Seismoelectric Exploration: An Intro to New Methods

Seismoelectric Exploration

An Intro to New Methods

Seismoelectric Exploration

An Intro to New Methods

Samir Ganaka

Seismoelectric Exploration

An Intro to New Methods

Samir Ganaka

ISBN - 9789361523861

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Preface

The recent technologies have made a huge impact on humans, and now the techniques are used in the exploration of the ground levels and oil wells on the earth. Hence the book is made to understand how the exploration is done using the different methods by various countries worldwide.

Here the earth is being studied by geophysics, and various other studies are involved in learning the various methods in exploring the earth. Thus, the land and marine habitats are studied and help us understand the various advantages of seismic exploration in geophysics.

The various surveys are carried out to know about the geophysical status of the ground and water and oil levels in the earth era. The recent advancements made in geosciences have given new hope for geologists in conducting researches.

This seismoelectric exploration is done to study earthquakes, and the seismic waves are created due to the disturbances, and the instruments obtain the data, and the study is done to know the various resources underneath.

The data obtained is thus used in various studies to carry out research and know the various benefits, thus understanding the disturbances caused to the earth's crust. There are several disadvantages along with advantages.

The habitats of the animals are disturbed and also cause various pollutions, and thus the earth is shaken due to the various activities during the exploration. But the data obtained help the humans in the various levels.

The instruments used and how the survey is carried out in recent times and the past the methods are discussed, and various methods and experiments used are studied and discussed in the chapters.

The book is made in point of the various seismoelectric exploration methods carried out in various countries, and the methods used are discussed, and the advantages of exploration to mankind and disadvantages to the animals.

Table of CONTENT

Chapter 1. Seismoelectric Exploration Process 1

1.1 Seismoelectric exploration 1

1.2 Seismoelectric exploration approaches in different countries 7

1.3 Modern seismoelectric exploration approaches made during the

recent times 10

1.4 Summary 14

1.5 Exercise 15

References 16

Figure Resource 18

Chapter 2. Various Seismoelectric Exploration 19

2.1 Types of seismoelectric exploration 19

2.2 Instrumentation of seismoelectric exploration 28

2.4 Advantages of seismoelectric exploration 32

2.5 Disadvantage of seismoelectric exploration 33

2.6 Summary 34

2.7 Exercise 36

References 37

Figure Resource 38

Chapter 3. Seismoelectric Exploration Role In Geophysics 39

3.1 Important role of seismoelectric exploration 39

3.2 seismoelectric exploration equipment 41

3.3 Seismoelectric exploration survey methods 48

3.4 Recent study made on seismoelectric exploration 50

3.5 Summary 53

3.6 Exercise 55

References 55

Figure Resource 56

Chapter 4. Research On Seismoelectric Exploration 58

4.1 Various benefits of seismoelectric exploration 60

4.2 Guidelines by Researchers about seismoelectric exploration 62

4.3 Articles on seismoelectric exploration 69

4.4 Summary 70

4.5 Exercise 71

References 71

Chapter 5. Seismoelectric Exploration Findings 75

5.1 Theories on seismoelectric exploration 77

5.2 Experiments on Seismoelectric Exploration 81

5.3 Applications of seismoelectric exploration 86

5.4 Summary 89

5.5 Exercise 90

References 91

Figure Resource 92

Chapter 6. Ground Water Investigation Using The Seismoelectric Method 93

6.1 Groundwater and underground excavation 98

6.2 statistical information on seismoelectric exploration 105

6.3 Aqua seismoelectric groundwater survey 111

6.4 Summary 124

6.5 Exercise 125

References 125

Figure Resource 127

Chapter 7. Experimental Surveys On Seismoelectric Exploration 128

7.1 Seismoelectric exploration in India 139

7.2 Seismoelectric exploration in England 143

7.3 Seismoelectric exploration in Russia 176

7.4 Summary 180

7.5 Exercise 181

References 182

Figure Resource 183

Chapter 8. Anatomy Of Seismoelectric Exploration 184

8.1 Projects on seismoelectric exploration 186

8.2 Seismoelectric monitoring of producing oil wells 190

8.3 Aquifer exploration using seismoelectric 192

8.4 Summary 195

8.5 Exercise 197

References 197

Figure Resource 199

Glossary 200

Index207

An Overview on Seismoelectric Exploration

Seismoelectric methods are based upon the physical properties of the earth that produce electrical signals from seismic waves. Electrokinetic sounding (EKS) is one such method that has great potential for hydrogeological studies as the signal arises from the movement of pore fluids under seismic excitation.

