Groundwater Geophysics (eBook)

Groundwater Geophysics – a Tool for Hydrogeology 5
Contents 7
Authors 15
1 Petrophysical properties of permeable and lowpermeable rocks 18
1.1 Seismic velocities 18
1.1.1 Consolidated rock 19
1.1.2 Unconsolidated rock 21
1.1.3 Clay and till 24
1.2 Electrical resistivity 25
1.2.1 Archie´s law – conductive pore fluid and resistive rock matrix 25
1.2.2 Limitations of Archie´s law – conducting mineral grains 29
1.3 Electric Permittivity (Dielectricity) 33
1.4 Conclusions 37
1.5 References 38
2 Seismic methods 40
2.1 Introduction 40
2.1.1 What type of waves is applied in seismic exploration? 40
2.1.2 How can seismic waves image geological structure? 41
2.1.3 How are seismic waves generated and recorded in the field? 44
2.1.4 What kind of seismic measurements can be performed? 46
2.1.5 What kind of hydro-geologically relevant information can be obtained from seismic prospecting? 46
2.1.6 What are the advantages and disadvantages of seismic measurements compared to other methods? How do seismics and other geophysical measurements complement each other? 48
2.2 Seismic refraction measurements 48
2.2.1 Targets for seismic refraction measurements 49
2.2.2 Body wave propagation in two-layer media with a plane interface 50
2.2.3 Seismic refraction in laterally heterogeneous two-layer media 55
2.2.4 Consistency criteria of seismic refraction measurements 58
2.2.5 Field layout of seismic refraction measurements 61
2.2.6 Near surface layering conditions and seismic implications 63
2.2.7 Seismic interpretation approaches for heterogeneous subsurface structures 66
2.2.8 Structural resolution of seismic refraction measurements 75
2.3 Seismic reflection imaging 80
2.3.1 Targets for seismic reflection measurements 81
2.3.2 Seismic reflection amplitudes 82
2.3.3 Concepts of seismic reflection measurements 84
2.3.4 Seismic migration 91
2.3.5 Field layout of seismic reflection measurements 94
2.3.6 Problems of near surface reflection seismics 96
2.3.7 Structural resolution of seismic reflection measurements 97
2.4 Further reading 99
2.5 References 99
3 Geoelectrical methods 102
3.1 Basic principles 102
3.2 Vertical electrical soundings (VES) 104
3.2.1 Field equipment 107
3.2.2 Field measurements 107
3.2.3 Sounding curve processing 109
3.2.4 Ambiguities of sounding curve interpretation 110
3.2.5 Geological and hydrogeological interpretation 114
3.3 Resistivity mapping 115
3.3.1 Square array configuration 117
3.3.2 Mobile electrode arrays 119
3.3.3 Mise-à-la-masse method 121
3.4 Self- potential measurements 122
3.4.1 Basic principles of streaming potential measurements 122
3.4.2 Field procedures 123
3.4.3 Data processing and interpretation 124
3.5 2D measurements 126
3.5.1 Field equipment 126
3.5.2 Field measurements 127
3.5.3 Data Processing and Interpretation 128
3.5.4 Examples 130
3.6 References 133
4 Complex Conductivity Measurements 136
4.1 Introduction 136
4.2 Complex conductivity and transfer function of waterwet rocks 137
4.3 Quantitative interpretation of Complex conductivity measurements 140
4.3.1 Low Frequency conductivity model 140
4.3.2 Complex conductivity measurements 142
4.4 Relations between complex electrical parameters and mean parameters of rock state and texture 147
4.5 The potential of complex conductivity for environmental applications 155
4.5.1 Organic and inorganic contaminants 155
4.5.2 Monitoring subsurface hydraulic and migration processes 158
4.5.3 Geohydraulic parameters 161
4.6 References 166
5 Electromagnetic methods – frequency domain 172
5.1 Airborne techniques 172
5.1.1 Introduction 172
5.1.2 Theory 173
5.1.3 Systems 179
5.1.4 Data Processing 182
5.1.5 Presentation 183
5.1.6 Discussion and Recommendations 187
5.2 Ground based techniques 187
5.2.1 Slingram and ground conductivity meters 187
5.2.2 VLF, VLF-R, and RMT 191
5.3 References 193
6 The transient electromagnetic method 196
6.1 Introduction 196
6.1.1 Historic development 196
6.1.2 Introduction 198
6.1.3 EMMA - ElectroMagnetic Model Analysis 199
6.2 Basic theory 199
6.2.1 Maxwell’s equations 200
6.2.2 Schelkunoff potentials 201
6.2.3 The transient response over a layered halfspace 203
6.2.4 The transient response for a halfspace 205
6.3 Basic principle and measuring technique 206
6.4 Current diffusion patterns 208
6.4.1 Current diffusion and sensitivity, homogeneous halfspace 208
6.4.2 Current densities, layered halfspaces 211
6.5 Data curves 213
6.5.1 Late-time apparent resistivity 213
6.6 Noise and Resolution 214
6.6.1 Natural background noise 214
6.6.2 Noise and measurements 216
6.6.3 Penetration depth 217
6.6.4 Model errors, equivalence 218
6.7 Coupling to man-made conductors 220
6.7.1 Coupling types 221
6.7.2 Handling coupled data 222
6.8 Modelling and interpretation 224
6.8.1 Modelling 224
6.8.2 The 1D model 224
6.8.3 Configurations, advantages and drawbacks 225
6.9 Airborne TEM 226
6.9.1 Historical background and present airborne TEM systems. 226
6.9.2 Special considerations for airborne measurements 228
6.10 Field example 233
6.10.1 The SkyTEM system 233
6.10.2 Inversion of SkyTEM data 236
