Marc A. Vallée

Estimating depth and model type using the continuous wavelet transform of magnetic data

The continuous wavelet transform has been proposed recently for the interpretation of potential field anomalies. Using Poisson wavelets, which are equivalent to an upward continuation of the analytic signal, this technique allows one to estimate the depth of burial of homogeneous field sources and to determine the nature of the source in the form of a structural index. Moreau et al. (1999) accomplish this by successively testing the least‐squares misfit on a log–log plot of the wavelet transform amplitude versus the sum of the depth and the dilation (upward continuation height). We extend this methodology by analyzing the ratio of the Poisson wavelet coefficients of the first and second orders. For simple pole sources, this ratio at one dilation is enough to estimate the depth and index uniquely; but for extended sources of finite size, we must analyze the variation of the estimates with dilations. The technique gives good results on synthetic and field examples.

Metalliferous mining geophysics — State of the art after a decade in the new millennium

Mining exploration was very active during the first decade of the twenty-first century because there were numerous advances in the science and technology that geophysicists were using for mineral exploration. Development came from different sources: instrumentation improvements, new numerical algorithms, and cross-fertilization with the seismic industry. In gravity, gradiometry kept its promise and is on the cusp of becoming a key technology for mining exploration. In potential-field methods in general, numerous techniques have been developed for automatic interpretation, and 3D inversion schemes came into frequent use. These inversions will have even greater use when geologic constraints can be applied easily. In airborne electromagnetic (EM) methods, the development of time-domain helicopter EM systems changed the industry. In parallel, improvements in EM modeling and interpretation occurred; in particular, the strengths and weaknesses of the various algorithms became better understood. Simpler imaging schemes came into standard use, whereas layered inversion seldom is used in the mining industry today. Improvements in ground EM methods were associated with the development of SQUID technology and distributed-acquisition systems; the latter also impacted ground induced-polarization (IP) methods. Developments in borehole geophysics for mining and exploration were numerous. Borehole logging to measure physical properties received significant interest. Perhaps one reason for that interest was the desire to develop links between geophysical and geologic results, which also is a topic of great importance to mining geologists and geophysicists.

Inversion of airborne time-domain electromagnetic data to a 1D structure using lateral constraints

We invert time-domain airborne electromagnetic data in such a way as to obtain a model that varies slowly along the profile. This is achieved by modifying a typical one-dimensional inversion algorithm to include lateral constraints. The lateral constraints are included as a roughness matrix that is solved simultaneously with the Jacobian matrix in an iterative eigenparameter inversion. In this case, multiple soundings along a line are all solved simultaneously. The lateral constraints can be applied to the resistivities and the thicknesses, both separately and together. We apply these techniques in two situations where airborne geophysical data are applied to near-surface exploration. The first case is in a resistive environment where we are interested in quantifying a superficial conductive overburden. In this case, lateral constraints improve the geological image compared with those images obtained using unconstrained layered-earth inversion. In the second case, we want to map the thickness of a resistive aquifer covering a saline layer. In this case, we show how varying the weights on the lateral constraints can change the image of the thickness of the aquifer. The presence of numerous cultural artefacts makes the inversion problematic. Application of a first-difference constraint did the best job at removing culture but resulted in oversmoothing of the hydrogeology. The use of a second-difference lateral constraint gave a good rendition of the hydrogeology but did not suppress the culture as well as the first-difference constraint.

Gridding aeromagnetic data using longitudinal and transverse horizontal gradients with the minimum curvature operator

The measurement of gradients of the earths magnetic field from an airborne platform has been advocated for many years. In 1954 Wickerham proposed the use of what is essentially a gradient along the flight direction (longitudinal gradient). Hoods 1965 suggestion was to measure the vertical gradient using a total-field magnetometer mounted in a tail stinger and a second magnetometer in a towed bird below the tail stinger. In fact, the system finally built by Hoods group at the Geological Survey of Canada used two tail stingers separated by 2 m. The advantages of this type of total-field vertical gradiometry were advocated so convincingly that vertical gradiometers was also developed for helicopter surveying. One of the key developments that allowed the collection of high-quality vertical-gradient data from a fixed-wing aircraft was the development of aircraft compensation schemes for gradiometer arrays. Subsequent installations generally measured a vertical gradient and a transverse gradient (perpendicular to the flight direction). The longitudinal gradient is either calculated by taking the spatial derivative of a single sensor traverse, or by averaging two wing tip sensors and subtracting this from a tail sensor. Marcotte et al. (1992) give a number of examples where having both horizontal gradients can help in the interpretation of magnetic data. They suggest that both gradients can be used to (1) determine whether the geology is two-dimensional; (2) estimate the strike direction, and (3) help calculate the vertical gradient. Interestingly, Marcotte et al. demonstrate that the vertical gradient derived from the horizontal gradients gives data with noise levels one tenth of that seen on the measured vertical gradients. Other advantages of measured horizontal gradients are that they can be used: (1) to derive a total field that is free of diurnal effects; (2) to derive the magnetic gradient tensor; (3) to estimate the depth …

Application of Occam's inversion to airborne time-domain electromagnetics

Airborne time-domain electromagnetics (ATDEM) methods are regularly used for mining, hydrocarbon, and groundwater exploration. A large quantity of data is collected along survey lines from an aircraft, and there is an incentive to interpret these data in a systematic way. When the geology is appropriate, the use of 1D inversion methods is justified. Among these methods are: conductivity-depth transform (CDT) (Wolfgram and Karlik, 1995), layered-earth inversion (Sattel, 1998), Zohdys method (Sattel, 2005), and Occams inversion (Constable et al., 1987; Sattel, 2005). These methods either require considerable tuning to get realistic results, are limited to step response data, or require considerable experimentation with the initial guess to ensure a reasonable result. The advantage of the Occams algorithm is that it can be easily adapted to different ATDEM methods and is not strongly dependent on the initial guess. Furthermore, there are not a lot of parameters to tune in order to get a reasonable result....

