Electromagnetic Methods in Geophysics E-Book

Fabio Giannino

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 545

Veröffentlichungsjahr: 2021

Beschreibung

Discover the utility of four popular electromagnetic geophysical techniques In GeoRadar, FDEM, TDEM, and AEM Methods, accomplished researchers Fabio Giannino and Giovanni Leucci deliver an in-depth exploration of the theory and application of four different electromagnetic geophysical techniques: ground penetrating radar, the frequency domain electromagnetic method, the time domain electromagnetic method, and the airborne electromagnetic method. The authors offer a full description of each technique as they relate to the economics, planning, and logistics of deploying each of them on-site. The book also discusses the potential output of each method and how it can be combined with other sources of below- and above-ground information to create a digitized common point cloud containing a wide variety of data. Giannino and Leucci rely on 25 years of professional experience in over 40 countries around the world to provide readers with a fulsome description of the optimal use of GPR, FDEM, TDEM, and AEM, demonstrating their flexibility and applicability to a wide variety of use cases. Readers will also benefit from the inclusion of: * A thorough introduction to electromagnetic theory, including the operative principles and theory of ground penetrating radar (GPR) and the frequency domain electromagnetic method (FDEM) * An exploration of hardware architecture and surveying, including GPR, FDEM, time domain electromagnetic method (TDEM), and airborne electromagnetic (AEM) surveying * A collection of case studies, including a multiple-geophysical archaeological GPR survey in Turkey and a UXO search in a building area in Italy using FDEM /li> * Discussions of planning and mobilizing a campaign, the shipment and clearance of survey equipment, and managing the operative aspects of field activity Perfect for forensic and archaeological geophysicists, GeoRadar, FDEM, TDEM, and AEM Methods will also earn a place in the libraries of anyone seeking a one-stop reference for the planning and deployment of GDR, FDEM, TDEM, and AEM surveying techniques.

Leseprobe

Electromagnetic Methods in Geophysics

Applications in GeoRadar, FDEM, TDEM, and AEM

Fabio Giannino

IDS GeoRadar s.r.l.Pisa, Italy

and

Giovanni Leucci

Institute of Heritage Sciences (ISPC)

National Research Council of Italy (CNR)

Lecce, Italy

This edition first published 2022© 2022 John Wiley & Sons, Inc.

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Library of Congress Cataloging‐in‐Publication Data

Names: Giannino, Fabio, author. | Leucci, Giovanni, author.Title: Electromagnetic methods in geophysics : applications in GeoRadar, FDEM, TDEM, and AEM / Fabio Giannino and Giovanni Leucci.Description: Hoboken, NJ : Wiley, 2022. | Includes bibliographical references and index.Identifiers: LCCN 2021027149 (print) | LCCN 2021027150 (ebook) | ISBN 9781119770985 (hardback) | ISBN 9781119770992 (adobe pdf) | ISBN 9781119771005 (epub)Subjects: LCSH: Magnetic prospecting. | Ground penetrating radar.Classification: LCC TN269 .G433 2022 (print) | LCC TN269 (ebook) | DDC 622/.153–dc23LC record available at https://lccn.loc.gov/2021027149LC ebook record available at https://lccn.loc.gov/2021027150

Cover Design: WileyCover Image: © Fabio Giannino

PREFACE

Geophysical techniques have many environmental, archaeological, forensic, geological, geotechnical, or engineering applications, as well as in the oil and gas, the mining industry, or for general academic research. Because of this, all the aspects connected with the logistics, designing, data collection, analysis, interpretation, and visualization, must be evaluated on a case‐by‐case basis.

Regardless of the geophysical technique deployed for a specific subsoil exploration campaign, the final objective is always to search for the variations of a specific physical property of the system to be investigated, and to infer the possible anthropogenic or natural factor(s), which caused the variation itself.

The properties which, more often, are the main subject of an applied geophysics measurements campaign are the electrical conductivity (electrical resistivity), the seismic waves velocity propagation (either in their vertical and/or horizontal component), the EM waves velocity propagation, the dielectric constant, the magnetic field, the gravity acceleration, and so on. Measuring of the abovementioned properties, in boreholes, on surface, in the sea, or airborne, is carried out with instrumentation specifically designed and manufactured, and the data gathered through these tools, are analyzed by means of specific software, whose proper use allows even very small variations of a given quantity to be highlighted. The survey and laboratory/office operations addressed to the acquisition and analysis of such specific data, result in those geophysical methodologies known as Geoelectrical techniques, EM induction methods, seismic methods (seismic refraction, seismic reflection, seismic tomography, MASW, Re.Mi.), ultrasound, Ground Penetrating Radar (GPR), Magnetic method, Micro‐gravimetry, and so on.