6.10.3 Processing of SkyTEM data 236
6.10.4 The Hundslund Survey 237
6.11 References 241
7 Ground Penetrating Radar 244
7.1 Electromagnetic wave propagation 245
7.1.1 Electric permittivity and conductivity 245
7.1.2 Electromagnetic wave propagation 247
7.1.3 Reflection and refraction of plane waves 249
7.1.4 Scattering and diffraction 251
7.1.5 Horizontal and vertical resolution 251
7.1.6 Wave paths, traveltimes, and amplitudes 252
7.1.7 Estimation of exploration depth 255
7.2 Technical aspects of GPR 256
7.2.1 Overview of system components 256
7.2.2 Antennas and antenna characteristics 256
7.2.3 Electronics 258
7.2.4 Survey practice 260
7.3 Processing and interpretation of GPR data 262
7.3.1 General processing steps 262
7.3.2 Examples for GPR profiling and CMP data 263
7.4 References 267
8 Magnetic Resonance Sounding 270
8.1 Introduction 270
8.2 NMR-Principles and MRS technique 270
8.3 Survey at Waalwijk / The Netherlands 278
8.4 Survey at Nauen / Germany with 2D assessment 282
8.5 Current developments in MRS 286
8.6 References 288
9 Magnetic, geothermal, and radioactivity methods 292
9.1 Magnetic method 292
9.1.1 Basic principles 292
9.1.2 Magnetic properties of rocks. 295
9.1.3 Field equipments and procedures 297
9.1.4 Data evaluation and interpretation 299
9.2 Geothermal method 303
9.2.1 The underground temperature field 306
9.2.2 Field procedures 307
9.2.3 Interpretation of temperature data 308
9.3 Radioactivity method 309
9.4 References 311
10 Microgravimetry 312
10.1 Physical Basics 312
10.2 Gravimeters 313
10.3 Gravity surveys and data processing 315
10.3.1 Preparation and performance of field surveys 316
10.3.2 Data processing 319
10.4 Interpretation 324
10.4.1 Direct methods 324
10.4.2 Indirect methods 328
10.4.3 Density estimation 330
10.5 Time dependent surveys 331
10.6 References 333
11 Direct Push-Technologies 338
11.1 Logging tools 338
11.1.1 Geotechnical tools 339
11.1.2 Geophysical tools 341
11.1.3 Hydroprobes 343
11.1.4 Hydrogeochemical tools 345
11.1.5 Miscellaneous other tools 347
11.2 Sampling tools 348
11.2.1 Soil sampling tools 348
11.2.2 Soil gas sampling tools 348
11.2.3 Groundwater sampling tools 349
11.3 Tomographic applications 349
11.4 Permanent installations 352
11.5 Conclusions 352
11.6 References 354
12 Aquifer structures – pore aquifers 358
12.1 Pore aquifers – general 358
12.1.1 Definition 358
12.1.2 Porosity – a key parameter for hydrogeology 358
12.1.3 Physical properties of pore aquifers 360
12.1.4 Geophysical survey of pore aquifers 361
12.2 Buried valley aquifer systems 365
12.2.1 Introduction 365
12.2.2 Geological and hydrological background 367
12.2.3 Methods 368
12.2.4 Discussion and Conclusion 376
12.3 A Large-scale TEM survey of Mors, Denmark 380
12.3.1 Study area – the island of Mors 380
12.3.2 Hydrogeological mapping by the use of TEM 382
12.3.3 Data collection and processing 384
12.3.4 Results and discussions 386
12.3.5 Conclusions 396
12.4 Groundwater prospection in Central Sinai, Egypt 398
12.4.1 Introduction 398
12.4.2 Geological and hydrogeological aspects 399
12.4.3 Field work and interpretation 401
12.4.4 Groundwater occurrence 407
12.5 References 408
13 Aquifer structures: fracture zones and caves 412
13.1 Hydraulic importance of fracture zones and caves 412
13.2 Geophysical exploration of fracture zones: seismic methods 414
13.3 Geophysical exploration of faults and fracture zones: geoelectrical methods 419
13.4 Geophysical exploration of fracture zones: GPR 429
13.5 Exploration of faults and fracture zones: Geophysical passive methods (self-potential, gravity, magnetic, geothermal and radioactivity methods) 430
13.6 Geophysical exploration of caves 435
13.7 References 437
14 Groundwater quality - saltwater intrusions 440
14.1 Definition 440
14.2 Origin of saltwater intrusions 440
14.3 Electrical conductivity of saline water 443
14.4 Exploration techniques 446
14.5 Field examples 446
14.5.1 Saltwater intrusions in the North Sea region 447
14.5.2 Saline groundwater in the Red Sea Province, Sudan 450
14.6 References 453
15 Geophysical characterisation of aquifers 456
15.1 Definition of hydraulic conductivity and permeability 456
15.2 Hydraulic conductivity related to other petrophysical parameter 457
15.3 Geophysical assessment of hydraulic conductivity 460
15.3.1 Resistivity 460
15.3.2 Seismic velocities 463
15.3.3 Nuclear resonance decay times 464
15.4 Case history: Hydraulic conductivity estimation from SIP data 467
15.5 References 472
16 Groundwater protection: vulnerability of aquifers 476
16.1 General 476
16.2 Vulnerability maps 476
16.3 Electrical conductivity related to hydraulic resistance, residence time, and vulnerability 480
16.4 Vulnerability maps based on electrical conductivity 483
16.5 References 487
17 Groundwater protection: mapping of contaminations 490
17.1 The brownfields problem 490
17.2 Mapping of waste deposits 491
17.3 Mapping of abandoned industrial sites 493
17.4 Mapping of groundwater contaminations 497
17.4.1 Anorganic contaminants 498
17.4.2 Organic contaminants 500
17.5 References 502
Index 506

