Electromagnetic Methods in Geophysics. Fabio Giannino
Introduction
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.
2 Electromgnetic (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