6th International AEM Conference & Exhibition

Two computer programs have been written to perform rapid TEM inversion in a 3D framework. Both programs operate on a 3D geological model, which facilitates integrated interpretation. In greenfields areas the starting model can degenerate into a discretised homogeneous half-space, to permit “unconstrained” inversion. VPem1D performs 1D inversion at each TEM data location. It is therefore a hybrid 1D/3D approach, ideal for quasi-layered environments. VPem3D performs 3D inversion on time-integrated (resistive limit) data, and is well suited for interpretation of compact conductive targets. Because the programs can invert a geological model, they permit a variety of inversion styles. For example, if one or more geological units are considered uniform in conductivity, the optimal conductivities can be determined for the entire survey area via homogeneous unit inversion. If some geological units are variable in conductivity, heterogeneous unit inversion can be applied to those units alone. Alternatively, because geological interfaces are captured in the model, geometry inversion can be used to adjust the shape of contacts or structures. For example, interpretation of cover thickness is a natural application for VPem1D. Both programs are applicable to data from variety of systems including (but not limited to) Geotem, Tempest, VTEM, Spectrem, SkyTEM, MegaTEM and Hoistem. This paper will illustrate some of the different inversion options on Spectrem data sets from Australia and Brazil.

Modification in the decay behaviour of vertical magnetic fields recorded by a typical HTEM system is studied considering a plate conductor in a conducting host. Good to excellent conductors sustain the induced eddy currents and thus yield slower decay rates in the recorded TEM response. For plate (or sheet like) conductors this behaviour can be seen clearly in the conductance aperture diagrams for various TEM systems. As the plate conductance increases, the amplitudes get smaller and yield slower decay rates. Thus it becomes difficult to discriminate between very good to excellent conductors. Measurements of ‘on time’ response or ‘B’ field instead of the ‘dB/dt’ field may improve the conductance discrimination to some extent. Present study reveals that current channelling due to host significantly modifies the decay of fields recorded over good to excellent conductors. For comparison, the responses of the host medium alone and the plate conductor in the absence of conducting host (air) is also computed. As the host medium conductivity increases its response dominates the response from the plate conductor to later times. Response in the presence of a very resistive (10,000 Ω.m) host is found close to that from the plate in air. However, the conductance aperture diagrams for various host medium resistivity values reveal that there is a divergence in various channel amplitudes for very good to excellent conductors. This is particularly observed for cases when the resistivity of the host medium is towards the higher values. For the lower values of the host medium resistivity, however, the response of even very good plate conductors is dominated by the currents in the host medium. The study suggests that a mildly conducting host medium may help in discriminating the conductivity of very good to excellent plate conductors.

