Dual Frequency Induced Polarization Method: An Effective Approach for Mineral Exploration

By Jishan He

Dual Frequency Induced Polarization Method: An Effective Approach for Mineral Exploration provides the first English-language text on the successful but little-known dual frequency induced polarization method. Engineers and technicians in geophysical exploration will appreciate learning about this enhanced method in sections that comprehensively explain the basic principle, method, technology and application of the dual frequency induced polarization method. Chapters cover the mathematical basis, observation parameters, instrument principle, application essentials, field work methods, technology, interference factors and their overcoming methods, ore prospecting and engineering application examples, and more.

In particular, the needs of frontline engineering and technical personnel are addressed in chapters specifically related to field work. Researchers, engineers and technicians working in geophysical exploration will discover a thorough and detailed description of all aspects of the dual frequency induced polarization method.

• Comprehensively discusses the basic principle, method, technology and application effect of dual frequency IP method
• Covers field work methods, techniques, interference factors and overcoming methods, ore prospecting, and engineering application examples, etc.
• Considers the needs of front-line engineering and technical personnel

• Language: English
• Publisher: Elsevier Science
• Release date: Nov 15, 2023
• ISBN: 9780443221378

Jishan He

Professor in School of Geosciences and Info-physics at Central South University. He was elected as one of the first academicians of the Chinese Academy of Engineering in 1994, and once served as president of Central South University of Technology, member of the presidium of the Chinese Academy of Engineering, and now is a lifelong member of the American Association of Exploration Geophysics. He established and developed the theory and method of Dual-frequency IP method and Pseudo-random signal electromagnetic method. His theories, methods and instruments have been applied throughout China, and a large number of minerals have been found.

Book preview

9780443221378_FC

Dual Frequency Induced Polarization Method

An Effective Approach for Mineral Exploration

First Edition

Jishan He

Central South University, Changsha, China

Table of Contents

Cover image

Title page

Copyright

Biography

About the author

Preface

Chapter 1 Principles of dual frequency induced polarization method

Abstract

1.1 Introduction

1.2 Physical and chemical interpretation of IP phenomenon

1.3 IP characteristics of various ores and rocks

1.4 Frequency characteristics and measurements of induced polarization

1.5 IP characteristics of rocks (ores) in frequency domain and affecting factors

Chapter 2 Equivalence principles and parameters in induced polarization measurements

Abstract

2.1 Macroscopic approaches for studying IP and LTI systems

2.2 Equivalence principles of induced polarization

2.3 Measurement parameters in frequency domain IP methods

Chapter 3 Measurement methods and characteristics of DFIP

Abstract

3.1 Measurement plan of DFIP

3.2 Waveforms of dual frequency currents

3.3 Anomalies in DFIP

3.4 Anti-interference ability of DFIP

3.5 Characteristics of DFIP

Chapter 4 DFIP instrumentation

Abstract

4.1 Parameters and performance characteristics of DFIP instruments

4.2 Work principles of SQ-3C portable IP instrument

4.3 Structure of DFIP instruments

4.4 Maintenance and service

Chapter 5 Removal and utilization of electromagnetic induction coupling in IP measurements

Abstract

5.1 Classification of EM induction couplings

5.2 Capacitance coupling and its representation in IP measurements

5.3 EM coupling and its time characteristics in IP measurements

5.4 Principles of chopping wave decoupling and selection of the width of chopping wave

5.5 Chopping wave effects on DF wave distortion and EM decoupling

5.6 Direct, simultaneous, and respective extractions of IP and EM effects

Chapter 6 Special performance of nonlinear effect in dual frequency spectrum induced polarization