The list outlined is obviously partial, but on the other hand may be useful to give an idea of the macro‐areas within which the most commonly used geophysical techniques can be allocated, with respect to the industry, the academic research, or in the professional services activities.

Tools deployed for the data acquisition, as well as the software to be used for the purposes of data processing and visualization, improved very much in the last three decades, and became rather complex and of a specific use. Moreover, the possibility to apply a specific technique for the purposes of a given project, cannot disregard the knowledge of the theoretical basis on which every geophysical technique is founded on. As a direct consequence of this, a poor awareness on the hardware, the software, and the theoretical aspects, will most likely lead to a geophysical approach, whose possibility of technical success is low.

Another very important factor in planning and performing a geophysical survey, is the collection of the greater possible amount of information relating to the nature, dimension, geometry, and burial depth of the “target” of the measurements campaign, and the (geological) background where the “object” is imbedded. In this case, the word “target” means every buried feature determining a contrast in physical properties detectable by the technique. The knowledge of this, can also positively contribute to selecting the geophysical technique, which increases the probability to maximize the differences of a given physical property between the background and the target. Hence, the possibility to locate the latter with a higher degree of confidence and increasing the resolution of the final result.

Many geophysical techniques are based on the propagation in the space of electromagnetic (EM) waves (wave field methods), and a unique classification of them can be difficult and, to a certain extent, of no use. However, in order to optimize the results deriving from their application to project‐related issues, it is of a paramount importance to have a clear idea about the existing EM techniques and what are the characteristics differentiating one from another. To do this, it may be used as a classification factor, at a first instance, the fact that a known amplitude and frequency transmitter may be used as a source of EM energy, as it happens for the TDEM methods, FDEM methods, as well as in the GPR techniques; on the other hand, also the interaction with the subsoil of natural sources of EM energy can be used as EM energy, as in the magnetotelluric methods (MT), the audio magnetotelluric (AMT), or in the audio frequency magnetotelluric (AFMAG). Under this point of view, EM techniques may be classified into active, the former, and passive, the latter.

Another way to differentiate one EM technique or one group of EM techniques, is the fact that the EM signal generated from an AC current having a known amplitude and frequency content, is emitted continuously (FDEM methods), or that the transmitting coil spreads out a transient EM signal of known amplitude and frequency, and that a receiver coil measures the time decay of the emitted EM signal interacting with the subsoil, after that the transmitter is been switched off (TDEM methods).

EM methods can be further classified based on the relative position between transmitter and receiver coils: these can have a fixed or variable relative distance. On the basis of this aspect we may distinguish the Turam method (D.S. Parasnis, 1979), the Slingram, the ground conductivity meter (GCM), the Very Low Frequency (VLF). Furthermore, P.V. Sharma (1996), introduced an EM methods classification, depending on the fact that the primary EM field may be continuous, transient, or broadband; according to this we may have:

EM continuous wave field methods (FDEM)

EM transient field methods (TDEM)

Magnetotelluric methods (MT)

Regardless, all EM methods are deployed with the common aim of characterizing the subsoil on the base of its electrical conductivity and dielectrical constant.

As is known, the purpose of these methods is to deduce the physical properties of the Earth and its internal constitution from the physical phenomena associated with it. On the other hand, the objective is to investigate, with a very high resolution and a relatively smaller scale, more superficial features present in the Earth's crust. Typically, the investigation of these characteristics provides an important contribution to practical problems, such as oil exploration, the identification of water resources, mining exploration, pollutant research, bridge and road construction, and civil engineering. The presence of bodies or structures in the subsoil is highlighted by measuring at the surface variations of some physical parameters in the subsoil itself. In practice, some measurements of a given physical field (i.e. electromagnetic) are carried out at the surface of a given area. If the subsoil were perfectly homogeneous, regardless of the position in which the measurement is carried out, the same value of the measured physical parameter would always be obtained. Assuming, instead, that in a certain position of the subsoil there is a body with different physical properties compared to the surrounding material, when the measuring instrument passes in correspondence with the body, the measured value tends to deviate from the unperturbed value, and the observed physical field assumes a value, defined as anomalous, i.e. a variation with respect to the reference value relative to an homogeneous situation (anomaly).

