A typical site layout is shown in Figure 3.9. Trees and bushes provide shelter from the Sun and prying eyes, and can therefore be good places to place the box containing the datalogger and batteries.
54 Planning a field campaign
Depending on digging conditions and intended recording time, it may be worthwhile to shield the datalogger further from temperature variations by burial or coverage with fallen branches, etc.
Having chosen a suitable position for the datalogger, the pos- itions of the electrodes can be sited. This can be achieved accurately using siting poles and a geological compass. Some MT practitioners advocate measuring a standard dipole length (e.g., 50 m) during the siting process, whereas others estimate the distance between electrodes and measure the actual distances subsequently. It can be difficult to obtain accurate measurements of the electrode separ- ations using a conventional measuring tape, particularly if there is vegetation to be circumvented. However, in practice, a high degree of accuracy is not necessary, since minor errors contribute an
Logger
Batteries Fluxgate unit
N Electrode
S Electrode
E Electrode W Electrode
3-component fluxgate head or 3 orthogonally aligned induction coils.
Pa d l o c k e d case holding datalogger, fluxgate unit and power source.
Burying the cable helps to protect it from being dragged or chewed by livestock.
Rocks can be used to anchor cables.
Tree provides shelter for datalogger etc.
Shrub used to anchor cable before attaching to electrode.
Figure 3.9Typical layout of an MT station.
3.8 Step-by-step guide to installing an MT station 55
insignificant static-type shift to the impedance magnitudes. Dipoles are normally aligned in the north–south (N–S) and east–west (E–W) directions, and may be configured to form a + or anL. If a + is chosen, then four electrodes are required, whereas for anL only three electrodes are required, because one electrode is common to both dipoles. If the Earth were 1-D, then the + configuration would have the advantage that if one of the dipoles were to fail (owing, for example, to a cow chewing through the cable), then electric fields would still be registered for the other dipole, and 1-D modelling would still be viable. An advantage of theL-configuration is that having a common electrode makes parallel connection (Figure 3.10) of telluric lines impossible. The risk of connecting telluric lines to the
(b)
S N W E
N
S W E
Earth
S N W E
N
S
E W
Earth
(c) N
S E
W
(d)
S N W E
N
S
E
Earth
W
N
S
E
Earth
W
S N W E
(a) N
S W E
N
S W E
N
S E
W Figure 3.10(a) Field
configuration in which electrodes () are connected to a datalogger (box) to establish a pair of orthogonal dipoles that form a +. (b) Accidental jumbling of the N and E telluric cables results in two parallel dipoles oriented NE–SW. (c) Field configuration in which electrodes are connected to a datalogger to establish a pair of orthogonal dipoles that form an L. The S and W telluric inputs are inter- connected at the datalogger. (d) Accidental jumbling of the telluric cables results in two dipoles that are oriented at 458to each other.
In this case, orthogonal directions of the electric field can be retrieved
mathematically.
56 Planning a field campaign
incorrect input is also minimised by labelling telluric cables accord- ing to their directions as they are laid out.
The electrodes should be buried below the surface to a depth that mitigates temperature variations (see Section 3.1.3). Waterproof plugs should be used to connect electrode wires to the telluric cables.
We recommend anchoring the telluric cables, particularly close to where they are connected to the electrodes. This can be achieved by tying cables to a shrub or rock, etc. (Figure 3.9). It can be a good idea to measure the contact resistance of the electrodes using a voltmeter.
If contact resistances exceed the krange, then this may indicate a poor contact (or a dried-out electrode). A metal stake can be used to earth the telluric inputs. After connecting the telluric cables to the appropriate inputs of the datalogger, the voltages between paired electrodes can be compensated. The appropriate level of the pre- amplifier should also be selected.
The fluxgate magnetometer, or induction coils should be buried at least 5 m from the datalogger. If induction coils are used, then it is necessary to orientate them using a compass, and to level them using a spirit level. If a fluxgate magnetometer is used then the fluxgate head should be approximately orientated and levelled. Thereafter, fine adjustments can be made using screw mechanisms (Figure 3.1).
The fluxgate head is connected via a coaxial cable to an electronic unit from which it receives power, and to which it transmits its output signal. Compensation (see section 3.1.4) may also be facili- tated via this unit, which is achieved by centring three needles (one for each magnetic component). Typically, fine adjustments to the fluxgate head are made until they-component of the magnetic field requires zero compensation, and a spirit level indicates that the head is level. Subsequently, thex-andz-components are compensated at the electronic unit.
With the sensors in place, information such as site name, pre-amplifier values and telluric lengths can be entered into the datalogger. This information will then be available in the header of the datafile to be produced. The date and universal time (UT) should also be correctly set (e.g., using a Global Positioning System (GPS) receiver).
You are ready to record data.
3.8 Step-by-step guide to installing an MT station 57
Chapter 4
From time series to transfer functions:
data processing
The digital time series collected during an MT survey can easily total a few Gigabytes, but the data that+ will finally be interpreted with numerical modelling schemes typically consist of a few hundred numbers per site that represent the frequency-dependent transfer functions. The reduction is referred to as ‘data processing’. One time series can simultaneously contain information about many periods and, therefore, about many penetration depths, and the first step in data processing involves a Fourier transforma- tion from the time domain to the frequency domain. Data reduction is then achieved by stacking data falling within particular spectral bands in the frequency domain: both neighboured frequencies from the same segment (window) of a time series, and similar frequencies from sequential time series windows can be stacked.
What is the exact meaning of ‘transfer function’? The Earth is regarded as a linear system that responds to an input process (e.g., a time-varying magnetic field) via a predictable output process, (e.g., a time-varying elec- tric field). The transfer function is the ratio of these processes, and because the system is linear, the transfer function does not depend on the amplitude of the input process. The estimation of the transfer functions can be hind- ered by noise in the input and output processes: e.g., electric and magnetic fields which are not naturally induced but are rather associated with our technical civilisation. If the noise level is low, or the noise has a Gaussian distribution (the frequency of occurrence, plotted as function of the size of the noise, delineates a Gaussian bell), then a least-square estimation (in which the squared residuum, which is not explained by the linear system, is minimised) is sufficient. Otherwise, the occurrence of outliers might force us to apply a ‘robust’ processing scheme. Most robust processing schemes
58
operate in an iterative manner, and use some measure of the departure of an individual contribution from the average to down-weight outliers in the next iteration. What is an outlier? As an example, suppose that an attempt is made to estimate the average height of pupils in a school class, but the sample includes four children (1.0, 1.1, 1.1, 1.2 m, respectively) and a basketball player (2.0 m). In this example, the basketball player would be considered to be an outlier. We finally discuss a few ways of displaying the transfer functions, as well as the advantages and disadvantages of various visualisation aids.