Effect of temporal and spatial variations of the primary signal on VLF total‐field surveys

Most of the airborne and ground VLF instruments presently used measure the total‐field response in addition to field ratios. Results of surveys using these instruments are adversely affected by spatial and temporal variations in the VLF primary field. Until now, the nature of such variations has not been studied from the point of view of geophysical surveying practice. Spatial variations are analyzed using radio propagation models. The most important result is the identification of primary field minima where surveys would be unreliable. Their dependence on the transmitter location is rather complex, and modeling should be carried out before specifying VLF stations for a survey area. Spatial and temporal variations have been studied using field monitoring of the transmitted signal. The results of field experiments indicate that the nature of the received VLF fields changes significantly even over moderate distances (20–30 km) and that data cannot be reliably corrected over larger distances. This observatio...

Variations of the VLF-EM primary field: Analysis of airborne survey data, New Brunswick, Canada

Variations in intensity of the primary VLF-EM field have a significant effect on the quality of airborne and ground surveys in which the total field is measured. An analysis of the results of a recent VLF-EM survey and base station records in New Brunswick, Canada, revealed major variations in the primary VLF-EM field due to changes in the transmitting power and to solar flares. To study the effect of systematic primary field variations over the survey area, VLF-EM field intensity was calculated using radio propagation model based on uniform conductive earth and anisotropic‐layered ionosphere. The model correctly predicted a decrease in the primary field generated by the NAA transmitter by about 40 percent in a direction perpendicular to a survey line. Such a change is sufficiently important to require repeated calibrations of the receiver between the survey lines. The model also indicated that the phase difference between contributions of the ground and sky waves should vary over the survey area. Such va...

New developments in AEM discrete conductor modelling and inversion

Discrete conductor models like sphere and plate were introduced in the 1950s as modelling tools in airborne electromagnetic (AEM) survey interpretation. In the last 20 years, with the development of inversion techniques, they have been integrated into parametric inversion programs. The recent advent of powerful workstations makes them useful tools for interactive AEM interpretation. Different problems have been encountered in the implementation and application of discrete objects as modelling and inversion tools. The sphere response is modelled using a sum of spherical functions. Assuming that the radius of the sphere is small compared to the distance between the transmitter and receiver to the centre of the sphere, the response can be approximated by using only the first term of the solution. This approach is reviewed for modelling the response of a conductive sphere in free space or buried in a layered earth. Plate modelling is based on spectral methods or the integral equation method, which provide different techniques for estimating the response of a plate in free space. A comparison of the results of these techniques show differences attributed to the different discretisation methods. A case history from Abitibi, Canada, shows that plate inversion using two different inversion methods provides useful information when the target is a plate-like conductor in a resistive environment. In the last 20 years, sphere and plate models have been integrated in parametric inversion programs which are used today for interactive interpretation of airborne electromagnetic surveys on powerful workstations. Different problems encountered in the implementation and application of these models are discussed and a case history from Abitibi, Canada, is presented.

Case history of combined airborne time-domain electromagnetics and power-line field survey in Chibougamau, Canada

Exploration for volcanogenic massive sulfides requires good geologic understanding. Geologic knowledge often is limited by a lack of outcrops. This is especially true in Canada under residual glacial covers. Geologic information must therefore be complemented by information obtained using means such as geophysical and geochemical observations. Electromagnetic (EM) methods extend lithological understanding to depths beyond the overburden. Massive sulfides are highly conductive and, depending on their depth and volume, may be detected easily by airborne EM surveys. They are more often equant than graphitic sediments, which typically have longer strike length. Current EMtechniques that identify massive sulfides operate in the frequency or time domain, the latter being more common. Additional information can be provided by using power-line fields as a source of EM signals when the powerlines are appropriately located in the area of interest. We have worked in an active exploration area near Chibougamau, Canad...

Correlation between aeromagnetic data rejection and geomagnetic indices

Predicting the rejection of aeromagnetic data would be a useful tool for aeromagnetic survey planning. To relate aeromagnetic survey requirements to geomagnetic activity monitoring and prediction, we analyzed the relationship between the rejection of aeromagnetic data as it is measured during surveys and the variations in existing geomagnetic indices. The magnetic data were collected at Canadian magnetic observatories during 2001 and covered the polar cap, auroral, and subauroral zones. The geomagnetic indices were global and local indices. The global indices included the Kp, ap, and Dst indices. The local indices were the three-component hourly ranges, the three-component maximum rate of change, and the Pc3 pulsation index. The goodness of fit was used to compare the results between the different indices at different locations. In general, there was some correlation between global geomagnetic indices and the rate of rejection of aeromagnetic data. Good correlation with a global index was obtained with the daily mean of the Ap index for a station located in the subauroral zone. The best correlation was obtained with local indices and particularly with the Pc3 index amplitude. From these results we conclude that forecasting Pc3 index amplitude would be a useful tool for planning aeromagnetic surveys.