Since each EM method is sensitive to the contrast of particular physical parameters (electrical conductivity, relative dielectric constant, etc.) of the object under investigation with respect to the surrounding environment, it is understandable that the greater or lesser effectiveness of the one with respect to the other depends on the extent of the contrast of the corresponding physical parameters. Therefore, the choice of the most suitable EM methods for a particular problem is strongly dependent on the objective and is essentially guided by the identification of the physical parameters of the object to be identified that present the greatest contrast with the host environment, and therefore they allow greater ease of detection, as well as considerations of an economic and logistical nature.

EM methods are often used in combination. Thus, for example, the search for illegal landfill takes place at an early stage with the use of GPR and FDEM methods. The ambiguities resulting from the results of a single method can be removed by considering the results obtained by using a second method. For example, the reflections in a GPR survey due to the presence of a wall or a buried pipe could be similar (a hyperbole shaped reflection). By integrating the GPR survey with another EM survey, this ambiguity can be solved considering that relatively low conductivity values could be associated with the wall, while relatively high conductivity value could be associated to the buried pipe.

It is important to stress that, although an interpretation of the results of the, here described, EM methods requires relatively advanced mathematical treatments, initial information, as will be shown in the book, can be obtained from the simple observation of the acquired data.

More in general, the methodological characteristics of the EM techniques, leads to a number of advantages, as it follows:

High degree of horizontal resolution in mapping apparent electrical conductivity

: data acquisition and management software to be interfaced with electro‐magneto meters, allowing the sampling frequency to vary. This allows for a very high number of data points to be collected while walking along pre‐selected acquisition alignments.

High degree of horizontal and vertical resolution in mapping EM reflection events.

Reducing the data acquisition time (i

.

e. the field work)

: As the EM method is based on the EM induction principle, no contact between sensors and the soil to be investigated is required, as normally happens for geoelectrical methods, where steel rods must be embedded on the surface of the field to be investigated in order for the current to be injected. This occurrence allows for the EM data to be collected while walking, or driving, or flying along acquisition lines pre‐defined within the survey area.

Survey cost reduction

: due to the previous point, it follows that the data acquisition costs dramatically reduce, compared to other geophysical techniques to be deployed for the same purposes and over the same areas.

As for any geophysical technique, also EM methods shows some limitations:

Instruments calibration before each survey operation

: measurements of the secondary EM fields due to the interaction between the Primary EM field generated by the transmitter coil and the subsoil, is performed through a ratio with respect to a reference signal. For this reason, a test EM measure over an area where no EM anomalies should be located, has to be done prior to the commencement of the actual survey.

Vertical resolution is limited in FDEM and TDEM:

the electrical conductivity datum to be collected, refers to a volume of subsoil located at the medium point between the transmitter and the receiver, and it must be considered as an “apparent” conductivity datum.

Dynamic Range is reduced:

as highlighted in McNeill (1980) for induced EM methods, when the subsoil shows very low electrical conductivity values (i.e. very resistive subsoil) it is rather difficult to induce electric current in the subsoil by the use of a electro‐magneto meter, capable to generate, in turn, eddy current large enough for a secondary magnetic field (induced) to be measured by a receiver coil with a dynamic range between 1 and 100 mS/m.

Some of the fields of applications allowing for the intrinsic properties of the EM to be enhanced along with their expected final results, are:

Mapping of saline water intrusion

Buried metallic utilities mapping

UXO mapping

Cavity search (under given subsoil conditions)

Utility mapping

Mapping of pollutants plumes

Mapping of un‐authorized landfill areas

Forensic geophysics

Archaeological geophysics

Buried metal search, in general

Mineral resource research.

Obviously, this is only a partial list of the potential application of the EM methods and may not be considerate complete; each geophysical measurement campaign should be designed and planned to take into account criteria strictly project specific and target oriented.

Throughout the book, an in‐depth view into the theory and application of four Electromagnetic geophysical techniques known as Ground Penetrating Radar (GPR), Frequency Domain Electromagnetic (FDEM), Time Domain Electromagnetic (TDEM) and Airbone Electromagnetic (AEM) shall be given. Also, each technique shall be considered in its general aspects related to economical, planning, and logistic aspects that are an integral part of the deploying activity on site.

As a further aspect that we attempt highlighting in this book, is that the output of each technique should/could be considered also in terms of its potential integration with the output of other source of information, collected either below the ground and above the ground, in a further effort of digitizing the global information describing the whole surrounding, in a common point cloud containing much information of a different nature, and for potentially different applications and use. This general concept takes place in what is nowadays known as Smart‐Cities, where many sources of information are collected by many sensors, analyzed together, and made available to stakeholders for the optimization, maintenance, and use of assets being part of a urban or industrial context.