Abstract

6.1 Electrochemical mechanism of nonlinear effect of IP effect

6.2 Equivalent circuit at interface between mineral and solution

6.3 Overpotential response of equivalent circuit

6.4 Theoretical calculations and model experimental results of IP frequency spectrum curves of cathode and anode

6.5 Nonlinear effect of DFIP spectrum

Chapter 7 Spatial distribution characteristics of DFIP anomaly

Abstract

7.1 Surface polarization field of a buried sphere in a homogeneous alternating current field

7.2 IP anomaly of a sphere with impregnated spherical shell in a homogeneous electric field

7.3 Vein ore body in a homogeneous alternating electric field

7.4 Surface-polarized spherical body in a point-source field

7.5 IP field of a surface polarized infinitely long cylinder

7.6 Experimental dipole profiling curves of several IP bodies with regular shape

7.7 Comparison of several profiling arrays

7.8 Anomaly characteristics of IP sounding

Chapter 8 Field working

Abstract

8.1 Electrode arrays

8.2 Selection of observation frequencies

8.3 Power supply system

8.4 Measurement circuit

8.5 Electrode effects and EM coupling in IP measurement

8.6 Interference and their removal in IP measurements

8.7 Evaluation of measurement accuracy in IP measurement

8.8 Measurements of rock electrical parameters F and ρ

8.9 The illustrations of observation result

8.10 Some experiences in field measurements

Chapter 9 Applications of DFIP method

Abstract

9.1 The application of DFIP in gold and silver ore exploration

9.2 Application of DFIP in exploration of copper, lead, and zinc polymetallic ores

9.3 Application of DFIP in other minerals

9.4 Application of DFIP in groundwater exploration

9.5 Applications of DF or TF (triple-frequency) phase measurements

Index of figures and tables

Bibliography

Index

Copyright

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Biography

Unlabelled Image

Jishan He is a professor in the School of Earth Science and Info-physics at Central South University. He was elected to be the founding member of the Chinese Academy of Engineering in 1994. He was the president of Central South University of Technology (1992–97), the vice president of the Nonferrous Metals Society of China, the vice president of the Geophysical Society of China, the director of the Energy and Mining Department of the Chinese Academy of Engineering, and the chairman of the Hunan Provincial Association for Science and Technology. He is now the honorary chairman of the Hunan Provincial Association for Science and Technology, a member of the fifth presidium of the Chinese Academy of Engineering, and a lifelong member of the Society of Exploration Geophysicists.

He proposed the closed addition rules of the three element set, realized the fast recursive coding of the 2n series of pseudorandom signals, and created the electrical method system of pseudorandom signals. He established the flow field fitting method for the high-resolution detection of the piping and leakage inlet of a dam, which provides essential technical support and a scientific emergency decision-making basis to detect hidden dangers in reservoirs and dams in flood seasons. He unified the definition and algorithm of the full-region resistivity of the frequency-domain electromagnetic method, created the wide field electromagnetic method, and opened a new research field for electromagnetic exploration. He created China's first national key discipline characterized by a geoelectric field and observation system, invented a series of geoelectric field observation instruments and equipments, and made significant contributions to China's resource exploration and engineering exploration.

The theoretical system of exploration geophysics characterized by the pseudorandom signal electromagnetic method and the dual-frequency induced polarization method, which he founded and developed, is internationally recognized as a major event in the field of applied geophysics.

His research achievements and academic thought have provided important guiding significance for the development of geophysics in China.

About the author

Jishan He was born in September 1934 during the Japanese invasion of China. War-related interruptions led to the cessation of his primary and secondary education, as did his 5 years spent working in a tungsten mine. He studied at the Department of Applied Geophysics of Changchun Geologic Institute. After graduating in 1960, he joined the Central South University of Mining and Metallurgy, now called Central South University (CSU), where he dedicated himself to the advancement of geophysics.

Professor He established China’s first national key discipline of a geoelectric field and observation system. His invention of geoelectric field observation equipment led to significant contributions to resource exploration and geophysical engineering in China. He also developed and established the exploration geophysical theory system, featuring the pseudorandom signal electromagnetic method and the dual-frequency IP method, which is recognized as a major achievement in the field of applied geophysics worldwide.

His accomplishments led to his appointment as a founding member of the Chinese Academy of Engineering (CAE) in 1994. He has also served as the president of the former Central South University of Technology (now CSU), vice chairman of the Nonferrous Metals Society of China, vice chairman of the Chinese Geophysical Society, director of the Energy and Mining Engineering Department of CAE, and chairman of the Hunan Association of Science and Technology. He is currently a professor of geophysics at CSU, honorary chairman of the Hunan Association of Science and Technology, and a lifetime member of the Society of Exploration Geophysicists.

The author has been dedicated to research on applied geophysical theory, methods, and observation systems for an extended period. The author founded and developed the dual-frequency induced polarization theory, and the subsequently designed dual-frequency induced polarization instrument has since become one of the fastest, most precise, compact, and cost-effective instruments for mineral resource surveying worldwide. The main purpose of this book is to discuss the development and success of the dual frequency induced polarization method for mineral exploration in China.

Due to the author’s limited knowledge, this book may inevitably have errors. We welcome corrections from readers.

