INTRODUCTION TO APPLIED GEOPHYSICS E X P L O R I N G THE S H A L L O W S U B S U R F A C E
H. Robert Burger Anne F. Sheehan Craig H.Jones SMITH COLLEGE UNIVERSITY OF COLORADO UNIVERSITY OF COLORADO
W. W, NORTON & COMPANY NEW YORK • LONDON
CbntentSi 1
i T ' t t l J
Computer Materials XIX
xxii
1. Approaching the Subsurface m i "'-a: Are the Options?
U £ : T e Fundamental Considerations • 3 defining Objectives ZM imitations &5 ~-e Advantage of Multiple Methods
1 2
3 4 5
Seismic Exploration: Fundamental Considerations
I
. . . J
2 i *
Seismic Waves and Wave Propagation Wave Terminology
E astic Coefficients Seismic Waves Seismic Wave Velocities
Ray Paths in Layered Materials
Huygens' Principle
Fermat's Principle
Reflection
Refraction
Sneirs Law
E'* cal Refraction
E -"-action Wlave Arrivals at the Surface
? 9
11
14 18
21 21
22
23 25
2?
28 29
35
2.3 Wave Attenuation and Amplitude 39
2.3.1 Spherical Spreading 40 2.3.2 Absorption 40
2.3.3 Energy Partitioning 42
2.3.4 Additional Factors 46 2.4 Energy Sources 4?
2.4.1 Source Types 4?
2.4.2 Source Considerations 49
2.5 Seismic Equipment 50 2.5.1 Signal Detection 50
2.5.2 Signal Conditioning 53 2.5.3 Sionnl Rprnrriino 56
2.6 Summary 60
Problems 61
References Cited 63
3 Seismic Expioratlon: The Refraction Method 55 3.1 A Homogeneous Subsurface 65
3.2 A Single Subsurface Interface 6?
3.2.1 Derivation of a Travel-Time Equation 62
3.2.2 Analysis of Arrival Times 20
3.2.3 Determining Thickness 22
3.2.4 Crossover Distance 23
3.2.5 Critical Distance 24
3.2.6 Constructing a Travel-Time Curve from a Field Seismogram 26 3.2.2 Using REFRACT 28
3.2.8 The Mohorovicic Discontinuity 29 3.3 Two Horizontal Interfaces 81
3.3.1 Derivation of a Travel-Time Equation 81 3.3.2 Determining Thickness 83
3.3.3 Critical Distance 85 3.3.4 Analyzings Second Field Seismogram 82
3.4 Multiple Interfaces 90
3.5 Dipping Interfaces 91
3.5.1 Analyzing the Problem 91
3.5.2 Derivation of a Travel-Time Equation 92
3.5.3 Determining Thickness 99 3.6 Multiple Dipping Interfaces 100
3.6.1 Travel-Time Equation 100
3.6.2 Analyzing Field Seismograms 102
' t e t a M e a l Subsurface 106 HIiilBiZbnes: The Low-Velocity Layer 10?
HWiBiZDnesiThe Thin Layer i l l
•Jill Jyterying Velocity 114
iiiinfau- Discontinuities 116
VcMag-Time Method 120
•••I m i l III 12? M M R m t Method 12? ^ f l a d n g a n d t h e Generalized Reciprocal Method 129
HdHPtacedures 129 3lSclBCtion and Planning Considerations 129 i p i p B K n t Considerations 130
iamRaneSpread Geometries and Placements 131
--'•rr-'irxs Using Seismic Refraction 136 issachusetts 13B
••easlem New Hampshire 13? : t : o s a l S i t e 138
--sa. Arizona 139
141 : 53 Cited 14? ^: Reading 148
Sasmic Exploration: The Reflection Method 149 • I ^ T i i i i iiifi I interface 150
JfeiRBffiFLECT 150 HBmanDnofa Travel-Time Equation 151
UnaAgpsof Arrival Times 156 ' - — ; "'r/e-Gut 160
-g Velocity and Thickness 163 _ - -ex- - t ^ Method to a Field Seismogram 165
5 -:rizontallnterfaces 16?
