INTRODUCTION TO APPLIED GEOPHYSICS

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1 INTRODUCTION TO APPLIED GEOPHYSICS EXPLORING THE SHALL0W SUBSURFACE H. Robert Burger Anne F. Sheehan Craig H.Jones VERSITY OF COLORADO VERSITY OF COLORADO W. W. NORTON & COMPANY NEW YORK LONDON

2 Contents Preface Preface to Computer Materials XIX xxii Approaching the Subsurface What Are the Options? Some Fundamental Considerations Defining Objectives Limitations The Advantage of Multiple Methods Seismic Exploration: Fundamental Considerations Seismic Waves and Wave Propagation Wave Terminology Elastic Coefficients Seismic Waves Seismic Wave Velocities Ray Paths in Layered Materials Huygens' Principle Fermat's Principle Reflection Refraction Snell's Law Critical Refraction Diffraction Wave Arrivals at the Surface ?

3 2.3 2;3.i S Wave Attenuation and Amplitude Spherical Spreading Absorption Energy Partitioning Additional Factors Energy Sources Source Types Source Considerations Seismic Equipment Signal Detection Signal Conditioning Signal Recording Summary Problems References Cited ? Seismic Exploration: The Refraction Method G A Homogeneous Subsurface A Single Subsurface Interface Derivation of a Travel-Time Equation Analysis of Arrival Times Determining Thickness Crossover Distance Critical Distance Constructing a Travel-Time Curve from a Field Seismogram Using REFRACT The Mohorovicic Discontinuity Two Horizontal Interfaces Derivation of a Travel-Time Equation Determining Thickness Critical Distance Analyzing a Second Field Seismogram Multiple Interfaces Dipping Interfaces Analyzing the Problem Derivation of a Travel-Time Equation Determining Thickness Multiple Dipping Interfaces Travel-Time Equation Analyzing Field Seismograms 65 6? ? X

4 The Nonideal Subsurface Hidden Zones: The Low-Velocity Layer Hidden Zones: The Thin Layer Laterally Varying Velocity Interface Discontinuities The Delay-Time Method Other Methods Wave Front Method Ray Tracing and the Generalized Reciprocal Method Field Procedures Site Selection and Planning Considerations Equipment Considerations Geophone Spread Geometries and Placements Corrections to Data Applications Using Seismic Refraction Whately, Massachusetts Southeastern New Hampshire Waste Disposal Site Maricopa Area, Arizona Problems References Cited Suggested Reading ? Seismic Exploration: The Reflection Method A Single Subsurface Interface Using REFLECT Derivation of a Travel-Time Equation Analysis of Arrival Times Normal Move-Out Determining Velocity and Thickness Applying the x Method to a Field Seismogram Multiple Horizontal Interfaces The Dix Equation Determining Velocities Determining Thicknesses Further Discussion of the Dix Method Analyzing a Field Seismogram Containing Multiple Reflections Dipping Interface Derivation of a Travel-Time Equation Determining Dip, Thickness, and Velocity xi

5 4.3.3 Determining Dip, Thickness, and Velocity Another Approach A Return to Normal Move-Out Determining Dip, Thickness, and Velocity Yet Another Approach Acquiring and Recognizing Reflections from Shallow Interfaces The Optimum Window Multiple Reflections Diffractions Common Field Procedures Equipment Considerations Geophone Spreads Split Spread Common Offset Common Depth Point Computer Processing of Reflection Data The Static Correction Correcting for Normal Move-Out Velocity Analysis A Velocity Analysis B Velocity Analysis C Stacking CDP Gathers Migration Waveform Adjustments Seismic Sections: Time Sections and Depth Sections Applying the Seismic Reflection Method Whately, Massachusetts Meers Fault, Oklahoma Cavity Detection Other Applications 252 Problems 254 References Cited 262 Suggested Reading 263 Electrical Resistivity Introduction Applied Currents Natural Currents A Brief History Chapter Goals Basic Electricity Current Flow in a Homogeneous, Isotropic Earth xii

6 5.3.1 Point Current Source Two Current Electrodes Two Potential Electrodes A Single Horizontal Interface Current Distribution Current Flow Lines and Current Density Apparent Resistivity Qualitative Development of the Resistivity Pattern over a Horizontal Interface Quantitative Development of the Resistivity Pattern over a Horizontal Interface Using RESIST Multiple Horizontal Interfaces Vertical Contact Constant-Spread Traverse Expanding-Spread Traverse Two Vertical Contacts, Hemispherical Structures, and Dipping Interfaces Two Vertical Contacts Hemispherical Structures Dipping Interfaces Field Procedures, Equipment Electrode Configurations Surveying Strategies Other Considerations Quantitative Interpretation of Apparent Resistivity Curves Electrical Resistivities of Geologic Materials Empirical Methods Analytical Methods Curve Matching Analytical Methods Automated Curve Matching Applications of Electrical Resistivity Surveying Applications Related to Aquifers Applications Related to Contamination Applications in Mapping Karst and Geologic Structures Other Applications Other Electrical Methods Induced Potential (IP) Spontaneous Potential (SP) Telluric and Magnetotelluric Methods 340 Problems 341 References Cited 346 Suggested Reading 347 xiii

