Note Outline: Geophysical Surveying Using Gravity

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Introduction

• Gravitational Force
• Gravitational Acceleration
• Units Associated With Gravitational Acceleration

Gravity and Geology

• How is the Gravitational Acceleration, g, Related to Geology?
• The Relevant Geologic Parameter is not Density, but Density Contrast
• Density Variations of Earth Materials
• A Simple Model

Measuring Gravitational Acceleration

• How do we Measure Gravity
• Falling Body Measurements
• Pendulum Measurements
• Mass and Spring Measurements

Factors that Affect the Gravitational Acceleration

Overview

Temporal Based Variations

• Instrument Drift
• Tides
• A Correction Strategy for Instrument Drift and Tides
• Tidal and Drift Corrections: A Field Procedure
• Tidal and Drift Corrections: Data Reduction

Spatial Based Variations

• Latitude Dependent Changes in Gravitational Acceleration
• Correcting for Latitude Dependent Changes
• Variation in Gravitational Acceleration Due to Changes in Elevation
• Accounting for Elevation Variations: The Free-Air Correction
• Variations in Gravity Due to Excess Mass
• Correcting for Excess Mass: The Bouguer Slab Correction
• Variations in Gravity Due to Nearby Topography
• Terrain Corrections

Summary of Gravity Types

Quiz 1 - Sources of Noise

Isolating Gravity Anomalies of Interest

• Local and Regional Gravity Anomalies
• Sources of the Local and Regional Gravity Anomalies
• Separating Local and Regional Gravity Anomalies
• Local/Regional Gravity Anomaly Separation Example

Gravity Anomalies Over Bodies With Simple Shapes

• Gravity Anomaly Over a Buried Point Mass
• Gravity Anomaly Over a Buried Sphere
• Model Indeterminancy
• Gravity Calculations over Bodies with more Complex Shapes

Quiz 2 - Factors Controlling the Shape of Gravity Anomalies

Study and Discussion Questions