Appendix 4 Magnetotelluric Electromagnetic Methods
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APPENDIX 4 MAGNETOTELLURIC ELECTROMAGNETIC METHODS A4.1 Introduction methods where an artificially created electromagnetic field is used, but the principles of the method remain the same. As described in Section 5.7.1.1, frequency domain electro- magnetic (FDEM) methods are now rarely used in mineral exploration. They still find use in applications requiring A4.2 Natural source magnetotellurics shallow conductivity information, usually surveys associ- ated with environmental monitoring. Exceptions are EM As described in Atmospheric noise in Section 5.4.2.1, there fi methods that rely on natural electromagnetic fields produced are natural time-varying electromagnetic elds originating by natural sources, i.e. the magnetotelluric (MT) methods from the magnetosphere, the region around the Earth (Chave and Jones, 2012; Simpson and Bahr, 2005) and the which includes the atmosphere and the ionosphere, and audio-frequency magnetic (AFMAG) method (Ward et al., at higher frequencies lightning strikes. The time variations 1966). In the case of MT methods, very deep penetration is occur over a wide frequency range (see Fig. 5.26), although ‘ ’ possible, greater than is achievable with the resistivity and there is a dead zone around 0.1 Hz and another around fi fl other EM methods (see Chapter 5), making them attractive 2 kHz. These elds induce circular current ow systems, ’ for exploring for deep targets. As these are passive geophys- several thousand kilometres across, within the Earth s crust fl ical methods (see Section 1.2), only receivers are required. and mantle. The currents ow as horizontal layers and are The relatively simple survey equipment and logistics is an known as telluric currents. fi fi advantage of these methods. However, as with all passive The electric eld (E- eld) of the telluric currents is fl geophysical methods, the use of a natural ‘transmitter’, horizontal, so the currents ow as large horizontal sheets fi fi whose characteristics vary with time and may not always and their associated magnetic eld (H- eld) is also hori- be ideal for the survey objectives, can be a disadvantage. zontal. In accordance with the skin effect phenomenon (see Figure 5.26 shows the natural EM spectrum, which for Section 5.2.3.1), lower frequencies penetrate deeper, the active source electromagnetic methods is a source of noise. actual depth range depending on the conductivity vari- For natural source EM methods this is the ‘primary’ mag- ations. The amplitude, phase and directional relationships fi fi netic field. The full range of frequencies shown in Fig. 5.26 between the E- eld and H- eld are measured for may be exploited (see Fig. 5.1). The lowest frequencies, individual frequencies over a range of frequencies, which down to 0.0001 Hz, are used in academic studies of the allows electrical soundings to be obtained, i.e. variations in Earth’s deep interior and for regional exploration targeting. electrical properties with depth. By making recordings at At higher frequencies extending into the audio range multiple locations, pseudosection and/or pseudovolume (10 Hz to 20 kHz), the method is referred to as audio datasets can be produced. magnetotellurics (AMT) and is used to map shallower features, from tens of metres to several kilometres depth. A4.2.1 Survey practice The airborne AFMAG method makes measurements at fi similar frequencies. The two horizontal components of the electric eld, EX and Most natural source measurements are made on the EY, are measured using two electric dipoles, each of up to ground, but airborne AFMAG measurements are also 1000 m in length, although a few tens of metres is more possible, specifically in the form of an MT-related method common. Non-polarising electrodes are used (see Section called ZTEM. Here we describe the natural source EM 5.4.1). The two or possibly three components of the mag- fi methods and also describe controlled source magnetotelluric netic eld, i.e. the perpendicular horizontal HX and HY 2 Magnetotelluric electromagnetic methods components and the vertical HZ component, are measured A4.3.1 Acquisition of CSAMT data with coils or, for lower-frequency measurements, with The source of the artificial signal is a large electric dipole magnetometers. In MT/AMT the natural ‘transmitter’ is produced by a pair of current electrodes typically 1–3km in effect located at an infinite distance from the survey apart (see Section 5.6.3). The current is transmitted over a area. The electromagnetic wave at the survey area is, there- range of frequencies, usually powers of 2 in the range 8 to fore, a plane-wave, i.e. the field lines are straight and 8192 Hz. The dipole is located a large distance, typically parallel to each other, unlike the curved field found closer 5–12 km, from the survey area so that the current flow in to the source (typical of other electrical and EM methods). the