More than 25 years of professional experience, collected in over 40 Countries world‐wide, for academic, research, professional, and industrial purposes, results in this manuscript that rather then enter into the deep details, aims at describing the optimal use of a limited number of geophysical techniques and its implementation to several application, demonstrating their flexibility.

REFERENCES

Parasnis, D.S. (1979).

Principles of Applied Geophysics

. Third edition, Chapman and Hall.

Sharma, P.V. (1997).

Environmental and engineering geophysics

. Cambridge University Press.

ACKNOWLEDGEMENTS

The authors are grateful and wish to offer thanks for their support, comments, suggestion, review, to:

Lara De Giorgi, Ivan Ferrari and Francesco Giuri, Institute of Heritage Science (National Research Council of Italy).

Alberto Bicci, President of IDS GeoRadar s.r.l. Part of Hexagon.

Vincenzo Sapia, Istituto Nazionale di Geofisica e Vulcanologia (RU Applied Geophysical Measurements Laboratory, Italy).

Andrea Viezzoli and Antonio Menghini, Aarhus Geofisica s.r.l.

Prof. Enzo Rizzo, University of Ferrara (Italy).

Section IIntroduction to Electromagnetic Theory

1Introduction

The framework of the following pages is structured into three sections. In the first section, the theoretical basis on which the GPR, FDEM, TDEM, and AEM techniques are founded, shall be illustrated without entering into the very deep physical and mathematical aspects, which are beyond the purposes of this text. However, the theoretical aspects shall be treated with a detail allowing the Reader to have a sufficient familiarity with those features that makes the methods themselves particularly suitable for specific applications. This will also allow the reader to comprehend how the EM instruments are built by the manufacturer, worldwide. This specific aspect is treated in the second section, where the system’s hardware architecture is illustrated, as well as showing how the instrumentation is designed and manufactured with the aim of maximizing the capability to detect the variation of physical properties of the subsoil, down to a given depth.

Also in the same second sections, all the aspects connected with the design of a survey campaign related to the EM methods will be analyzed, in order to reach the best achievable compromise between the client’s requirements and technical specifications, the survey area’s logistics, the available assets, and the need to collect high quality data. Eventually, still within the context of the description of an EM survey, all the most relevant aspects connected to the data acquisition, analysis, visualization, and interpretation, shall be discussed.

The third section is dedicated to the applications, and several case histories shall be illustrated. These will be proposed with the aim of highlighting the technical and practical aspects that may be of interest for the geophysicist approaching these techniques. Cases illustrated in this section were selected with the aim of covering a wide geographical context but, at the same time, the largest possible number of different applications including archaeological and monumental heritage study, utility mapping, rebars detection, water leakage mapping, bridge deck study, mineral exploration, geological and hydrogeological mapping.

In the same sections all those aspects are illustrated related to non‐technical parts involving the logistics and the handling of a survey in terms of its organization and implementation, even when the shipping of material overseas is part of the campaign.

2Electromgnetic (EM) Theory: An Outline

2.1. GROUND PENETRATING RADAR (GPR): OPERATIVE PRINCIPLES AND THEORY

2.1.1. General

The Ground Penetrating Radar (GPR), also known as Georadar is one of the most widely accepted and used geophysical methods for the exploration of the shallow subsurface, especially but not limited to civil engineering, geological studies, utility mapping, environmental, or archaeological applications. Its ability to provide, easily and quickly, high‐resolution and continuous information on the uppermost few meters (up to tens of meters) of the natural or man‐made surface, heavily contributed to the increasing popularity of this method and to its expanding role among the shallow geophysical techniques in the last two decades. Nevertheless, the same reasons could make this method highly subjected to misuse.

For a successful application of the GPR technique, as well as any other methods for underground mapping, it is necessary not only to understand its fundamental principles, but also its general characteristics and limitations in relation to the practical application for which it is required its deployment on site; this information, addresses the user to develop suitable field and post‐acquisition procedures for the specific problem at hand.

Dissertation on the theoretical basis, practical guidelines, as well as numerous case histories on GPR studies in various fields of applications, can be found in recent literature, such as books (Leucci, 2019; Conyers, 2004; Conyers, 2013), geophysical handbooks (Campana and Piro, 2008; Reynolds, 2011; Persico et al., 2018), Proceedings and Special Issues of geophysical journals (as those devoted to the biennial International Conference on GPR held since 1986), and numerous research papers.

Although in earlier times GPR data were generally used and interpreted as they were collected (the so‐called raw data), they are now routinely subjected to digital data‐processing, interpretation, and display techniques aiming to further enhance the visibility of meaningful signals in the raw data, and to help in understanding their three‐dimensional relationships. Due to the close kinematic similarity with seismic reflection methods, most of the processing and visualization techniques currently available in GPR processing software are a direct adaptation of the seismic ones.

