Attenuation analysis of ground penetrating radar data acquired over a crude oil spill

188 Pages·2004·31.26 MB·English

ATTENUATION ANALYSIS OF GROUND PENETRATING
RADAR DATA ACQUIRED OVER A CRUDE OIL SPILL

by

Bethany L. Burton

ABSTRACT

Hydrocarbon spills in the subsurface are a widespread problem and have been extensively
studied with ground penetrating radar (GPR). A change in apparent amplitude response has been
repeatedly observed over areas of known spills. This research focuses on determining the
dominant attenuation mechanism in a multi-offset GPR dataset acquired at a crude oil spill site
outside of Bemidji, Minnesota.
A frequency spectrum attribute analysis was performed to test if particular spectra
attributes (peak frequency, half-amplitude width, and low and high frequency slopes) would
indicate the presence of oil. Several factors were applied to the data to account for non-
attenuation related changes in amplitude: radiation pattern, antenna-ground normalization, and
geometrical divergence. This analysis exhibited no consistent trends that would indicate the
location and presence of the oil if it were not already known.
In the final analysis, attenuation and loss tangent were calculated after additionally taking
into account the water table reflection coefficient and the estimated transmitted wavelet’s
amplitude spectrum. The analysis was performed on several, selected reflectors and all exhibited
trends that are indicative of conductive losses over the low range of calculated frequencies and
scattering losses over the high range. The loss tangents of the contaminated water table reflectors
shift upward compared to the uncontaminated water table reflectors’ loss tangents at lower
frequencies, indicating an increase in the conductive losses. This increase is supported by
specific conductance measurements made on water samples and by a DC resistivity survey. An
EM-31 dataset, however, contains less than 1% variation in conductivity over the site. If these
data are considered, then the shift upward of the contaminated loss tangent at low frequencies
may be due to a decrease in the permittivity caused by the oil replacing the water in the pores.
The lack of a similar shift upward at the higher frequencies may indicate a decrease in the amount
of scattering from a compressed capillary fringe. To better determine the dominant mechanism
causing the observed shift upward in the loss tangents, this same analysis could be performed on
a dataset acquired with an antenna of a different frequency, or a multi-polarization GPR survey
could be acquired to exploit the depolarization caused by scattering at the capillary fringe.
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TABLE OF CONTENTS

ABSTRACT.......................................................................................................................iii
LIST OF FIGURES..........................................................................................................vii
LIST OF TABLES.............................................................................................................xi
ACKNOWLEDGMENTS................................................................................................xii

INTRODUCTION...............................................................................................................1

Chapter 1 GROUND PENETRATING RADAR..............................................................5

1.1 Theory.....................................................................................................................5
1.2 Velocity Analysis....................................................................................................8
1.3 Attenuation Analysis.............................................................................................10

1.3.1 Geometrical Divergence............................................................................12
1.3.2 Angular Dependent Gain Functions...........................................................13
1.3.3 Amplitude Spectrum of Transmitted Wavelet...........................................13
1.3.4 Reflection Coefficients..............................................................................14
1.3.5 Electromagnetic Loss Mechanisms............................................................17
1.3.6 Scattering Losses.......................................................................................19

1.3.6.1 Surface Scattering........................................................................20
1.3.6.2 Volume Scattering.......................................................................23

1.3.7 Dominant Mechanism Determination........................................................26

Chapter 2 BEMIDJI RESEARCH SITE DESCRIPTION...............................................30

2.1 Site History...........................................................................................................30
2.2 Geologic and Hydrgeologic Setting......................................................................31
2.3 Well and Core Data...............................................................................................33

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2.3.1 Oil Biodegradation.....................................................................................35
2.3.2 Contaminant Plume Migration...................................................................36
2.3.3 Core Sample Resistivity Measurements....................................................40

2.4 Previous Geophysical Surveys Performed............................................................41

2.4.1 GPR Surveys..............................................................................................41
2.4.2 Electromagnetic Surveys...........................................................................41