In principle, the method should directly map changes in hydraulic permeability, rock porosity, and fluid chemistry. Several researchers have recently tried to exploit the phenomenon in groundwater problems where conventional methods worked poorly. However, publications of successful case histories to support the theory are rare.

This may be due to the very weak amplitudes of the electrical signals generated from the seismic wave, which are millivolts to nanovolts in magnitude, and the presence of cultural noise, which is usually much greater in magnitude.

Scientists demonstrated electrokinetic responses from formations more than 50 meters deep in two test areas in Western Australia. One is over a saline palaeo channel, and the other is over a freshwater aquifer. The seismic waves were generated from a sledgehammer source, and acoustic and electrical data were recorded by a seismic acquisition system.

Seismic refraction and reflection data provide seismic velocity information for depth conversion and support the seismoelectric data. The data were then compared to borehole logs to find what physical contrasts were detected. Significant hydrogeological boundaries were detected up to 50 m deep in saline groundwater conditions and at least 100 m deep in freshwater aquifers.

Electrokinetic effects induced by seismic waves are of interest for their sensitivity to the pore fluid content, porosity, and permeability of porous and fractured rocks and sediments. Two dominant types of signals, known as coseismic and interfacial seismoelectric effects, have been predicted by theoretical modeling and confirmed by measurements since the early 1990s.

The development of practical applications for these phenomena has progressed more slowly due to the challenges involved in their routine measurement. The common sources of noise encountered and approaches that have been developed to deal with them. Recent trends in surface and borehole field experiments are also briefly discussed in The following chapters.

Seismoelectric and electro-seismic interface responses resulting from the electrokinetic effect are useful for studying the subsurface medium's properties. Researchers investigated the characteristics of these interfacial signals generated at irregular subsurface interfaces considering hydrocarbon exploration scenarios.

Adopting several typical models, the electro seismic and seismoelectric wavefields are calculated using the finite-difference frequency-domain method. Besides the well-known electro seismic and seismoelectric signals created at a flat interface, the scattered seismic wave and scattered electromagnetic (EM) wave can also be generated by EM and seismic sources subsurface scattering points, respectively.

When an electric source is applied for excitation, the waveforms recorded by horizontally- or vertically-aligned receiver array indicate that electro seismic interface responses nearly do not change with the source locations and can directly delineate the shapes and morphologies of the corresponding interfaces.

Simulations of seismoelectric wavefield show that both the interface seismoelectric responses and scattered EM waves display flat events in the electric record. It is not easy to distinguish them if we do not know the realistic underground structure. The electro seismic interface responses seem more promising than the seismoelectric interface responses for imaging the subsurface interfaces based on simulations.

The electro seismic signals can be directly used to delineate the shapes and morphologies of subsurface complex interfaces. And electro seismic interface responses seem more promising than seismoelectric interface responses in imaging the subsurface interfaces.

Considering hydrocarbon exploration scenarios, we apply the frequency-domain finite-difference method to calculate the electro seismic and seismoeletric wavefields in typical irregular models.

The electro seismic and seismoelectric interface responses generated at those complex hydrocarbon subsurface interfaces are studied.

Chapter 1. Seismoelectric Exploration Process

1.1 Seismoelectric exploration

Over the last few decades, there have been significant advances and innovations in energy exploration and development. Seismic survey technology has been one of the greatest driving forces behind the energy exploration and development revolution, providing some of the greatest investment returns.

From the exploration phase to the first well drilled and throughout the asset’s life, seismic survey technology is making its mark across the entire lifecycle of energy development.

For oil and gas exploration, images of the earth’s subsurface created from geophysical data can shed light on potential drilling hazards to ensure it is as safe, reliable, and efficient as possible.

Analysis of subsurface in advance of drilling is only available through seismic techniques and assists in designing well trajectories that can reach the resource reservoir while avoiding any hazardous zones that could cause potentially serious issues.

