P-waves are a type of body wave.
- P-waves are compressional and can travel through solids and liquids, while S-waves are shear waves and can only travel through solids. (Correct answer)
Correct answer: P-waves are compressional and can travel through solids and liquids, while S-waves are shear waves and can only travel through solids.
P-waves are compressional (longitudinal), meaning particle motion is in the same direction as wave propagation, allowing them to travel through solids, liquids, and gases. S-waves are shear (transverse) waves, with particle motion perpendicular to the direction of propagation. Shear motion cannot be sustained in fluids, so S-waves can only travel through solids. P-waves are also faster than S-waves.
Question 5: A gravity survey is conducted over a buried, high-density ore body surrounded by lower-density host rock. After applying the standard Free-Air, Bouguer, and terrain corrections, which of the following would be the expected gravity anomaly directly over the ore body?
- A positive Bouguer anomaly. (Correct answer)
- A negative Bouguer anomaly.
- No significant anomaly because the corrections remove all local effects.
- A positive Free-Air anomaly but a negative Bouguer anomaly.
Correct answer: A positive Bouguer anomaly.
The Bouguer anomaly reflects variations in the density of the subsurface rocks after corrections for latitude, elevation (Free-Air), and the gravitational effect of the rock mass between the station and a datum (Bouguer correction). A body of rock that is denser than the surrounding rock will create a local mass excess, resulting in a slightly stronger gravitational pull and thus a positive Bouguer anomaly.
Question 6: A seismic refraction survey is performed to determine the depth to bedrock. The resulting travel-time graph for the first arrivals shows two distinct linear segments. The first segment has a steep slope (low velocity), and the second has a shallower slope (high velocity). What does the 'crossover distance' on this graph represent?
- The point where the direct wave and the reflected wave arrive at the same time.
- The shallowest depth at which the bedrock can be detected.
- The point where the S-wave from the bedrock first arrives.
- The distance from the source where the refracted wave from the bedrock becomes the first arrival at the geophone. (Correct answer)
Correct answer: The distance from the source where the refracted wave from the bedrock becomes the first arrival at the geophone.
In a simple two-layer case, geophones close to the source will first detect the direct wave traveling through the slower top layer. Geophones farther away will first detect the wave that has been critically refracted along the faster, deeper layer. The crossover distance is the specific source-to-geophone distance where these two waves arrive simultaneously. Beyond this distance, the refracted wave is the first arrival.
ASBOG - Association of State Boards of Geology Geophysics and Seismology Questions and Answers
An airborne magnetic survey is being used for regional geologic mapping.
Which of the following geologic features is most likely to produce a strong, positive magnetic anomaly?