Chapter 3 GPR DATA ACQUISITION AND PRE-PROCESSING..............................44

3.1 Data Acquisition...................................................................................................44
3.2 Data Pre-processing..............................................................................................47

Chapter 4 VELOCITY ANALYSIS................................................................................54

4.1 Semblance Plot and Constant Velocity Stack Analyses.......................................55
4.2 Constant Velocity Panel Analysis.........................................................................57
4.3 Complex Trace Attribute Analysis on Stolt Migrated Data..................................65
4.4 Discussion.............................................................................................................69

Chapter 5 ATTENUATION ANALYSIS........................................................................73

5.1 Frequency Spectra Attribute Analysis..................................................................74

5.1.1 Non-Attenuation Related Factors Affecting Amplitude............................74

5.1.1.1 Angular Dependent Gain Functions.............................................76
5.1.1.2 Antenna-ground Coupling Normalization...................................78
5.1.1.3 Geometrical Divergence..............................................................79
5.1.1.4 Reflection Coefficients and Transmitted Wavelet Spectrum.......81

5.1.2 Attribute Analysis Methodology................................................................83
5.1.3 Results........................................................................................................86
5.1.4 Discussion..................................................................................................89

5.2 Attenuation and Loss Tangent Determination......................................................97

5.2.1 Amplitude Spectrum of Transmitted Wavelet...........................................97

5.2.1.1 Airwave and Groundwave Separation.........................................98
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5.2.1.2 Forward Modeling Comparison of Two Wavelets....................103

5.2.2 Reflection Coefficients............................................................................106
5.2.3 Attenuation and Loss Tangent Analysis Methodology............................107
5.2.4 Results......................................................................................................114
5.2.5 Discussion................................................................................................123

Chapter 6 SUMMARY AND CONCLUSIONS............................................................131

REFERENCES CITED....................................................................................................136
Appendix A RADIATION PATTERN ANALYSIS.....................................................142
Appendix B MATLAB CODES AND SU SCRIPTS....................................................170
Appendix C RAW AND BINNED GPR DATA...........................................................174

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LIST OF FIGURES

1.1 Plots of reflection coefficients for a perpendicular-polarized electric field..............18

1.2 Surface scattering patterns and a schematic of volume scattering............................21

1.3 Graph of normalized radar cross section for a single spherical particle versus
normalized incident wavelength.............................................................................25

1.4 Plot of conductive and volume scattering losses in terms of loss tangents as a
function of frequency.............................................................................................28

1.5 Flow chart of the general steps necessary to perform the velocity and attenuation
analyses...................................................................................................................29

2.1 1991 aerial photograph of the Bemidji research site with the spill-related
features overlaid.....................................................................................................32

2.2 Map of the well positions at the Bemidji site............................................................34

2.3 Specific conductances and total dissolved solids (TDS) of water samples
collected in June 1987 within the North Oil Pool..................................................37

2.4 Topographic map of the Bemidji site showing the positions of the North Pool
(A-A’) and South Pool (B-B’) transects shown in Figure 2.5...............................38

2.5 Subsurface oil distributions of the North and South Oil Pools.................................39

2.6 Three parallel, single antenna, 80 MHz profiles acquired over the spray zone
and South Oil Pool in January 1985......................................................................42

3.1 1991 aerial photograph with spill site features and positions of the two GPR
profiles acquired in January 2000 overlaid............................................................45

3.2 Photograph of the acquisition of the W602 Line.......................................................46

3.3 South Pool and W602 Lines 0.60 m offset profiles...................................................49

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3.4 South Pool Line 0.81 m offset before and after flattening the first arrival event
and after re-zeroing................................................................................................50

3.5 Images of the 1.63b, 1.83, and 2.03 m offset files before and after first arrival
drift correction........................................................................................................51

3.6 An expanded pre-processing flow chart of processes performed on data before
velocity or attenuation analyses..............................................................................53

4.1 Expanded velocity analysis flow chart of processes performed on the data.............56

4.2 Sample South Pool Line CMPs.................................................................................58

4.3 Sample constant velocity panel from W602 Line with pick velocities.....................60

4.4 South Pool Line stacking velocity profile from the constant velocity panel
analysis...................................................................................................................61