Companies also use seismic to perform hazard surveys to look for geologic hazards on the sea bottom and in the shallow subsurface that could affect drilling a well. Once oil or gas is found and a reservoir is being developed and produced, seismic images increase the understanding of the reservoir’s characteristics and optimize development plans. The use of seismic data leads to more efficient oil and gas extraction requiring fewer wells while increasing hydrocarbons production.

Land seismic operations are similar to marine operations in that the energy sources are acoustic energy generated using vibrators mounted on trucks or low-impact charges placed in shot holes that truck-mounted or portable drills have drilled.

The receivers are typically geophones like small microphones pushed into the soil to measure the ground motion.  Onshore seismic are used in sensitive locations without damaging buildings or the environment.

Seismic data is vital in identifying potential hydrocarbon resources and is also an important source for planning the exploration and development of complex geological structures containing unconventional hydrocarbon resources.  The term unconventional refers to the methods used and the types of rock from which oil and natural gas are produced. 

Regardless of the production process or where they come from, unconventional oil and natural gas are essentially the same as their conventional counterparts and depend on seismic survey data.

As technological advances have enabled the extraction of oil and natural gas in complex geological structures, advancements in seismic survey technology have allowed for the discovery and identification of tight gas and other sweet spot formations that are more difficult to discover with 2D land seismic.

Land seismic data acquisition uses primarily two types of seismic sources, non-impulsive vibroseis vehicles or an impulsive energy source such as a low-impact charge—that generate acoustic waves which propagate deep into the earth.  Each time an acoustic wave encounters a change in the rock formation, part of the wave is reflected on the surface, where an array of sensors records the returning signals.

Seismoelectric exploration definition

The seismoelectrical method is based on the generation of electromagnetic fields in soils and rocks by seismic waves. This technique is still under development, and in the future, it may have applications like detecting and characterizing fluids in the underground by their electrical properties, among others, usually related to fluids (porosity, transmissivity, physical properties).

When a seismic wave encounters an interface, it creates a charge separation at the interface, forming an electric dipole. This dipole radiates an electromagnetic wave that can be detected by antennae on the ground surface.

As the seismic waves stress earth materials, four geophysical phenomena occur:

1. The resistivity of the earth materials is modulated by the seismic wave;

2. Electrokinetic effects analogous to streaming potentials are created by the seismic wave

3. Piezoelectric effects are created by the seismic wave and

4. High-frequency, audio-, and high-frequency radio frequency impulsive responses are generated in sulfide minerals

The dominant application of the electro seismic method is to measure the electrokinetic effect or streaming potential. Electrokinetic effects are initiated by sound waves passing through porous rock, inducing the rock matrix and fluid’s relative motion.

The ionic fluid’s motion through the capillaries in the rock occurs with cations preferentially adhering to the capillary walls so that applied pressure and resulting fluid flow relative to the rock matrix produces an electric dipole. In a non-homogeneous formation, the seismic wave generates an oscillating fluid flow and a corresponding oscillating electrical and EM field. The resulting EM wave can be detected by electrode pairs placed on the ground surface.

However, P-waves moving through a solid containing some moisture also generate an electric phenomenon called coseismic waves. The coseismic waves travel with P-waves and are not sensitive to the electrical properties of the subsurface. The dipole antenna cannot distinguish electrokinetic signal from the coseismic signal, so it records them both, and coseismic waves must be removed while processing field data to be able actually to interpret the electrokinetic effect

At the moment, there is not a field routine operation method, but in scientific studies, an array of several dipole antennas is placed along a straight line to record seismoelectric waves and an array of geophones placed between dipole antennas to record seismic wave arrivals. Geophones are necessary to suppress coseismic waves from the seismoelectric signal so that the electrokinetic effect can be separated and studied.

Figure 1.1 Seismoelectric exploration

Seismoelectric effects are electromagnetic signals that arise when seismic waves stress earth materials. At least four are of interest to geophysicists:

1. the modulation by seismic stress of the resistivity of the earth through which steady currents flow

2. seismically induced electrokinetic effects analogous to streaming potentials

3. the piezoelectric effect and

4. highly non-linear processes

5. that generate high audio frequency and radio frequency impulsive responses in sulfides.