Jishan He, School of Geosciences and Info-Physics,Central South University, Changsha,Hunan, China

Preface

Of geophysical exploration methods, the induced polarization (IP) method is the most effective one in mineral resource exploration. Compared with resistivity methods, it has an important advantage in that distinct anomalies can only be induced by the electronic conductors rather than by the topography or heterogeneity of nonpolarizable rocks. Studying the variation of the IP effect with time is called the time domain IP method while studying the variation of the IP effect with frequency is called the frequency domain IP method. Both the time characteristics of the charging and discharging curves in the time domain IP show a frequency dependence. Therefore, the frequency domain IP is, in essence, equivalent to the time domain IP. However, they differ in method so that in practice, there are many differences between them. For the time domain IP, the equipment is heavy and field measurements are costly, so its application is limited. However, the frequency domain IP has essential advantages such as portable equipment and strong anti-interference ability, and thus high efficiency and low costs. However, in history, it was the variable frequency method that was used to carry out frequency domain IP measurements; this has disadvantages such as low measurement accuracy and low efficiency so that its application is still limited.

As early as 1934, S. Rose discovered that the effective conductivity of rocks, when measured using alternating current, becomes a complex variable. Prior to 1950, all electrical stimulation methods employed time-domain measurements. In the 1950s, China introduced time-domain electrical stimulation methods from the Soviet Union, and in the 1960s, frequency-domain electrical stimulation methods were introduced and referred to as alternating current electrical stimulation, while time-domain methods were renamed as direct current electrical stimulation. However, some methods claiming to be direct current electrical method actually used bidirectional short pulse currents, which clearly belong to alternating current electrical stimulation, making the term direct current induced polarization method inappropriate. Because AC electrical stimulation essentially studies the temporal variations of stimulation effects, it is necessary to induced polarization method as time-domain induced polarization method. Methods such as frequency-domain induced polarization method, phase induced polarization method, complex resistivity method, spectral induced polarization method, and dual frequency induced polarization method all investigate the relationship between electrical stimulation parameters and frequency, and naturally fall into the category of frequency-domain induced polarization method.

In 1950, I.S. Collett and H.O. Seigel proposed a method to measure the IP effect by using alternating current with different frequencies. In the autumn of 1950, J.R. Wait successfully carried out the first field experiment in frequency domain IP in the United States. In his monograph Variable Frequency Method, he thoroughly discussed and analyzed the principles and field measurements. Since then, the method in which the frequency is varied to measure the IP effect is called the variable frequency method, and Wait is called the father of the variable frequency method. In the 1960s, Saizhen Zhang et al. introduced the variable frequency IP method into China by translating the corresponding overseas literature, which led to a wave of people making variable frequency IP method equipment in a number of universities and field exploration teams. The author of this book based in the Central-South Institute of Mining and Metallurgy invited Hongji Yuan, Chunzhi Tang, Mingxian Li, and Lei Zhang to set up a research and development center for developing variable frequency IP method equipment. In 1977, the DBJ-1 variable frequency IP potential instrument was successfully developed. It was officially approved by the Geological Bureau of China and manufactured by the Shanghai Geological Instrument Factory in 1978. It was the first-generation variable frequency domain IP instrument officially manufactured in China; it performed better than the frequency domain IP instrument, P-670, made in Canada. However, it was not widely accepted and used in China because it was still a variable frequency instrument. In principle, as mentioned above, the essential advantages in frequency domain IP over time domain IP are lighter equipment and a strong anti-interference ability, leading to high accuracy and high efficiency. However, those advantages are all neutralized because frequency has to be varied during measurements.

The author of this book then realized that there was no future in just copying the overseas ideas, and that those advantages in frequency domain IP can only be fully exploited if a better IP measurement plan than variable frequency can be proposed and a more advanced frequency domain IP instrument with its own intellectual property could be developed. In fact, since 1976, the author of this book has been studying the dual frequency IP method, for which an instrument was needed. A dual frequency current transmitter with 0.3 Hz and 3.9 Hz currents was then developed with self-raised funds. Trough water tank and field experiments were successfully carried out by supplying a dual frequency current using the transmitter and measuring the high and low potential differences using a self-made variable frequency receiver. Afterward, a dual frequency receiver was designed that consisted of five parts: the common channel, the high frequency channel, the low frequency channel, the logic control circuit, and the digital display circuit. Xuehui Wang, Jiaju Chao, and Chenfang Tian were invited to take part in a circuit test. In early 1978, a dual frequency receiver was successfully developed, and thus the first dual frequency instrument was born. Because it is used to measure the dual frequency amplitude, it is, therefore, called the dual frequency amplitude frequency instrument. It has essential advantages such as high portability, high efficiency, strong anti-interference ability, and high measurement accuracy.