In this study, the SkyTEM system was used to map key functional elements of the hydrogeological system critical to the identification and assessment of managed aquifer recharge (MAR) sites and potential groundwater resource targets. A suite of customized interpretation products were produced through integration of AEM data with data from 60 sonic-cored holes and 40 new rotary mud and. Data obtained from this program drilling includes: sedimentary facies, textures (including grain size), mineralogy, redox state and whole rock geochemistry; hydrogeophysical data (nuclear magnetic resonance (NMR), induction and gamma logs); hydraulic data (from slug and pump tests), hydrochemistry (from groundwater and porefluid samples), and hydrodynamic data from the monitoring of groundwater levels in 40 boreholes pre- and post-flooding in 2010-2011. Customised interpretation products developed through integration of these datasets include: maps of the sedimentary system including palaeochannels with favourable hydraulic properties; hydrostratigraphy; confining aquitards; textural classes within the sedimentary system; tectonic elements; zones of inter-aquifer leakage and groundwater flow pathways; groundwater salinity, aquifer transmissivity; MAR storage volumes; and groundwater storage estimates in various water quality classes (0-600; 600-1200; and 1200-3000 mg/L). The products were integrated with other datasets (including time series vegetation condition, surface geomorphology, flood inundation) for MAR risk assessments of various options and targets, with a priority target identified and positively assessed. Volume estimates for fresh, acceptable and brackish groundwater stored within discrete targets was particularly challenging due to the hydrogeological complexity. The methodology used relied upon a multi-scale approach. Salinity class thresholds were estimated by comparing the pore fluid data with the AEM response. Bulk volumes for each water quality class in a target were then calculated using these thresholds on an AEM depth slice basis, which had been mapped into textural classes. Gravimetric water and textural data from sonic cores were compared with borehole NMR data and laboratory effective porosities (from Lexan-encapsulated core) to estimate an effective porosity range for each textural class. These were used to convert the depth-slice bulk volume estimates to stored groundwater volumes. Sensitivity analysis revealed that there are significant uncertainties in volume estimates using this approach. There can be orders of magnitude differences in volume calculation depending on the AEM inversions used, with significant variations also found depending on the salinity thresholds used, and the uncertainties linked to the effective porosity estimation. This study has demonstrated the importance of selecting the most appropriate AEM system and optimizing the AEM inversions for generating a wide range of customized interpretation products. For estimating groundwater quality and volumes, there are large uncertainties due to the inherent issues with individual measurement methods, compounded by integration, scaling and extrapolation issues. Such mapping and estimates of groundwater salinity and volumes are still useful guides, but ultimately, the assessment of groundwater resources identified using these datasets requires numerical groundwater and solute transport modeling . Such modeling is needed to determine the duration and rates of supply possible from the identified targets, and to assess potential environmental and resource impacts from prolonged extraction during drought conditions.

The new Askaf mine with its estimated 3 billion ton of iron ore near Zouerate in northern Mauritania needs a guaranteed water supply of 11 million litres/day. The Touadeni Basin to the east has the potential to deliver this if the geology of the basin can be properly understood. Aeromagnetic data allowed the identification of faults and thrusts on the adjacent Reguibat Shield and tracing them into the Touadeni Basin. Two eastwards dipping conductive horizons in the Basin could be identified on TEMPEST CDI’s and existing borehole information allowed them to be correlated with dolomite layers. The contact between the dolomite and underlying sandstone-shale layers has frequent water strikes and were selected as targets especially where evidence existed of faulting. Tau maps played a major role in choosing the targets.

In this abstract, we present the theoretical background for the geophysical EM analysis with arbitrarily oriented magnetic dipole. We study the case of airborne EM measurements over an inclined ground. This context can be encountered if the measurements are made in mountain area. We show in particular that transient central loop helicopter borne magnetic data should be corrected by a factor proportional to

The conductive regolith present in most of Australia compromises the use of airborne electromagnetic (AEM) data, limiting the method's depth of penetration and its ability to unveil basement mineral anomalies under conductive cover. Here we describe an approach where by incorporating the highly conductive EM response of the 'overburden' rather than avoiding it in the interpretation; we increase the understanding of the cover's architecture and the broader exploration scenario. A better understanding of regolith distribution and stratigraphy has significant impact in the planning of drilling targeting and geochemical interpretation of surface anomalies. Detection of potential mineral targets, data driven geological sections and constrained regolith architecture such as thickness and main stratigraphical units can be achieve by: 1) operating a deep penetrating AEM system such as SPECTREM, 2) applying an inversion algorithm which can simultaneously resolve a same 1D model from X and Z component data and address unknowns around the system's geometry and 3) combining ancillary data,

The challenge in developing airborne electromagnetic systems was followed by the appearance of a wide variety of different kinds of such systems. Even in the case of considering active inductive systems with only carried transmitters. Sometimes it seems that these different airborne electromagnetic systems are based on completely different laws of physics. But all of them, Time-Domain and Frequency-Domain, with different transmitter-receiver geometry, with use of different primary field waveform – are based on the same principles: time-variable magnetic field generated by any transmitter induces eddy currents in buried conductors and the secondary field of these currents measured by an inductive receiver can give us information about the geology structures. The objective of this paper is to review the variety of systems from different points, especially in terms of signal structure, to analyse advantages and limitations of many existing systems and to suggest an approach that can make possible the ability to use the achievements in all of them during developing a new one. An attempt to realize this lead to the appearance of the system EQUATOR. The practice of using the system EQUATOR in both isolating and conductive environment for different type of targets fully confirms effectiveness of this approach.