Figure 1.2 Seismic frequency

Seismically induced electrokinetic effects arise in porous media because of the electric double layer at a solid-liquid interface – a layer of ions adsorbed on the solid matrix and a parallel, diffuse layer of counter-ions in the pore fluid. Part of the diffuse layer is free to move with the pore fluid. Thus, the motion of pore fluid relative to solid that accompanies P-wave propagation leads to a macroscopic separation of electrical charge between compressed and rarefied regions in the seismic wavefront.

This charge separation gives rise to coseismic electrical fields that are contained within the traveling seismic wave and exhibit amplitudes dependent on the electrical and mechanical properties of the host medium. Also, abrupt distortions in the charge distribution caused by the P-wave impinging on a boundary can give rise to the second type of seismoelectric effect, which is distinct in that it radiates as an EM field.

Such interfacial seismoelectric effects will be received essentially simultaneously by widely separated antennas at an arrival time given by the one-way seismic travel time to the interface. In the case of a horizontal interface, the source can be approximated as a vertical electrical dipole centered on the interface directly below the shot.

Horizontal grounded dipole receivers located on the earth’s surface will observe a signal with opposite polarity on opposite sides of the shot and with maximum amplitude at an offset equal to one-half of the interface depth. Of the two types of seismoelectric signals, the one which has received the most attention as a potential exploration tool is the interfacial effect.

Although the weaker of the two can provide important information about formations at depth, while the coseismic can only provide information about the soil or rock properties in the vicinity of the electrical receivers.

Measurements of interfacial effects have been reported by several investigators, and a handful of reports have demonstrated that they can indeed be used to map boundaries. The reported amplitudes of interfacial effects measured using grounded dipoles on the earth’s surface have ranged from an exceptionally strong 1 mV/m for the case of an interface at 3 m depth and a sledgehammer source to as low as 60 nV/m for the case of an interface at 68 m depth and a 1 kg explosive source. Most recently, a 40 kg accelerated weight drop source and a 26 channel recording system to map the water table and interfaces in the overlying vadose zone in a sandy aquifer near Perth.

Figure 1.3 Seismic source

The clear interfacial signals emanating from the water table at about 14 m depth had amplitudes on the order of 1 mV/m. In our experience, it is desirable and presently possible to achieve background noise levels of 0.1 mV/m or less in stacked, processed shot records acquired for near-surface applications a much lower noise floor of 0.1 nV/m might eventually be achievable in final stacked seismoelectric sections. Apart from ambient and instrumentation-related noise sources, coseismic effects can be a significant source of interference in surveys targeting interfacial effects. The coseismic effects are broadly analogous to surface wave interference in seismic reflection records.

They may be attenuated relative to interfacial effects that appear flat on shot records using wavefield separation (velocity) filtering and multi-channel stacking, provided an adequate number of channels is available.

Alternatively, in some applications, such as borehole or cross-hole surveys, source-receiver geometries may be selected to ensure coseismic effects do not arrive until after the interfacial signals of interest. In the last section, we discuss the benefits of vertical seismoelectric profiling in boreholes as a complement or alternative to surface surveys. The approach offers several advantages in S/N and can also use the coseismic signal for borehole logging.

The most common type of data acquisition system used to acquire seismoelectric signals is seismographs. The high dynamic range and channel capacity of modern exploration seismographs open up many avenues for S/N improvement through post-processing. These systems on their own, however, are not suitable for the systematic acquisition of high-quality seismoelectric data.

Seismographs are designed to be interfaced to geophones, which present low source impedances of a few hundred ohms. As such, the input impedance of the seismograph is commonly no higher than about 20 kΩ.

Unfortunately, electrode contact resistances in most soils are significantly greater than geophone impedances, and thus a voltage divider problem can arise as earth currents are partially shunted through the seismograph inputs. For a source impedance of 20 kΩ at the dipole, not uncommon at many sites, only half the voltage will be seen at the acquisition system.

Figure 1.4 the data of Seismoelectric exploration

Furthermore, as the source impedance rises, bandwidth is reduced due to the low-pass filter formed by the source impedance and the cable and seismograph’s combined input capacitance typical filter responses we would expect given our Geode seismograph’s input impedance of 20 kW in parallel with 20 nF and a seismic cable capacitance of 100 pF/m. The filter would vary with dipole source impedances of 1 to 50 kW for a 100 m length of cable, while the lower plot shows the more subtle effect of cable lengths ranging from 5 to 100 m.