In 1979, to promote the dual frequency IP method, the author of this book invited Guangda Du and Jinglebo Weng from the Geophysical Exploration Team in Heilongjiang province, Zonglan Wang from the Geophysical Exploration Team in Liao Ning province, Zhenxiang Yu from the Geophysical Exploration Team in Yunnan province as well as Guangshu Bao and Youshan Zhang at the same institute to form a nonofficial research and development network. Later, Yicheng Bai and Huiru Pu from the Geological Exploration Team in Ganshu province also joined. As the author of this book was teaching the principles and instrumentation of dual frequency IP, other members were learning and assembling instruments until one dual frequency amplitude frequency instrument was made for every province involved. These instruments were then taken to the provinces and all were successful in field measurements. In 1980, the instrument was officially approved by the Geological Bureau, Ministry of Metallurgical Industry of China, and manufactured by the Sanming Secondary Wireless Equipment Factory in Fujian province. It was promoted and so widely applied in China that a large number of mineral deposits were found. As a result, it was awarded the third prize of the National Award for Technological Invention in 1985.

Being a self-initiated project that has received national-level recognition, is already a great honor, and many believe that there is no need to continue further; however, the author deeply understands that the Dual frequency IP method is particularly suitable for the national conditions of China and can help uncover more mineral resources in the country. The advantages of the Dual frequency IP method have not been fully acknowledged yet, and if we were to stop at this point, it would be a premature abandonment. Therefore, the author intends to continue seeking funding for further research and promotion. He made the following achievements in instrumentation: in 1983, he invented the anticoupling dual frequency IP instrument in which induction coupling can be automatically eliminated by using chopping waves; in 1987, he invented the F-1 frequency domain (spectrum) instrument that can provide a series of dual frequency waves from 0.028 to 32 Hz and measure the dual frequency amplitude and phase or real and imaginary components–this can be done not only for dual frequency IP but also for frequency spectrum IP; and in the 1990s, he invented the pseudorandom IP instrument; in the meantime, the author carried out a series of studies in the theory, method, and corresponding technologies including special nonlinear phenomenon in dual frequency IP, a study of the automatic elimination of induction coupling, and a study of the field working procedure and corresponding technologies. He then drew up the standard for field measurements. Thus, a whole system of dual frequency IP has been established, which is called the dual frequency induced polarization method, in which the advantages of the frequency domain IP can be fully exploited. As a result, the instruments have been promoted and applied in every province, city, and autonomous region in China, except for Taiwan. As a result, large quantities of various natural resources have been found including gold, silver, copper, zinc, molybdenum, manganese, coal, and underground water. Moreover, it has been used in solving engineering problems. In 1995, the work Dual Frequency Induced Polarization Method and its Applications in China was awarded the second prize of National Award for Science and Technology Progress.

In 1996, the key projects in the ninth 5-year plan, the study of a large-power IP system for deep exploration and the study and development of a pseudorandom signal electromagnetic method and corresponding multifunctional instrument, were assigned by the Ministry of Science and Technology of China. We developed medium- and large-power transmitters and corresponding methods and technologies so that a measuring depth of greater than 500 m could be achieved by dual frequency IP. While in pseudorandom signal IP, multifrequency currents can be supplied and multiparameters can be measured simultaneously. The former was awarded the outstanding project in the ninth 5-year plan by the Ministry of Science and Technology of China while the latter was awarded the first prize for Science and Technology Progress by the State of Bureau of the Nonferrous Metal Industry in 2002.

