The effective depth(s) of detection of a helicopter-borne, frequency-domain EM system are related to the frequency and sensitivity of the system. Sensitivity is controlled by the quality of the electronics, the quality of the data processing, and the coil separation.

Frequency
The depth of exploration of the system in ground of constant conductivity is a function of the inverse of the square root of the EM frequency - a frequency 4 times lower penetrates approximately 2 times deeper. The depth of maximum sensitivity is also roughly proportional to the inverse square root of frequency. This means that for a system to have maximum sensitivity through a range of depths and conductivities, it must have a wide range of frequencies. The Dighem Resistivity Bird has frequencies of 380 Hz, 1400 Hz, 6200 Hz, 25 kHz, and 102 kHz. This multiple of 270 between lowest and highest frequencies delivers data sensitivity to both very shallow layers, and deep layers at the same time, and over complex geology.

The frequency of the EM system also affects the conductivity of the ground to which the system is most sensitive. A broad range of frequencies provides a system which delivers good quality data over a broad range of ground conditions.

Figure 1 shows a graph of estimated depth of exploration for the five frequencies employed by the Dighem Resistivity system. One can see, for example, that at a ground resistivity of 20 ohm metres (conductivity 50mS/m), the five frequencies are sampling at depths ranging from about 5m (101 kHz) to about 80m depth. This will provide an excellent measure of the distribution of conductivity over this depth range.

A smaller range of EM frequencies will result (all else being equal) in a system with less sensitivity over a broad depth range. A lower high frequency will be less sensitive to shallow or resistive targets. A higher low frequency will have less depth penetration in conductive conditions.

Coil Separation
The depth of detection, and the fineness of resistivity resolution depend in part on the ability to resolve weak signals, and subtle changes in signal, in the presence of the noise. The smaller the actual ppm value is, for the same ppm noise level, the finer the system resolution. (Frequency-domain systems measure the measured secondary field in parts-per-million of the transmitted primary field.) This creates an apparent contradiction: the more powerful the transmitted field, the better, but the smaller the primary field interfering with the measurement at the receiver, also the better. This is best accomplished by having the receiver as far as practical from a powerful transmitter. The farther away the receiver is from the transmitter, the lower the primary field at the receiver will be, allowing for the detection of smaller secondary fields. (When the system altitude is significantly higher than the transmitter-receiver separation, the response from the earth depends only on the transmitter power and frequency.)

The decrease in primary field is a function of the coil separation cubed. Thus for an increase in coil separation from 5m to 8m, the decrease in received primary field would be (8/5)3=4.1 times. The anomaly responses, measured in ppms, would be 400% bigger.

Tuned Coils
The sensitivity of an EM system is dependent on its ability to transmit powerful fields, and detect the weak signal in the presence of natural electromagnetic noise. The most efficient transmitter coil is one which is tuned to the frequency being transmitted. The most effective receiver coil is also one which is tuned to the frequency being measured, so that it is more sensitive to the signal desired, and less sensitive to noise. (Detecting the signal through the strong primary is described in "Coil Separation", above.)

A system which uses a single transmitter and receiver coil for its entire range of frequencies must use a broad-band coil, not tuned to any specific frequency It cannot be made as sensitive to each and every frequency measured over a broad range and will have less efficient transmission, and more noise pick-up. This results in a lighter, but less sensitive EM system.

The Dighem^VRES system uses 5 individual coil pairs, one for each transmitted frequency. The number of turns of wire in the coils varies by a range of 10 times between low and high frequency, providing the maximum power and sensitivity at each frequency.

Data Processing
To achieve maximum sensitivity from a data set it must be processed with a set of algorithms which can separate the desired target signal from the real-world noise. This noise is most often geological and operational. Geological noise usually stems from changes in the geology other that the change which is the target of the survey. For example, when looking for water through variation in conductivity, changes in the soil type and depth may also vary the conductivity, and mask the presence of water. Operational noise may include variations in signal as the system altitude varies, or external EM noise such as power lines.

An extensive suite of data processing algorithms, developed through millions of kilometres of data processing, and backed by years of experience processing and interpreting these data are necessary to get the best results from the data. Conductivity calculation algorithms must produce a close approximation of the ground conductivity, removing the effects of altitude and variations in the near - surface conductivity. Transforms and inversions convert the EM data to an accurate representation of the distribution of resistivity in three dimensions for the production of vertical sections or conductivity blocks.

In its thirty year history, DIGHEM has collected data for over 1.5 million line kilometres of HEM surveys. During the last two years alone, approximately 350,000 line-kilometres of survey have been flown by DIGHEM systems. Since the origin of the apparent conductivity calculations, pioneered by Dighem, (Fraser, 1978), Fugro has been at the fore-front of data processing technology. The apparent resistivity algorithm developed by Dighem is now the world standard for frequency-domain HEM data processing. Other significant papers published include: Layered earth resistivity mapping (Fraser, 1990), Magnetite Mapping with a multicoil airborne electromagnetic system (Fraser, 1981), The differential parameter method for multifrequency airborne resistivity mapping (Huang and Fraser, 1996), Magnetic permeability and electrical resistivity mapping with a multi-frequency airborne EM system (Huang and Fraser, 1998).

References:

Fraser, D.C. 1978, Resistivity mapping with an airborne multicoil electromagnetic system, Geophysics, v43, p. 144-172
Fraser, D.C. 1981, Magnetite Mapping with a multicoil airborne electromagnetic system, Geophysics, v47, p. 1579-1593
Fraser, D.C. 1990, Layered earth resistivity mapping: in Fitterman, D.V., Ed., Development and applications of Modern Airborne electromagnetic surveys, U.S. Geol. Surv. Bull. 1925, p. 53-64
Huang, H. and Fraser D.C., 1996, The differential parameter method for multifrequency airborne resistivity mapping Geophysics, v61, p. 100 - 109
(Huang H., and Fraser D.C., 1998, Magnetic permeability and electrical resistivity mapping with a multi-frequency airborne EM system. Exploration Geophysics, v29 p. 249-253