AEM System Research Articles

Investigation of Hazardous Waste Sites and their Environment Using the BGR Helicopter-Borne Geophysical System

The Federal Institute for Geosciences and Natural Resources of Germany (BGR) completed a research and development project aiming at optimizing its helicopter-borne geophysical system for high resolution site characterization. The overall objective was to adapt the existing helicopter-borne geophysical system used for groundwater and mineral exploration to survey conditions where the anomalies to be recorded are much smaller. The BGR helicopter-borne system permits simultaneous electromagnetic (AEM), magnetic (AMAG), and gamma-ray surveying. At the suggestion of the BGR, the AEM system manufactured by Geoterrex-Dighem, Toronto, Canada, was improved compared with the Dighem III system formerly used. The new system operates at five frequencies and the transmitter and receiver dipole moments are increased up to 25%. In addition, the system is now calibrated during flight. The sensitivity to waste objects was augmented by reducing the sensor heights from more than [Formula: see text] (AEM) and [Formula: see text] (AMAG), respectively, to less than [Formula: see text] about ground level by means of installing a magnetic sensor and a laser altimeter in the AEM bird. Enhanced spatial resolution was achieved by decreasing the sampling distance along line from about [Formula: see text] and by reducing the line separation from about a hundred meters to less than [Formula: see text] due to better navigational and positioning instruments. The modified system was tested over two former military training areas south of Berlin, Germany. Special surveys to locate steel drums, scrap metal, steel pipes, petrol tanks, ordnance, buried at depths from [Formula: see text] were carried out with nominal bird heights of [Formula: see text] and flight-line spacings of [Formula: see text]. Due to the extremely weak AEM and magnetic anomalies produced by these materials, suitable detection algorithms were developed to recognize and to identify these weak anomalies. They were tested using an airborne data set collected over an area where thousands of anomalies had been found. Many of them were subsequently verified on the ground. More than 90% of the anomalies selected for verification could be confirmed either by visual inspection of the ground surface or ground geophysical surveying or excavation. The modified AEM system not only allows better detection of waste but also better investigation of the environment. The AEM data could be reliably inverted to resistivity vs. depth sections using multi-layer inversion procedures. These resistivity data provide information about the hydrogeology and lithology, e.g., the depth of the groundwater table or the distribution of clay and silt. Thus, AEM can be successfully used for the hydrogeological and geological mapping of the near surface.

Depth of penetration of frequency-domain airborne electromagnetics in resistive terrains

The depth of investigation of a fixed-wing frequency-domain airborne electromagnetic system is studied. Starting from the definitions for plane-wave EM and a homogeneous half-space, the analysis is extended to dipole fields and a layered earth, and finally to more realistic two-and three-dimensional conductive sources. The main factors affecting the depth of penetration are coil configuration, operating frequency and signal-to-noise ratio of the AEM system. The analysis is based on forward modeling of AEM responses in the frequency domain using existing software for conductive ID, 2D, and 3D bodies. The AEM system analysed as an example is a vertical coaxial, frequency-domain system flown in a DC-3 aircraft. Summary diagrams for different model geometries are constructed from the modeling results. Based on the analysis of the modeling results at the inductive limit, the depth of penetration in resistive terrain for the AEM system studied is between 85 m and 210 m depending on the geometry and the size of the conductor. Host rock and overburden of low but finite conductivity do not cause a significant change in penetration values. The screening and the attenuation effects from conductive overburden and conductive host rock will, however, decrease the penetration depth.

Personal views on the effectiveness of airborne EM in Australian mineral exploration: A client perspective

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.

On the effect of varying the pulse width to detect high conductance bodies

Theory, modelling and field observations have shown that good conductors have the strongest time-domain AEM response if the duration of the transmitter pulse is twice the time constant of the conductor. Because good conductors have long time constants, recent time-domain AEM system modifications have resulted in the pulse width increasing to 4 ms, and more recently, 6 ms. The theoretical analyses, modelling and data observations have all assumed that the secondary voltage response from the conductor is only measured in the off time. However, very high conductance bodies have off-time voltage responses which are asymptotically close to zero, so these bodies are undetectable, irrespective of the pulse width.However, theoretical analysis shows that high conductance bodies do have a significant voltage response during the transmitter on-time. The shape of this measured response is a half-sine pulse identical in shape to the transmitter current waveform. The on-time amplitude of a horizontal high conductance sheet is independent of the pulse width. As the conductance increases, the amplitude decreases, but does not disappear into the noise until the conductance is greater than about 3 x 105 S. This conductance is significantly greater than the upper-limit of conductances detectable with off-time AEM systems (1000 S).Measuring the secondary magnetic field rather than the voltage response results in a finite, non-decaying response in the off-time. The magnitude of the late-time response is proportional to the width of the pulse, assuming the peak dipole moment is the same. Thus, the off-time magnetic-field response of very high conductance bodies can be enhanced with a longer transmitter pulse. For extremely high conductances, the response at all delay times is zero.For a conductive half-space, the off-time voltage response is significant; however, the on-time voltage response is even greater. Like the thin-sheet case, the magnetic-field response is finite in the off-time and this response decays more slowly than the voltage response. Because the magnitude of the measured response decreases as the inverse square root of the conductivity, it is possible to detect half-spaces as conductive as 4 x 106 S/m.