From March to April 2000, comparison trials at three known profiles over the Shichao copper mine in Yanqin County, Beijing, were organized by the China Geological Survey Bureau. These trials involved a dozen instruments of different exploration methods. These trials showed not only that the anomalies expressed by dual frequency IP method were in good agreement with those of the known profiles but also that the dual frequency IP instrument had the advantages of high portability, high measurement speed, strong anti-interference ability, and high measurement accuracy. From the results of the comparison trials, the dual frequency IP method was taken as a very effective method for mineral exploration, and demonstration measurements were arranged by the China Geological Survey Bureau from 2000 to 2004. In 2000, the assigned demonstration measurement called high-efficiency reconnaissance in high mountain areas was carried out over the Shijuli mine, which is 4000 m above sea level, in the south of Qilian Mountain, Gangshu Province. In 2001, the assigned demonstration measurement called high-efficiency reconnaissance in high mountain areas was carried out in the tropical rainforest in Lijiang and Zhongdian, Yunnan Province, which is 3800 m above sea level with a relative height difference of up to 1500 m. In 2002, the Qulong mine in Tibet, which is about 5000 m above sea level and has no human habitation (snow mountain part) was selected for the assigned demonstration measurement called high-efficiency reconnaissance in tough mountain areas. Moreover, the Lulongqiao iron ore mine in Anhui Province, Jinchuan mine, Ganshu Province, and Tuwu mine in Xinjiang Province were selected for experiments in deep exploration. All these demonstrations and experiments were very well fulfilled and excellent geological results were achieved with a higher measurement accuracy than the variable frequency IP method and with a time only a third or two-thirds that by using the variable frequency IP method.

Although the electronic elements that are as advanced as possible are used and the regeneration period is as short as possible for the IP instruments made in Western countries, the whole plan is still based on the variable frequency IP method. Somebody might say that there are well-developed transportation systems in most countries so that portability might not be so important as in China, but that is not true. In 1995, geophysicists from the Beijing General Institute of Mining and Metallurgy were only able to take dual frequency IP instruments to a mountain area in Bolivia that was more than 4000 m above sea level. They successfully made measurements while a number of pieces of equipment made in Western countries were too heavy to be employed. Dual frequency IP instruments also clearly showed their advantages in countries such as Iran, Malaysia, and Australia.

Today, however, there are some misunderstandings that still prevent full applications of the dual frequency IP method:

(1)There is still an opinion that the time domain IP is the orthodox approach. This has been proved wrong as rigorous theoretical studies and a large number of comparisons of field measurements have verified that dual frequency IP is equivalent to time domain IP in expressing IP anomalies.

(2)The secondary potential difference of time domain IP is measured when the excitation current applied is removed in the time domain IP so that it is the pure anomaly from the ore body in the ground. In fact, it is an overall reflection of the ore body and surrounding rocks rather than a pure anomaly.

(3)Although the equipment used in time domain IP is heavy, these pieces can reach deeper in exploration. In fact, under the same field conditions, the current intensity required by time domain IP is 50th to 20th that of the dual frequency IP. That is, a dual frequency IP transmitter with a small power supply (e.g., output voltage = 400 V, output current = 1 A, output power = 400 W) is at least equivalent to a time domain IP transmitter with a much larger power supply (output voltage = 1500 V, output current = 25 A, output power = 37.5 kW), which has been verified by a large number of field measurements.

(4)Induction coupling in the frequency domain is stronger than that in the time domain IP, so that it is more difficult to correct it. This misunderstanding originated in the 1970s when the variable frequency IP was the mainstream frequency domain IP in which induction coupling was indeed very troublesome while it was to increase the delay time for data recording to avoid induction coupling in time domain. Considering that the secondary potential difference is measured when the excitation current applied is removed in the time domain, the induction coupling effect is strongest in the same time so that a large part of the information about the IP effect will be lost when the delay time is increased. In particular, the IP delay curve will be mixed with that of induction coupling when the time constant of the induction coupling is large, which is fatal for the time domain IP. However, the induction coupling and IP effects can be separated from their time characteristics in the dual frequency IP method.

The author of this book hopes that these misunderstandings can be clarified though the corresponding discussions and analyses in the book.

We will continue studying a series of theoretical problems in dual frequency IP. In the future, however, it is more important to pursue better field applications to find more natural resources and solve more engineering problems. When writing this book, therefore, more attention has been paid to the field measurements and corresponding technologies rather than complicated descriptions of physical principles to meet the needs of the field geophysical and geological workers.

The dual frequency IP method is proposed by the Chinese, the instruments are designed and made by the Chinese, and the method has been successfully applied in China. Over the 40-year period of developing the dual frequency IP method, we have been on a road full of hard times, challenges, progress, and joys. We have been supported, helped, and encouraged by so many colleagues, geophysicists, geologists, and corresponding official departments, in particular those working in field measurements. Here, the author of the book thanks them from the bottom of his heart.