The physical bases and mathematical foundations underlying these techniques are therefore available from seismic literature (Yilmaz, 1987) and most recently Persico, 2014. Nevertheless, although without presuming to furnish a deep examination and an exhaustive treatment of the theoretical and practical aspects of the GPR method, the main basic principles underlying the acquisition and processing of GPR data, needed for the comprehension of the tasks faced in the next chapters, are concisely exposed in the following pages.

2.1.2. Principles of the Method

The GPR technique is similar, in principle, to the seismic reflection technique but, instead of mechanical waves, it uses high frequency (10–2500 MHz) electromagnetic pulses to explore the underground.

A radar wave, emitted by a transmitting antenna (a transmitter antenna, or transmitter, is generally indicated with “Tx”) placed directly above the ground surface, propagates in the ground and it is partially reflected by any change in the electrical properties of the subsoil. The reflected energy is then detected by the receiving antenna (a receiving antenna, or receiver, is generally indicated with “Rx”). This basic concept is schematized in the simple sketch of Figure 2.1.1, below.

Georadar antennas have a relatively large frequency band, whose width is approximately equal to the center‐frequency, that is the frequency around which most of the pulse energy is concentrated. For example, if the center‐frequency of emission of the transmitter dipole is 600 MHz, the frequency band is approximately between 300 MHz and 900 MHz. However, the intrinsic characteristics of emission, primarily depends upon the manufacturers technical specifications and technology.

Most GPR equipment uses dipole antennas (identified by their center‐frequency or by the pulse width, approximately corresponding to the reciprocal of the center‐frequency) arranged either in monostatic or in bistatic configurations. In the first case (monostatic mode) the same antenna is used for transmission and reception and the Tx and Rx dipole are contained in the same antenna case and a fixed distance from each other. In the second case (the bistatic mode) there is a constant, small offset between the two antennas, that can be placed either in separated cases (as for the low‐frequency antennas) or inside the same box (as for the higher‐frequency ones).

Generally, the offset is sufficiently small that it can be practically neglected, and the last arrangement could be considered nearly monostatic. For both arrangements the usual data acquisition is the reflection mode, performed either as continuous profiling (moving the antennas along the profile at a slow, near constant towing speed) or as stationary point collection (shifting them stepwise).

Figure 2.1.1 Sketch of the basic components of a GPR system and principle of operation.

GPR data, properly amplified, are then recorded and displayed as a two‐dimensional section with the antenna positions (or midpoint positions in case of bistatic systems) in the horizontal axis (Figure 2.1.2 a) and the two‐way travel time in the vertical axis (Figure 2.1.2 b and c). This section can be considered a normal‐incidence time section (corresponding to the zero‐offset section of the seismic reflection), where the two‐way time is plotted vertically below the midpoint position, between the Tx and the Rx, even if the actual ray path is slanted, as for the reflection from dipping interfaces or from small‐size targets (diffraction).

Typically, the vast majority of commercial GPR, are built according to a monostatic architecture. However, GPR data can be acquired using other modes depending on the relative geometry of transmitter(s) and receiver(s). These acquisition modes are known as: The Common Mid‐Point (CMP) or Common Depth Point (CDP), the Wide‐Angle Reflection and Refraction (WARR), and the transillumination (Figure 2.1.3) The first two are mainly used for the electromagnetic (EM) wave velocity determination whereas the last is used in tomographic studies.

In general, any GPR is built to measures EM waves reflection events at a given time. This means that, once the EM signal is emitted by the Tx, it travels in the ground and when the wave encounters a reflector it is scattered back and recorded by the receiver. The time spent by the EM wave to travel from the Tx to the reflector and back to the Rx is known as two‐way travel time. Hence, the electromagnetic wave propagation velocity plays an important role in the GPR data analysis, because it allows the conversion of the two‐way travel time window into depth. The EM wave, propagates at a different velocity in different mediums, depending on their physical (dielectric) properties.

Beside CMP and WARR methods to estimate EM waves velocity, other methods can be used. They are (i) the location of objects at known depth, and (ii) the reflection from a source point. In the first method, two‐way travel time is the time that an electromagnetic wave takes to travel through the ground, from the transmitting antenna to the object and back to the receiving antenna (Figure 2.1.4).

Denoting the depth of the known object with a zknown and the velocity of the electromagnetic wave with v, the two‐way travel time for a monostatic configuration of the antenna is given by:

(2.1.1)