It is also important to ensure good isolation between channels, which can be a challenge given the normal design of seismographs’ input stage if the contact impedance is much larger than the input impedance. The use of buffering amplifiers resolves these problems and thus offers a signal with a consistent spectrum, greater channel isolation, and better signal strength.

1.2 Seismoelectric exploration approaches in

different countries

The first work that contributed to the development of the electro-seismic effect was done in 1944 by Frenkel. He described the relative flow of fluid to the matrix brought about by the passage of a seismic compression wave through the medium. He investigated the induced electric fields generated by this relative motion of fluid matrix interaction with Helmholtz-Smoluchowski equations.

However, his investigations did not fully explain this relationship. In 1964, Biot made further progress by developing theories that predicted a seismic wave movement through a saturated porous media. Various advances toward developing a general equation describing the link between the relative fluid matrix interaction, and the electromagnetic fields induced by this motion, were formulated between 1962 and 1994.

These developments include irreversible thermodynamic coupling effects in porous media and averaging fluid volume to determine the electro seismic effect’s governing equations.

In 1995, Haartsen and Pride explained the electromagnetic field induced by the fluid motion relative to a porous matrix as being generated by current dynamic imbalances. These current imbalances are generated by plane sheer waves moving across and interface between rocks with different electro seismic properties.

These net current imbalances induce an electromagnetic field that can be read at the surface as an interface response. However, suppose the plain sheer waves pass through a homogeneous saturated medium, with no interfaces of different electrokinetic properties. In that case, the net currents induced will be balanced and cancel each other out.

This essentially means there is no current flow induced by the relative motion of the fluid and matrix. This means no electromagnetic fields are induced that can be read at the surface. In 1997 Haartsen and Pride used their findings on electromagnetic interface response to investigate electro kinetic waves from single point sources in layered rock formations.

They discovered saturated media interfaces produced a response equivalent to that of a dipole-induced field on the interface directly under the seismic point source. In 1980, Chandler used a theoretical model and saturated core samples in laboratory experiments to relate the rise time of electro seismic signals to permeability. However, Haartsen in 1998 proved that the electro seismic response is a function of the salinity, porosity, and permeability of a porous elastic media.

Reflections and refractions of seismic waves at geologic interfaces within the earth were first observed on recordings of earthquake-generated seismic waves. The basic model of the earth’s deep interior is based on observations of earthquake-generated seismic waves transmitted through the earth’s interior

The use of human-generated seismic waves to map the geology of the upper few kilometers of the earth’s crust followed shortly after that and has developed mainly due to commercial enterprise, particularly the petroleum industry.

Seismic reflection exploration grew out of the seismic refraction exploration method used to find oil associated with salt domes. Ludger Mintrop, a German mine surveyor, devised a mechanical seismograph in 1914 that he successfully used to detect salt domes in Germany. He applied for a German patent in 1919 that was issued in 1926.

In 1921 he founded the company Seismos, which was hired to conduct seismic exploration in Texas and Mexico, resulting in the first commercial discovery of oil using the seismic refraction method in 1924. 

The 1924 discovery of the Orchard salt dome in Texas led to a boom in seismic refraction exploration along the Gulf Coast, but by 1930 the method had led to the discovery of most of the shallow Louann Salt domes and the seismic refraction method faded.

The Canadian inventor Reginald Fessenden was the first to conceive of using reflected seismic waves to infer geology. His work was initially on the propagation of acoustic waves in water, motivated by the Titanic sinking by an iceberg in 1912.

He also worked on methods of detecting submarines during World War I. He applied for the first patent on a seismic exploration method in 1914, issued in 1917. Due to the war, he was unable to follow up on the idea. John Clarence Karcher discovered seismic reflections independently while working for the United States Bureau of Standards on sound-ranging methods to detect artillery.

In discussion with colleagues, the idea developed that these reflections could aid in the exploration of petroleum. Many affiliated with the University of Oklahoma, Karcher helped to form the Geological Engineering Company, incorporated in Oklahoma in April 1920. The first field tests were conducted near Oklahoma City, Oklahoma,