Jishan He, Central South University, Changsha, China

Chapter 1 Principles of dual frequency induced polarization method

Abstract

This chapter explores the induced polarization (IP) effect in rocks and ores, which is a crucial factor in understanding their intrinsic properties. The study investigates the physical parameters to describe the IP effect in the time domain, which is the polarization η, and its difference from the apparent polarization ηs. The mechanisms of IP are divided into two categories: electronic conductor IP and ionic conductor IP. The chapter also describes the charge separation and transfer at the interface between the electronic conductor and the surrounding solution, which is also called surface charge polarization. The study presents equations to calculate the imaginary and real parts of the impedance. The research concludes that the conductive mineral ingredients in ores have strong effects on the IP properties, especially on the time constant τ.

Keywords

Induced polarization; Rocks; Ores; Electronic conductor; Ionic conductor

Outline

1.1Introduction

1.1.1Time domain induced polarization

1.1.2From time domain to frequency domain

1.1.3From variable frequency IP to DFIP

1.2Physical and chemical interpretation of IP phenomenon

1.2.1Induced polarization of electronic conductor

1.2.2IP of ionic conductor

1.3IP characteristics of various ores and rocks

1.4Frequency characteristics and measurements of induced polarization

1.4.1Frequency characteristics of induced polarization

1.4.2Testing method for frequency characteristics of rocks and ores

1.5IP characteristics of rocks (ores) in frequency domain and affecting factors

1.5.1Concept of mutual impedance and transfer function

1.5.2Frequency characteristics of several common rocks and ores

1.5.3Cole-Cole model

1.5.4Factors affecting frequency characteristics of rocks (ores)

The milestone of the dual frequency induced polarization (DFIP) method is the induced polarization phenomenon. For the induced polarization phenomenon, the research mainly focuses on its principles, observation, characterization, and the induced polarization parameters of different rocks (ore) bodies.

We will start from the basics of the induced polarization phenomenon. At the beginning, the measurement of the induced polarization effect was usually in the time domain; however, the pioneers gradually realized that it might be more convenient to transfer the battlefield to the frequency domain. The transformation from the time domain to the frequency domain gave birth to the variable frequency IP method and indirectly promoted the conception of the DFIP method.

1.1 Introduction

1.1.1 Time domain induced polarization

The IP phenomenon is a kind of physical chemistry reaction that takes place in a geological medium where an overvoltage is formed due to the charge separation inside the medium induced by external current. The phenomenon can be observed by a device, as shown in Fig. 1.1.

Fig. 1.1

Fig. 1.1 Induced polarization phenomenon.

Direct current I is supplied into the ground through two current supply electrodes A and B; the potential difference and its variation with time between the measuring electrodes M and N are measured. If a medium in the ground is homogeneous and nonpolarizable and the current applied is kept steady, the potential difference ΔV1 between M and N should be presented just as the horizontal broken line, as shown in Fig. 1.2. It is kept constant with the time, and it is called the primary potential difference. However, if there is a polarizable medium in the ground such as a metallic ore body, it will be induced due to an electrochemistry reaction that occurs when the external current applied flows through it. As a result, an additional current is produced, which causes an additional potential difference between M and N, called the secondary potential difference ΔV2(t). The potential difference ΔV(t) observed between M and N when the external current is being applied is called the total field potential difference, and it is the summation of the primary potential difference ΔV1 and the secondary potential difference ΔV2(t). The potential differences between M and N are schematically shown in Fig. 1.2.

Fig. 1.2

Fig. 1.2 Induced polarization phenomenon observed in time domain.

The polarization of an ore body (or any polarizable medium) is a process with time. It is fast in the beginning and slows down gradually, and approaches saturation with time. The time of the polarizing process Ts for a polarizable medium, which generally lasts from several seconds to several minutes, as well as the saturated value ΔVs (s stands for saturated), are closely dependent on the properties of the medium.

When the switch K in the power supply wire in Fig. 1.1 is switched off, the current applied through the electrodes A and B will be removed; the polarization of the ore body, however, still exists as the discharge continues. Accordingly, when the potential difference ΔV1 produced by the current applied has already disappeared, the potential difference ΔV2(t) due to induced polarization will not return to zero instantaneously. It will reduce rapidly in the beginning, then decay slowly and finally vanish several seconds to several minutes later depending on the properties of the medium, as curve b shows in Fig. 1.2. The phenomenon observed above is similar to the process of charging and discharging of a capacitor, so the curve of the total field potential difference ΔV(t) measured between M and N when the excitation current is being applied is called the charging curve while the secondary curve of potential difference ΔV2(t) measured after the excitation current is removed is called the discharging curve.