1?0
^ i : cities 1?0 , :-<nesses 1?4
: ; t of the Dix Method 1?5
- c :: Seismogram Containing Multiple Reflections 181 5 ace 182
- -r~ ?. e'-Time Equation 183
^ I : "-ickness, and Velocity 186
6 Exploration Using Gravity 349 6.1 Fundamental Relationships 350
6.1.1 Gravitational Acceleration 350
6.2 Measuring Gravity 351
6.2.1 Relative Measurements Using a Pendulum 351 6.2.2 Relative Measurements Using a Gravimeter 352
6.2.3 Absolute Measurements 355
6.2.4 International Gravity Standardization Net 1971 [IGSN71] 355
6.3 Adjusting Observed Gravity 356
6.3.1 Variation in g as a Function of Latitude 356
6.3.2 Correcting for the Latitude Effect 357
6.3.3 Elevation Correction l:The Free-Air Correction o r o
358 6.3.4 1— 1 /•* . * 'm Tl pv p. . *
Elevation Correction 2: The Bouguer Correction 360
6.3.5 Elevation Correction 3: The Terrain Correction 364
6.3.6 The Isostatic Anomaly 369
6.4 Basic Field Procedures 370
6.4.1 Drift and Tidal Effects 370
6.4.2 Establishing Base Stations 373
6.4.3 Determining Elevations 373 6.4.4 Determining Horizontal Position 375
6.4.5 Selecting a Reduction Density 376
6.4.6 Survey Procedure 376
6.5 Gravity Effects of Simple Geometric Shapes 378
6.5.1 Rock Densities 378
6.5.2 Gravity Effect of a Sphere 379
6.5.3 Gravity Effect of a Horizontal Cylinder 383
6.5.4 Gravity Effect of a Vertical Cylinder 386 6.5.5 p* r—pp P 1 1' i r ^ 1
Gravity Effect of an Inclined Rod 388
6.5.6 Gravity Effect of a Horizontal Sheet 390
6.5.2 GRAVMAG 397
6.6 Analyzing Anomalies 399 6.6.1 Regionals and Residuals 399
6.6.2 Trend Surfaces 402 f- f- o 6.6.3 Upward and Downward Continuation 403 6.6.4 Second Derivatives 407
6.6.5 Filtering 409
6.? Gravity Interpretation 409
6.2.1 Half-Maximum Technique 410
6.2.2 Second Derivative Techniques 411
6.7.3 Revisiting Some Bouguer Anomaly Values 413
6.8 Applications of the Gravity Method 415
M M K k Depths 415
S d K w t e e Voids 416
InN i l l Geometry 417 ftiMliiis 420
• i f i i i nces Cited 426
S^B^sted Reading 427
I ' C oration Using the Magnetic Method 429 - -• : = -nental Relationships 430
^ : -area 431 ^ -̂ : - i eld Strength 431
z ^ i : '-'oment 432
^ ; : -'Magnet izat ion 433
. : Susceptibility 434 I =:- : -otential 437
"2 Earth's Magnetic Field 438
~ : ^ S s - e n t s 438 . z , ; : j re of the Earth's Field 439
: :--:-= of the Earth's Field 441
: =: Sr.stions 444 : T i s . n g the Magnetic Field 448
i f o '-lagnetometer 446
; -- ecession Magnetometer 447 - - 0 ; Anomalies 44?
: a = : - eld Procedures 450 . z Z eanliness 450
:- --actions 450
- : lorrections 451 : ; -or Horizontal Position 452
£ a t - e - : Effects of Simple Geometric Shapes 454
- . • : .;;5otibilities 454 ; - : E-'-'ect of an Isolated Pole [Monopole] 456
: , - : E-'-ect of a Dipole 45?
: a - : E'Tect of a Sphere 465
z a - : E-'fect of a Thin, Horizontal Sheet 470 : T - ; E-'fects of Polygons with Infinite Strike Length (Using GRAVMAG) 478
£ - -e - : etation of Magnetic Data 482
a -sges and Advantages 482 a- .e'nterpretation Techniques 484
a - „ T i Techniques 484
?£.22 SInnp MpthnHc; 485
P.6.2.3 rnmnuTpr Mndplino 488
M p p i l U d l l U l l b UT l l l C | V | a g l l C l l U IVICLIIUU Ann
7.7.1 ArrhapnInoirpI Siirvpijc; M t L . l l a c U l i J t l i L . C 1 l U L I I V C U D
488
f.i.C U c L c L L l U n UT VUlUb d l l U W e l l L d b l l l ^ b zian
f .f. o r i o r i n t n n lanHti 1 lEnniTnolTi i U C I i 11 i IIV:: L d l l U i l l l u c U I I I C k l U
P r n h l p m Q 494
References Cited 496 Suggested Reading 49?
8 Electromagnetic Surveying 499 8.1 Electromagnetic Waves 500
8.1.1 Wavelengths 502
8.1.2 AC/DC 504 8.1.3 Electrical Properties of Geologic Materials 504
8.1.3.1 Electrical Resistivity and Conductivity 505
8.1.3.2 Dielectric Properties 505 8.1.4 Absorption and Attenuation 50? 8.2 EM Sounding 509
8.2.1 Near-Field Continuous-Wave Methods of Frequency Domain Electromagnetics [FDEM] 512
8.2.1.1 MovingTransmitter-Flus-Receiver System (Slingram) 512
8.2.1.2 Noncontacting Ground Conductivity Measurements 514 8.2.2 Other FDEM Systems 519 8.2.3 Time Domain Electromagnetics (IDEM) 519
8.3 EM Field Techniques 521 8.3.1 Profiling versus Sounding 521
8.3.1.1 Sounding 521
8.3.1.2 Profiling 523 8.3.2 Interpretation 523 8.4 Ground-Penetrating Radar 524
8.4.1 Radar Velocity 525 8.4.2 Data Acquisition 52?
8.4.3 GPR Velocity Analysis 533
8.4.3.1 Burial of Known Object 533
8.4.3.2 Walkaway Test 533
8.4.3.3 Diffraction Hyperbola 53? 8.5 Applications of Electromagnetic Surveying 539
8.5.1 Archaeological Surveys 539 8.5.1.1 Aztec Ruins, New Mexico 539
xvl CONTENTS
L i l 2 Ceren, El Salvador 541 " Geologic Applications 543
Snow and Ice Mapping 546
Environmental and Engineering Applications 546
Problems 548
References Cited 553
Suggested Reading 554
Appendixes Instructions for Using REFRACT Al
Instructions for Using REFLECT A6
iMtructions for Using RESIST A13
hMructions for Using GRAVMAG A17
IMtructions for Using DIFFRACT A24