4 Complex Conductivity Measurements (p. 119-120)

Frank Börner

4.1 Introduction

Geohydraulic properties of aquifers and soils are of general interest for environmental and geotechnical applications. Aquifer and soil characterizing parameters are used for the modelling the water flow and the migration of hazardous substances. Compared to the application of conventional direct methods, non-invasive geophysical measurements have the potential to predict parameter distributions more realistically. Additionally, such measurements are more cost-effective. Geoelectrical surveys have become an increasingly important tool in subsurface hydrogeological applications.

The spatial distribution of electrical parameters of the subsurface can provide valuable information for characterizing the heterogeneity of the groundwater and the soil zone. However, due to various petrophysical influences on electrical rock properties, the attempt to convert electrical conductivity variations to variations of geohydraulic parameters brings often ambiguous results. Geoelectrical measurements can contribute to, e.g.,

* the assessment of aquifer vulnerability and depth to watertable (Kalinsky et al. 1993, Kirsch 2000),

* the determination of catchment areas and aquifer characteristics (hydraulic conductivity, sorption capacity, dominant flow regime, Mazac et al. 1985, Boerner et al. 1996, Kemna 2000, Weihnacht 2005),

* the monitoring of water content and water movement (e.g. Daily et al. 1992, Gruhne 1999, Berger et al. 2001, Berthold et al. 2004, Liu and Yeh 2004),

* the monitoring of changes of water quality and mineral alteration connected with active remedial measures on contaminated sites as well as with natural attenuation processes (Grissemann and Rammlmair 2000, Atekwana et al. 2004).

The complex conductivity measurement is an innovative geoelectrical method which is sensitive to physico-chemical mineral-water-interaction at the grain surfaces. In comparison to conventional geoelectrics a complex electrical measurement can be provide besides conductivity also information on the electrical capacity and the relaxation processes in the frequency range below some kHz. At higher frequencies the electrical behaviour of rock is increasingly determined by physical interactions alone. Complex measurements can reduce the petrophysical ambiguity caused by the influence of textural and state properties. This advantage compensates for the at times expensive and time-consuming measurements. In the context of the general topic the following section focuses on the utilizable petrophysical potential of complex conductivity measurements to provide non-invasively

* geohydraulic parameters like the hydraulic conductivity or the sorption capacity, and

* information to the state and distribution of contaminants in the pore space.

Complex conductivity as well as IP phenomena and measuring techniques are reviewed by, e.g. Wait (1959), Sumner (1976), Klein et al. (1984) or Ward (1990). Olhoeft (1985) presented an overview of the properties and state conditions which influence complex conductivity spectra. The complex electrical properties of porous rocks were investigated by Buchheim and Irmer (1979), Vinegar and Waxman (1984) or Boerner (1992). The effect of elevated pressure on the broad band complex conductivity was discussed by Lockner and Byerlee (1985). Recently Slater and Lesmes (2002a), Klitsch (2004), Ulrich and Slater (2004) focused their research on unconsolidated material as well as on the distributed geometry of rock texture and related relaxation phenomena.