7 Exploration Using Gravity Fundamental Relationships Gravitational Acceleration Measuring Gravity Relative Measurements Using a Pendulum Relative Measurements Using a Gravimeter Absolute Measurements International Gravity Standardization Net 1971 [IGSN?1 Adjusting Observed Gravity Variation in g as a Function of Latitude Correcting for the Latitude Effect Elevation Correction 1: The Free-Air Correction Elevation Correction 2: The Bouguer Correction Elevation Correction 3: The Terrain Correction The Isostatic Anomaly Basic Field Procedures Drift and Tidal Effects Establishing Base Stations Determining Elevations Determining Horizontal Position Selecting a Reduction Density Survey Procedure Gravity Effects of Simple Geometric Shapes Rock Densities Gravity Effect of a Sphere Gravity Effect of a Horizontal Cylinder Gravity Effect of a Vertical Cylinder Gravity Effect of an Inclined Rod Gravity Effect of a Horizontal Sheet GRAVMAG Analyzing Anomalies Regionals and Residuals Trend Surfaces Upward and Downward Continuation Second Derivatives Filtering Gravity Interpretation Half-Maximum Technique Second Derivative Techniques Revisiting Some Bouguer Anomaly Values Applications of the Gravity Method xiv

8 Bedrock Depths Subsurface Voids Landfill Geometry Problems References Cited Suggested Reading ? Exploration Using the Magnetic Method Fundamental Relationships Magnetic Force Magnetic Field Strength Magnetic Moment Intensity of Magnetization Magnetic Susceptibility Magnetic Potential The Earth's Magnetic Field Field Elements Dipolar Nature of the Earth's Field Variations of the Earth's Field Dipole Equations Measuring the Magnetic Field Flux-Gate Magnetometer Proton-Precession Magnetometer Total-Field Anomalies Basic Field Procedures Magnetic Cleanliness Diurnal Corrections Elevation Corrections Correcting for Horizontal Position Magnetic Effects of Simple Geometric Shapes Rock Susceptibilities Magnetic Effect of an Isolated Pole (Monopole) Magnetic Effect of a Dipole Magnetic Effect of a Sphere Magnetic Effect of a Thin, Horizontal Sheet Magnetic Effects of Polygons with Infinite Strike Length (Using GRAVMAG) Interpretation of Magnetic Data Disadvantages and Advantages Quantitative Interpretation Techniques Half-Maximum Techniques xv

9 Slope Methods 485 7:6.2.3 Computer Modeling 488?.? Applications of the Magnetic Method Archaeological Surveys Detection of Voids and Well Casings Defining Landfill Geometry 493 Problems 494 References Cited 496 Suggested Reading Electromagnetic Surveying Electromagnetic Waves Wavelengths AC/DC Electrical Properties of Geologic Materials Electrical Resistivity and Conductivity Dielectric Properties Absorption and Attenuation EM Sounding ' Near-Field Continuous-Wave Methods of Frequency Domain Electromagnetics (FDEM) Moving Transmitter-Plus-Receiver System (SlingratrV Noncontacting Ground Conductivity Measurements Other FDEM Systems Time Domain Electromagnetics (TDEM] EM Field Techniques Profiling versus Sounding Sounding Profiling Interpretation Ground-Penetrating Radar Radar Velocity Data Acquisition GPR Velocity Analysis Burial of Known Object Walkaway Test Diffraction Hyperbola Applications of Electromagnetic Surveying Archaeological Surveys Aztec Ruins, New Mexico xvi 1

10 Ceren, El Salvador Geologic Applications Snow and Ice Mapping Environmental and Engineering Applications Problems References Cited Suggested Reading Appendixes A Instructions for Using REFRACT Al B Instructions for Using REFLECT A6 C Instructions for Using RESIST A13 D Instructions for Using GRAVMAG A17 E Instructions for Using DIFFRACT A24 Index A33 I xvil

3. Magnetic Methods / 62

3. Magnetic Methods / 62 Contents Preface to the Second Edition / xv Excerpts from Preface to the FirstEdition / xvii Mathematical Conventions / xix 1. Introduction / 1 Reference / 5 2. Gravity Methods / 6 2. I. Introduction /