Читать книгу Electromagnetic Methods in Geophysics - Fabio Giannino - Страница 10

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.