Block modelling as a check on the interpretation of stitched CDI sections from AEM data

AEM data collected over quasi-layered regolith structures is readily and conveniently interpreted using stitched 1D models. As a check on the validity of this process, a 3D modelling program was used to generate a ‘blocky’ regolith conductivity profile to characterise the gross conductivity structure seen on a stitched 1D model of a 12-km AEM survey line. The differences between model data based on ‘stitched 1D conductivities’ and the original field data highlight ‘interpretation errors’ resulting from a simplistic 1D assumption. In the case tested, large conductive zones (1 km or so in lateral scale) directly interpreted from a stitched 1D conductivity section underestimated the required local conductivity and overestimated the true lateral dimensions. Interspersed resistive areas are thus spatially more extensive than they appear on the stitched 1D model.Simple 3D modelling does not however automatically correct for survey deficiencies. For example, probable errors in bird location resulted in surficial conductors being modelled at a ‘false’ depth of 10 to 20 m using the nominal but imprecisely known AEM system geometry. Topography should also be modelled to account for all observed features, but this was not possible with the 3D-modelling program used.

Can Sulphides be Discriminated from Conductive Slates? A Case History at Dugald River

If an excellent conductor is located within a conductive host or halo, a counter-intuitive effect may be seen in impulse response results. The presence of the very conductive core can dramatically reduce the fitted time constant of the observed decay at early to mid delay times, and the response due to the slow decay of currents within the core itself may not become dominant until very late delays when the signal may lie beneath the noise level. With an on-time system, although the 'total' response may also lie below observable noise levels, a detectable secondary can easily be inferred since the total field measured is markedly different from the calculated primary field. The Dugald River zinc-lead-silver deposit in Northwest Queensland consists of a massive to brecciated sulphide lode hosted within conductive slates. Modelling of the observed borehole response of Utem and Sirotem data requires the presence of very conductive material with a decay constant of more than 200 ms. This would imply that the conductance of the lode must be well in excess of 500 S. This value may seem very high, but is consistent with a nominal thickness of 20 m and physical property measurements. Interpretation of the surface and borehole Utem data imply that a very conductive target, approximately coincident with the lode horizon, can be discriminated from the slates, and must extend to great depths. Borehole Sirotem data indirectly appear to support the premise that the lode is very conductive, but only with the benefit of careful modelling. By contrast airborne Geotem data using a 75 Hz base frequency is comparatively insensitive to excellent conductors; and all detected anomalies in the survey can be attributed to the response of the shales alone. This is a result of the relatively high frequency transmitter waveform and off-time measurement used in the AEM system.

DAMPED LEAST‐SQUARES INVERSION OF TIME‐DOMAIN AIRBORNE EM DATA BASED ON SINGULAR VALUE DECOMPOSITION1

AbstractAirborne electromagnetic (AEM) methods are increasingly being used as tools of geological mapping, groundwater exploration and prospecting for coal and lignite. In such applications, quantitative interpretation is commonly based on the layered‐earth model. A new approach, a damped least‐squares inversion with singular value decomposition, is proposed for interpretation of time‐domain, towed‐bird AEM data. Studies using theoretical and field AEM data indicate that inversion techniques are dependable and provide fast converging solutions. An analysis has been made of the accuracy of model parameter determination, which depends on resistivity and thickness distribution. In the common case of conductive overburden, upper‐layer resistivity and thickness are usually well determined, although situations exist where their separation becomes difficult. In the case of a resistive layer overlying a conductive basement, the layer thickness is the best‐determined parameter. In both cases, estimates of basement resistivity are the least reliable. Field data obtained with the Chinese‐made M‐l AEM system in Dongling, Anhui Province, China, were processed using the described inversion algorithm. The survey area comprised fluvial Cenozoic clays and weathered Mesozoic sediments. Inversion of AEM data resulted in accurate depth‐to‐bedrock sections and realistic estimates of the resistivities of overburden and bedrock which agree with the results of drilling and resistivity sounding.

Simplified prediction of transient electromagnetic response

In general the target response to an electromagnetic system is a complex function of target geometry and of the geometrical and electrical parameters of the apparatus used to make the measurements. Thus, the prediction of system performance is usually done with respect to a particular target. Under some circumstances, however, one can predict the effect of changing one of the system parameters by assuming that the typical conductor in a given area is well represented by a shorted single‐turn loop. This simple, classical concept is shown to be valid for evaluating the effect of primary pulse width on target response to the Input airborne electromagnetic system. The justification of our basic technique for predicting this aspect of AEM system performance is provided by the analysis of Input surveys done in two areas of the Precambrian Shield in Canada. Repeat surveys were done in each area with a 1 ms and a 2 ms primary pulse width. The change in system performance that could be expected by doubling the pulse width was correctly predicted after an analysis of the 1 ms data.