Transient Electro-magnetics (TEM) or Time-domain EM (TDEM) is one of the two main divisions of electo-magnetics. It is distinguished from the other main division, Frequency domain EM (FDEM), by the transmitter not being continuously on, as in FDEM, but pulsed on and off. This provides one great advantage to TEM in measuring the received “secondary” signal when the stronger “primary” transmission signal is off.
Principle of Operation
A loop of insulated wire, usually in the shape of a square or rectangle, with side dimensions of from 1 to 1,000 metres is laid out on the ground. By connecting it to a generator of electrical current, the passage of current in the loop, called the transmitter loop, develops a magnetic field which propagates into the earth. See Figure 1. After some time, which is usually some milliseconds, the current is switched off to terminate the primary magnetic field. (Hence the name “transient” or, “time domain”).
Due to Faraday’s law, the rapid switching off of the field induces currents to flow in the subsurface, called “eddy” currents. The peak current, migrates downward and outward from the loop with time at a velocity dependent on the conductivity of the subsurface, be it uniform or varying. When this current intersects a discrete conductor, currents are induced in it. They in turn, have a magnetic field, the secondary field, which can be detected on another loop, called the receiver loop (or since the transmitter loop is no longer transmitting it can be that same loop). This set-up can be likened to a common power transformer with primary and secondary windings and the core being like the conductive earth. By this means, we can detect conducting bodies or layers of different conductivity in the subsurface.
The receiver loop measures the time rate of change of the magnetic field, which intersects the loop. With a properly calibrated receiver, the units are volts per transmitted current. That is, the measurement is ‘normalised’ to the current, as its variation would alter the voltage alone. Sometimes this expression is also normalised to the area of the receiver loop as this also determines the value of the received signal and is important in comparing results from different loop sizes in the same place.
The secondary field decays with time so the characteristic receiver signal, recorded as a voltage, reduces in amplitude with time. This time is measured from when the transmitter was instructed to turn off and is called the delay time. The strength of the amplitude and the rate of decay provide information about the strength of conductivity and when readings are taken in different places, the physical shape and attitude (dip, etc.) of the conductor. With particular field operations and subsequent analysis, these parameters can be determined more accurately. Algorithms allow the voltages to be converted into conductivity values.
Presentation and Interpretation
As we have seen that later delay times can mean deeper depths probed, the measured decay curve can be converted into a variation with depth as a type of depth sounding at the measurement point. This is achieved with the one size of transmitter loop and is one of the advantages of the TEM method. Soundings are achieved without having to expand electrodes as in contact electrical methods. This has other advantages as well as the logistical one. (It is called “parametric sounding” as distinct from geometric sounding). When a number of soundings are measured, they can be plotted as a 2-D depth section sometimes referred to as a conductivity-depth section, or “CDI”. The determination of the thicknesses and conductivities of different layers uses a process known as inversion. This is commonly done by first calculating a starting model and varying the parameters until a best fit is obtained with the observed result. This process is also applied to discrete bodies of varying shapes. 3-D interpretation is now possible in some cases. As such a result is not necessarily unique, other independent information from other methods or known geology can be used to refine the final interpretation.
Traditionally the decaying voltage curve is usually compiled by measuring the voltage at specific times or averages of the voltage variation over time intervals. These intervals are called “windows” or “gates”. It is desirable to have many “samples” at close spacing so as to plot the variation closely. As the signal level becomes small at later times, it is usual to gradually increase the window width to measure it with more accuracy. The actual delay times measured range from zero to 100s of milliseconds. The FastSnap instead records full waveform data, time gates/series are are assigned during post processing.
To account for the possibility that some voltage on the loop may be a constant background, the first ON pulse of the transmitter is repeated but with the reverse polarity. (See Figure 2). By adding the resulting receiver signals together and taking an average value, the constant is cancelled out. This pair of readings is regarded as one measurement, called a “stack”.
As one stack is generally of poor signal-to-noise ratio (due to the method being a broadband receptor for stray fields) usually many stacks are taken and the instrument automatically averages them to give a final value. By this means, much unwanted noise is cancelled. Since each stack may take only a few tens of milliseconds, hundreds of stacks will still be finished in a few seconds.
The transmitter loop “moment”, or strength, determines the penetration depth for a given conductivity section. A stronger transmitter moment will ensure a stronger received signal for a given situation which allows its better detection. The moment, M is the product of the area of the loop, A, the amount of current, I and the number of turns of the loop, n. Mathematically, M = n.I.A. Of all these three factors, the one that gives the most improvement for a given increase is the area as it varies as the square of the side length of the loop. Usually the change of area has the least constraints of all three, usually. The current, often has a limited range and it is dependent on the resistance of the loop wire. Generally, there is no gain from having more than one turn on the transmitter loop as this will increase the total resistance and hence reduce the current by the same proportion.
Depth of penetration not only depends on the moment, but also on the value and variation of the conductivity in the section. As we have seen from the theory, conductivity at the surface, such as we have in Australia, slows down the penetration time. On the other hand, low conductivity at the surface and higher conductivity at depth can increase the penetration before noise takes over the signal. A very general rule is that the penetration is approx. equal to 2-3 times the side length of a square loop depending on the conductivity profile. A 25 m x 25 m loop will generally achieve a depth of about 50 m to 75 m. or, a 300 m x 300 m loop will possibly penetrate to 1,000 m depth.
The main factors in determining the strength, or “moment” of the receiver loop are the area of the loop and the number of turns. The current in this case is not controllable as it is determined by the amount of signal received. If a small receiver loop is desired for better resolution or for convenience in measuring a particular component of the field, this can be a coil with a moment made up of multiple turns, which compensate for the very small area of the coil. This moment is often referred to as the coil’s “effective area” and is equivalent to a loop of this area and with only one turn.
The receiver loop measures only the component of the total field that is perpendicular to the plane of the loop. Thus for a loop laid on the ground, it is the vertical component that is measured. If a component of the field other than vertical is required, then the plane of the receiver loop must be perpendicular to this direction. In this case, compact receiver coils described above can be used more easily than uprighting a large loop.
Since the actual received voltages are small, the unit of voltage used in practice is microvolts (µvolts). The unit of normalised voltage amplitude is then µvolts/amp. The practical unit of conductivity is milliSiemens/metre (mS/m), where 1 mS/m = 1,000 ohm-m of resistivity, 100 mS/m = 10 ohm-m, etc.
There are a number of different loop configurations that can be used. (See Figure 3 on page 4). Each has particular attributes for best achievement of the data and often also its interpretation. A summary of these advantages is given in Figure 3.
The measurement point of any reading is taken to be the centre of the receiver loop or the position of the coil if a compact multi-turn coil is used.
As for production, as measured by the number of readings in a given time, if a roll-along procedure is established by setting up the next loop during a measurement, generally the move to the next station can be made as soon, or soon after, the measurement is completed. The time interval between stations will depend largely on the size of the loops and the nature of the terrain.