Lecture 2a Gravity Method

Flashcards about the gravity method.

What is the standard value of gravitational acceleration on Earth's surface?

9.81 meters per second squared

What are geophysicists usually interested in when studying gravity?

Anomalies or small deviations from the background value.

Besides meters per second squared, what other units are geophysical gravity measurements described in?

Milligal and gravity unit (GU)

What is one milligal equal to?

10 to the minus five meters per second squared

What is one gravity unit (GU) equal to?

10 to the minus six meters per second squared.

What causes Earth's gravity to vary steadily across its surface?

The Earth's nonspherical shape and rotation.

What two factors can be used to derive the value of gravitational acceleration at Earth's surface?

Newton's law of gravitation and Earth's mass.

What are the two basic types of gravimeters used for surface-based measurements of gravity?

Absolute and relative gravimeters.

How do relative gravimeters work?

Measuring gravity relative to a previously made reference setting.

How does a relative gravimeter essentially work?

By balancing a mass on a cantilevered beam.

What is the range of the Lacrosse Romberg gravimeter?

About 50,000 GU

What are the corrections related to position in gravity data reduction?

Latitude correction, free air correction, and bouguer correction.

What does the free air correction account for?

Changes in gravity due to variations in the distance from the center of the earth.

What does the Bouguer correction account for?

Accounts for the amount of earth material under the instrument.

What are the primary time-related corrections in gravity measurements?

Ocean tides and instrument drift.

What can reduced gravity measurements be interpreted in terms of?

Variations in the density of the subsurface.

Which rock types are generally at the less dense end of the density spectrum?

Sedimentary rocks, followed by metamorphic and igneous rocks.

Why do ores exhibit distinctively high densities?

Metallic minerals increase their densities.

What can gravity measurements be used to understand on a regional scale?

Regional geological structure at depths down to a few hundred kilometers.

What are microgravity surveys best for identifying?

Near surface voids.