4.5 W602 Line stacking velocity profile from the constant velocity panel analysis.......62

4.6 South Pool Line smoothed stacking velocity profile.................................................63

4.7 W602 Line smoothed stacking velocity profile........................................................64

4.8 South Pool Line interval velocity profile..................................................................66

4.9 W602 Line interval velocity profile..........................................................................67

4.10 Sample instantaneous amplitude plots of a section of the W602 Line 0.60 m
offset Stolt migrated data........................................................................................70

5.1 Expanded flow chart of the steps involved in the frequency spectrum attribute
analysis...................................................................................................................75

5.2 Field radiation pattern in relative amplitude versus incidence angle for φ=90°........77

5.3 Stacking velocity profiles and corresponding geometrical divergence functions
for the South Pool and W602 Lines.......................................................................80

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5.4 Plots of reflection coefficients as a function of incidence angle with ranges of
incidence angles specific to Bemidji data for varying depths to reflectors
indicated.................................................................................................................82

5.5 Schematic of a frequency spectrum...........................................................................84

5.6 Schematic of a sample trace showing time-windows................................................87

5.7 South Pool Line full trace attributes that exhibit trends that correspond with the
position of the oil pool............................................................................................88

5.8 South Pool Line time-windowed trace frequency spectra half-amplitude width
attribute...................................................................................................................90

5.9 South Pool Line time-windowed trace frequency spectra low and high
frequency slope attributes......................................................................................91

5.10 South Pool Line 0.60 m offset profile showing positions of time windows 3
through 6.................................................................................................................94

5.11 Low and high frequency slopes for time windows 3 through 6 calculated from
the South Pool Line 0.60 m offset profile..............................................................95

5.12 A diagram showing the concept of how the infiltration of oil in the subsurface
affects the RDP contrasts........................................................................................96

5.13 Three binned, un-zeroed profiles from the W602 Line showing traces from
which air and groundwave wavelets extracted.......................................................99

5.14 Transmitted wavelet in air of the GSSI 300 MHz antenna and air and
groundwave wavelets extracted from three traces indicated in Figure 5.13........100

5.15 Frequency spectra plots that correspond to the recorded time wavelets shown
in Figure 5.14........................................................................................................101

5.16 Time wavelets and corresponding spectra of the estimated transmitted wavelet
into the ground......................................................................................................104

5.17 0.60 m offset profiles showing the positions of the selected reflectors for the
attenuation and loss tangent analysis....................................................................110

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5.18 2.23 m offset profiles showing the positions of the selected reflectors for the
attenuation and loss tangent analysis....................................................................111

5.19 5.15 m offset profiles showing the positions of the selected reflectors for the
attenuation and loss tangent analysis....................................................................112

5.20 Expanded flow chart of the steps involved in the attenuation and loss tangent
analysis.................................................................................................................115

5.21 Loss tangent and wavelet spectra plot for trace 870 (sample 59-93) of the
W602 Line 0.60 m offset profile..........................................................................117

5.22 Loss tangent and wavelet spectra plot for trace 1778 (sample 111-142) of the
South Pool Line 0.60 m offset profile..................................................................118

5.23 Loss tangent and wavelet spectra plot for trace 1778 (sample 161-196) of the
South Pool Line 0.60 m offset profile..................................................................119

5.24 Loss tangent plots for same three traces on square log-log scale............................121

5.25 Model of the first four wavelets to arrive at the receiving antenna.........................122

5.26 Loss tangents for three reflectors from uncontaminated zones of the South
Pool Line 0.60 m offset profile.............................................................................124

5.27 Loss tangents for three reflectors from contaminated and uncontaminated
zones of the South Pool Line 0.60 m offset profile.............................................125

5.28 Loss tangents for three reflectors from contaminated and uncontaminated
zones near the water table of the South Pool Line 0.60 m offset profile.............126

5.29 Loss tangents for three reflectors from below the water table in contaminated
and uncontaminated zones of the South Pool Line 0.60 m offset profile............127

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