survey area is chiefly horizontal. The transmitter dipole This simplifies the mathematics of calculating the resistiv- is usually orientated perpendicular to the geological strike ity of the subsurface and simplifies interpretation of the (Fig. A4.1) to orientate the electric field, and therefore the survey data (see Section A4.4). current flow, across strike. This is known as the transverse Time series (see Section 2.2) of E-field and H-field magnetic (TM) mode. Orientating the dipole parallel to the variations are recorded. Lower frequencies, i.e. greater geological strike so that the electric field is parallel to strike depth of investigation, require longer recording time. For is known as the transverse electric (TE) mode. AMT surveys, recording periods extend from tens of min- The receiver measures two signals simultaneously: the utes to a few hours. MT measurements typically require horizontal electric field in the direction parallel to the a recording period of a few days in order to investigate transmitter dipole (E ), and the horizontal component of down to mantle depths, and may continue for weeks or X the magnetic field in the direction perpendicular to the even months for studies of the deep interior. transmitter dipole (H ). The electric receivers are dipoles MT measurements are made on a regional scale with Y comprising two non-polarising electrodes (see Section 5.4.1) measurements forming an approximately regular traverse and typically 10–300 m in length. The magnetic receivers or grid, depending on site accessibility. AMT measure- are coils, i.e. dB/dt sensors (see Section 5.7.1.5). It is possible ments are made at the smaller prospect scale and usually to measure all the electric and magnetic components of comprise measurements made at regular intervals along the transmitted field with two perpendicular orientations survey traverses spaced at several times the station interval. of the transmitter dipole, i.e. EX, EY, HX, HY and HZ to obtain the full tensor (see Section 2.2.3). Full-tensor A4.3 Controlled source audio-frequency CSAMT measurements provide information about the magnetotellurics electrical anisotropy of the subsurface (see Section 5.3.1.4). In the commonly used broadside configuration (TM AMT measurements using a man-made source are known mode, Fig. A4.1), a survey comprises a series of readings as controlled source audio-frequency magnetotellurics at stations along one or more traverses parallel to the (CSAMT). The data provide information from depths of transmitter dipole, perpendicular to the regional strike. a few tens of metres to about 2 to 3 km. A detailed descrip- Ideally, measurements are made in the region where the tion of the CSAMT method, with several case studies, is E-field is approximately parallel, and the H-field perpen- provided by Zonge (1992). dicular, to the transmitter dipole. This is a roughly Transmitter (Tx) AMPS 45° - 60° Strike direction 4 or more Near-field skin depths zone Receivers E E E E Survey H Far-field Y H traverse E zone X Figure A4.1 CSAMT survey arrangement. The transmitter E field H dipole is oriented perpendicular to the geological strike field fi (TM mode). Electric (EX) and magnetic (HY) elds are measured in far-field zone. A4.3 Controlled source audio-frequency magnetotellurics 3 triangular zone whose apex is at the centre of the transmit- and parallel to each other (cf. MT/AMT surveys), which ter dipole and whose sides trend 45–60° to the dipole. Note greatly facilitates interpretation of the data. The responses that the length of the survey lines can increase with are nearly equivalent to those recorded in MT/AMT, and increasing distance from the transmitter as the width of so too are the methods of interpretation. When surveying this zone increases outward. The largest distance from the on homogeneous ground, the measured apparent resistiv- transmitter dipole at which readings can be made is ity is independent of transmitter–receiver geometry, and determined by the signal strength necessary to obtain a depth of penetration depends on the frequency and the stable reading at the receiver. resistivity of the subsurface. In the near-field these param- – The magnitude and phase of EX and HY are measured. eters are a function of transmitter receiver geometry, but The magnitudes of both parameters are normalised by the not frequency. In the transition zone the response is transmitter current ,and phase-shift measurements are ref- complex and dependent on the resistivity contrasts and erenced to the signal source by synchronising the receiver their complexity. Clearly, far-field data provide useful timing to that of the transmitter. information about the electrical properties of the sub- Resistivity variations in the subsurface mainly affect the surface; so as far as is possible, measurements should be fi fi fi E- eld, whilst the HY- eld varies quite smoothly over a made in the plane-wave far- eld region of the transmitter large distance.