ASBOG - Association of State Boards of Geology Geophysics and Seismology 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?

Accepted Answer

A buried granite batholith intruding into sedimentary rocks.

Answer

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?

Accepted Answer

The boundary between the crust and the mantle.

Answer

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?

Accepted Answer

The earthquake's epicenter is closest to station A.

Answer

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?

Accepted Answer

P-waves are compressional and can travel through solids and liquids, while S-waves are shear waves and can only travel through solids.

Answer

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?

Accepted Answer

A positive Bouguer anomaly.

Answer

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.

ASBOG - Association of State Boards of Geology Practice Test

ASBOG - Association of State Boards of Geology Practice Test

ASBOG - Association of State Boards of Geology Geophysics and Seismology Questions and Answers