The phenomenon described above is induced by applying an external current to a polarizable medium (it will not happen before the external current is applied), and it will not vanish immediately after the external current is removed. The phenomenon is therefore called induced polarization.

To display the whole induced polarization process, the charging time should be applied long enough (over 2 min, for instance) before the current is cut off. The variation of potential difference during the whole process of charging and discharging, namely, a full charging curve and a full discharging curve, should be measured; this is very important for studying induced polarization. However, in reconnaissance aimed at target finding, induced polarization characteristics are described with some parameters composed of several potential differences measured at several time points. The most widely used parameter in China is apparent polarization ratio ηs (s here is first in the phonetic alphabet of the Chinese character apparent), defined as

Equation    (1.1.1)

where (tioff) is the secondary potential difference measured at the time ti after the external current applied is cut off. Δt is a very short period of time while ΔV(T − Δt) is the total field potential difference measured just before the current is removed, and it is close to the saturated potential difference ΔVs. ηs is a relative physical quantity with no dimension.

Because ΔV2(tioff) ≪ ΔV(T − Δt), to improve the accuracy of ηs, for modern IP instruments, it is necessary to measure the integral of the secondary potential for a period of time, namely ∫t1t2ΔV2(t)dt. It is defined as

Equation    (1.1.2)

Where ms is called apparent chargeability, which is essentially the same as ηs. However, for the numerator, it is an integral of time, its dimension is second, and its practical dimension is a millisecond.

The method by which induced polarization is studied by measuring ΔV2(t) (or ∫t1t2ΔV2(t)dt) at several time points, that is, by measuring the variation of the secondary potential difference with the time, is called the time domain method.

1.1.2 From time domain to frequency domain

It can be seen by comparing curve a with curve b that the induced polarization phenomenon displays not only a secondary potential difference ΔV2(ton) that is increasing and approaches saturation during charging, but also a secondary potential difference ΔV2(toff) that approaches zero when discharging. According to theoretical study, the charging curve and the discharging curve are equivalent (the theoretical study is to be presented in Chapter 2).

A pure potential difference ΔV2(ton) can be obtained by taking out the primary potential difference ΔV1 (a constant) from curve a in Fig. 1.2. Then put curve a together with curve b after moving it to the right for a distance T. It is shown that the saturated value ΔV2(ton → ∞) of the charging curve is equal to the original value ΔV2(toff = 0) of the discharging curve; also, at any time when toff = ton, the increasing speed of the charging IP potential difference ΔV2(ton) and the decreasing speed of the discharging IP potential difference ΔV2(toff) have the same magnitude but an opposite sign. The charging curve and discharging curve are inverted images of each other, which implies that induced polarization can be studied not only by measuring ΔV2(toff) during charging but also ΔV2(ton) during discharging. The equivalent relation between the charging curve and the discharging curve can be seen clearly in the photograph record shown in Fig. 1.4.

These conclusions thus can be made from both Figs. 1.3 and 1.4:

(1)A whole discharging curve can be obtained providing that a whole charging curve is measured, and vice versa.

(2)A charging curve contains as much information as a discharging curve about the IP characteristics of a medium in the ground. No more new information can be obtained by measuring the two curves.

(3)A whole charging curve is equivalent to a whole discharging curve under the same conditions.

Fig. 1.3

Fig. 1.3 The symmetry of the charging curve and discharging curve.

Fig. 1.4

Fig. 1.4 The Record of the induced polarization phenomenon.

Secondary potential difference ΔV2(ton) is always superimposed with primary potential difference ΔV1 during charging, so they cannot be separated by measuring ΔV(t) only. To separate ΔV2(ton) and ΔV1, polarization should be induced by pulse currents with at least two different pulse widths, as shown in Fig. 1.5. The pulse width T/2 of one current should be so wide that ΔV2(ton) approaches saturation at the end of each single pulse. The potential difference measured here is

Equation

   (1.1.3)

Fig. 1.5

Fig. 1.5 Excite and measure the charging curve with a sufficiently wide and narrow pulse current.

The pulse width of the other current should be so narrow that time is not long enough to produce any significant IP potential difference. The potential difference measured here is

Equation    (1.1.4)

ΔV2(ton → T) is obtained after measurement from Eq. (1.1.3) minus that from Eq. (1.1.4).