In airborne time-domain EM (ATEM) the signal-to-noise ratio (SNR) is paramount for the detection of small responses from discrete conductors. In this paper it is examined how shortening the linear current turn-off can enhance the target responses significantly for discrete 3D conductors. For the objective of the paper two methods were applied (1) synthetic 2D thin sheet modelling as approximations to 3D discrete conductors and (2) comparison of real datasets collected over discrete conductors using two different ATEM systems having respectively a 200 s and a 1,200 s turn-off ramp. The findings of both the synthetic modelling and the real datasets show that the target response can be amplified by a factor 2 or even more if the length of the current turn-off ramp is shortened from 1,200 s to 200 s. The enhancement of the target response, and thereby the SNR, occurs for a large group of discrete conductors for which the time constant is comparable to or smaller than the duration of the current turn-off ramp. Shortening the current turn-off ramp will improve the capability to detect such conductors.

The SPECTREM AEM system [ Annan A.P. 1986 ] generates the step response of the ground to a repetitive sequence of transmitter steps switching from positive to negative and back again. Apart from the intervals where the transmitter is in transition it is always on. This means that the ground response and the transmitter signal are present in the receiver at the same time. In order to retrieve the secondary signal alone the transmitter signal must be removed from the receiver secondary signal. Unlike a ground system like UTEM [West G.F. 1984] which also measures the secondary step response and the primary field at the same time, both the receiver and transmitter positions are fixed during the recording and the amplitude of the transmitter can thus be computed from the geometry. For a towed bird system like SPECTREM, this method cannot be used to compute the amplitude of any component of the transmitter primary at the bird as the relative positions, and rotations, of the transmitter and receiver coils are constantly changing. For a particular component (say Z) at the receiver the amplitude of the primary signal can change by a few percent from one reading to another while the changes in the secondary (ground) signal are of the order of a few hundred ppm. That is, the secondary signal of interest is buried in a not dissimilar transmitter signal three orders of magnitude larger. The traditional method SPECTREM uses for the real time processing performed in flight is to assume that near the end of each half cycle, the secondary signal due to the transmitter transition (step at the start of the half cycle has more or less decayed away and this late time signal can be regarded as primary signal only, and when subtracted from all the preceding data in the half cycle the result is deemed to be the actual secondary signal. By choosing lower base frequencies for the transmitter the separation of this late time signal from the transmitter switch- over is increased and so better estimate of the primary field is obtained, as more of the secondary signal will have died away. At a base frequency of 25 Hz the transmitter signal changes sign every 0.02 seconds ( two transitions per cycle ). Assuming the secondary signal at 0.02 seconds is just at the noise level of 100 ppm then, for an inductive limit secondary signal amplitude of say 25% of the primary field the decay constant must be less than about 3700 microseconds. Any decay rate slower than this will have a secondary signal amplitude at 0.02 second delay time, greater than the noise level and by subtracting this (non zero) signal together with the true primary signal from the preceding data it will cause them to be under estimated. If one assumes a 50,000 microsecond decay constant and the same inductive limit amplitude as the previous example, the late time amplitude at 0.02 seconds delay would be is 167,580 ppm and so all the preceding secondary data will be reduced in amplitude by this amount when it is subtracted along with the true primary. This is likely to push the signal below the noise level for nearly all the data, apart from some at early times. This results in the anomaly being unlikely to be selected as a target for further work. So the really good conductor (think Nickel) is then effectively, rejected due to poor processing. Several techniques for computing a better estimate of the primary field will be demonstrated, as well as the one currently in use by SPECTREM, ( and currently being improved upon) together with illustrations on actual field data.