Exploration Geophysics - Volume 29, Issue 1-2, 1998

Helicopter electromagnetic surveys are being used to map saltwater intrusion in Everglades National Park, Florida. Using layered earth inversion methods, resistivity-depth slices are generated which provide information on the location of the freshwater/saltwater interface, as well as showing the influence of canals and roadbeds on the hydrologic regime. Through the use of well logs and time-domain electromagnetic soundings, correlations between formation resistivity and water specific conductance have been established. As a result, the interpreted resistivity maps can be used as estimated water quality maps with better coverage than is possible from well sampling alone. Repeat surveys provide means of assessing long term changes in the aquifer system resulting from changes in water management policies aimed at restoring the South Florida ecosystem.

An experimental campaign was carried out in 1991-1997 in the Baltic Sea, a body of brackish water with resistivities of 3 to 4 ohm-m, to test the ability of airborne and ground electromagnetic methods to map sea ice thickness. The measurements were made as part of the European research projects Baltic Experiment for ERS-1 and Local Ice Cover Deformation and Mesoscale Ice Dynamics.

Measurements on sea ice were carried out using a multifrequency, horizontal and vertical coplanar loop-loop system with a coil separation of 10 m. The purpose of these measurements was to obtain detailed information about conductivity properties of the ice, including keel ice rubble.

The first interpretation was made by using a layered-earth model. To correct for small variations in coil separation which primarily influences the in-phase component, coil separation was treated as an unknown parameter in the inversion. A three-dimensional model was used to interpret more complicated ice geometry in ridge areas. The results obtained were validated by drilling.

Airborne measurements were made with a vertical coplanar coil configuration where the coils were mounted on the wing tips of a Twin Otter aircraft, at a frequency of 3.1 kHz. A laser profilometer was used to map surface topography. Test measurements were made along 1 to 2 km long profiles. In good conditions the thickness accuracy achieved was ±0.2 m for undeformed ice, but worse for deformed ice with complicated geometry.

EM measurements on sea ice were found to be a practical tool if the detailed structure of ice and ridges is needed whereas AEM is superior if large coverage is essential.

The algorithms currently used to generate the apparent resistivity from helicopter EM data are not reliable in highly magnetic areas. This is because magnetic polarisation currents occur in addition to conduction currents, causing the computed resistivity to be erroneously high.

A new method for computing the apparent resistivity and apparent magnetic permeability has been developed for the magnetic conductive half-space. The inphase and quadrature responses at the lowest frequency are first used to estimate the apparent magnetic permeability. The apparent resistivity is then computed for all frequencies using the quadrature responses alone.

The EM response of magnetically permeable material is much greater for the inphase component than for the quadrature component. This means that the calculation of resistivity using the quadrature component at two frequencies is less subject to error from magnetic polarisation than if the inphase and quadrature components at a single frequency are used. The method allows the EM data to be portrayed as credible resistivity maps when magnetic polarisation currents occur in addition to conduction currents.

Using helicopter-borne frequency domain electromagnetic (HEM) data to map magnetite has advantages over the magnetic method. It is independent of the earth’s locally asymmetric inducing magnetic field, remanent magnetism, and magnetic anisotropy. It is also restricted to the notional “depth of exploration” of the HEM system, limiting detection to that which may be economically mined. A significant complication arises when interference between the magnetite polarisation and conductivity responses reduces the apparent magnetic content. This interference is significant over the conductive, magnetically polarisable ground common to the Hamersley Basin.

The current procedures for magnetite mapping with HEM are reviewed. Both problems and successes of the method are highlighted using a case study as an example. Replacing the data transformation by an inversion increases the sensitivity of the method, but at the cost of enhancing noise.

In any attempt to appraise the effectiveness of AEM in mineral exploration, the dominant questions relate to cost and credibility. Risk reduction is a key issue in assessing cost. Bedrock responses can normally be guaranteed with aeromagnetics, but this is not so with AEM. The risk of using AEM, considering that the costs are an order of magnitude higher, is significant enough to act as a deterrent when making decisions on how to explore. One way to reduce risk is to first fly wide spaced reconnaissance lines, and then fly detailed follow up only over those areas judged amenable to the survey system. Another way may be to pool resources through surveys conducted for multiple clients, including governments, so as to popularise conductivity and bedrock conductor maps, in the way magnetic images became popular in the 1980s and are now considered essential.

Credibility depends on the application. In ground water and environmental work there is wide agreement that the method is effective. However, in mineral exploration, results are mixed; in vast thinly covered areas such as the Canadian Shield, the record is good, but in areas of ubiquitous conductive cover, which include much of Australia, the record is poor. Credibility is affected by both emotional and technical issues. Emotional issues include scepticism over a lack of exploration successes in Australia, fear of staking ones reputation on a survey that doesn’t live up to expectations, hope but without any guarantees, confusion over which AEM system is best which can inhibit decision making, and even the personalities of the people involved. Technical issues include transmitter waveform and opinions on whether the theoretical blindness of impulse systems to highly conductive targets is worth the worry, motion induced false anomalies, the possibilities of which beg for continuous monitoring of bird location and orientation, and interpretation problems which recognises that an important aspect of mineral exploration is still “bump hunting” and that there is a need for more reliable discrimination between bedrock and overburden sources.

David Fountain’s talk on the history of airborne EM reminds me of Stephen Jay Gould’s book on development of pre-Cambrian life in the Burgess Shale, with multitude of phyla appearing and most dying out. AEM development has been similar; although there has been steady evolution and refinement of new systems, the phyla have been compressed into classes – deep-penetrating fixed wing, towed bird systems and more portable multiple-frequency helicopter systems. Big quantum jumps in AEM systems do not happen, rather there is slow and steady development. Magnetics is, by contrast, far more advanced. On a geological time scale, magnetics would be near-present and AEM at Palaeozoic.