The IP effect induced by pulse current is very dependent on the width of the pulse used. The pulse with a large width can polarize the medium in the ground adequately, thus producing an IP effect nearly saturated. The pulse with a middle width cannot polarize the medium in the ground adequately, so the IP effect measured will be less than that with a larger pulse width. A very narrow pulse current produces a negligible IP effect and only a primary ΔV1 can be measured (ΔV2 approaches 0). If the x-axis is for the pulse width and the y-axis is for the potential difference measured, a curve can be drawn as shown in Fig. 1.6. It is easy to see that it is the curve a in Fig. 1.2.

Fig. 1.6

Fig. 1.6 Relationship between pulse width T and potential difference Δ V .

The pulse current shown in Fig. 1.5 is cyclic, with a width of T/2, a cycle of T, and a frequency f = 1/T. Fig. 1.6 can be redrawn by using frequency as the x-axis; this can be achieved by transforming the measurements ΔV(T) to ΔV(f), as shown in Fig. 1.7.

Fig. 1.7

Fig. 1.7 Visualize the induced polarization phenomenon from the function of period T to the function of frequency f .

It can be seen from Fig. 1.7 that the IP intensity varies with frequency, thus it is a function of frequency. When the excitation current frequency is very low (a long cycle), the medium in the ground can be induced adequately, so a higher value of ΔV(f) is obtained and the secondary potential difference accounts for a larger proportion in the total field potential difference ΔV(f), which is equivalent to the ΔV(T − Δt) in the time domain. On the other hand, if the frequency is high enough that the medium in the ground can be barely induced, then a much lower value of ΔV(f) is obtained and the IP potential difference accounts for a much smaller proportion in the total field potential difference ΔV(f), which is equivalent to the ΔV1 in time domain. This method that takes IP as the function of frequency is called the frequency domain method.

It is easy to see that what is achieved by the time domain method in which the IP characteristics of a medium in the ground are valuated by measuring ηs (or ms) also can be done in the frequency domain. In this aspect, both methods are equivalent to each other. This will be theoretically verified in Section 2.2, Chapter 2.

1.1.3 From variable frequency IP to DFIP

Excitation currents with at least two different frequencies are needed to study the IP in the frequency domain, as shown in Fig. 1.7. To obtain a significant IP effect, one frequency (fL) must be so low that the medium in the ground can be induced adequately and a large enough proportion of IP potential difference (secondary potential difference) is contained in the measured ΔV(fL), which is equivalent to the ΔV(T − Δt) in the time domain. The other frequency (fH) must be so high that little polarization is induced, and the measured ΔV(fH) approaches ΔV1.

To distinguish from the glossary in the time domain, the apparent amplitude frequency ratio Fs (equivalent to percent frequency effect, PFE) is adopted to represent IP in the frequency domain, which is defined as

Equation

   (1.1.5)

Fs is a percentage of the amplitude change of the total field potential difference caused by frequency differences. Fs is not only dependent on the characteristics of the medium in the ground, but also on the frequency couple (fL fH). When fL approaches fH, it is obvious that Fs approaches 0 so that no IP information can be obtained.

Comparing Eq. (1.1.1) with Eq. (1.1.5), it is obvious that the apparent amplitude frequency ratio Fs and the apparent polarization ηs are essentially equivalent. ΔV(fL) is equivalent to ΔV(T − Δt) in the time domain and ΔV(fH) is equivalent to ΔV1 in the time domain. Accordingly,

Equation

   (1.1.6)

However, this is on the extreme condition that fL → 0, fH → ∞ and ton → ∞ , toff → 0. Generally speaking, fL, FH, and ton, toff cannot be in such an extreme condition that Fs is equivalent to but not exactly equal to ηs in practice.

In the traditional frequency domain IP methods, the currents of different frequencies are applied one by one. That is done to supply the low frequency current first and to supply the high frequency current second so that ΔV(fL) and ΔV(fH) have to be measured separately, then Fs is calculated by using Eq. (1.1.5). J.R. Wait, who is the first person to use this method, named it the variable frequency method.

The field equipment used in frequency domain IP is lighter, easier to be transported in the field, and of higher efficiency than those in the time domain IP. This is because in frequency domain IP, there is no need to measure the secondary potential difference after the excitation current is removed, but only the total field potential difference when the excitation current is being applied so that the current required is much lower than that in the time domain. A longer working time, however, is required due to double measurements and the relation between current supply and measurements is more complicated, which results in lower efficiency. Furthermore, it is difficult to achieve a high accuracy of measurements because the measurements are made at two different times, and interference is more complicated.

To overcome these disadvantages of the variable frequency method and take advantage of the frequency domain IP, the author of this