Hernan Ugalde

Mapping of apparent magnetic susceptibility and the identification of fractures: A case study from the Eye-Dashwa Lakes pluton, Atikokan, Ontario

In situ magnetic-susceptibility measurements are only possible on outcrops, which are often limited by overburden and water bodies. An alternative approach is to derive an apparent susceptibility map from total-magnetic-intensity (TMI) surveys, which was done in this study for the Eye-Dashwa Lakes pluton near Atikokan, Ontario. Susceptibility logs of cores directly link alteration to systematic changes in the amount and composition of magnetic minerals. The surficial distribution of alteration zones was originally estimated from a limited number of in situ magnetic-susceptibility measurements. Here, through forward modeling of the TMI data set, susceptibility data are used to validate the apparent susceptibility data set. The modeling accounts for the bathymetric surface of all lakes that cover the area. A two-step process of bulk and local-scale modeling was used to estimate apparent susceptibility patterns. Bulk magnetic susceptibility is used as an indicator of overall alteration content, and local-scale apparent magnetic-susceptibility values are computed using a forward-modeling routine. The new apparent magnetic data set indicates northwest and northeast linears, which are the same as those seen in previous studies.

Cluster analysis of Euler deconvolution solutions: New filtering techniques and geologic strike determination

Euler deconvolution often presents the problem of filtering coherent solutions from uncorrelated ones. We have applied clustering and kernel density distribution techniques to a Euler-generated data set. First a kernel density distribution algorithm filters uncorrelated Euler solutions from those consistently located near an anomalous magnetic-gravimetric source. Then a fuzzy c -means clustering algorithm is applied to the filtered data set. The computation of cluster centers reduces the size of the data set considerably, yet maintains its statistical consistency. Finally, the computation of eigenvectors and eigenvalues on the cluster centers yields an estimate of the geologic strike of the anomalous sources responsible for the observed geophysical anomalies. Therefore, we can obtain an improved strike and depth estimation of the magnetic sources. Although the algorithm can filter and cluster any Euler data set, we recommend obtaining the best solutions possible before any clustering. Hence, we have used ...

Magnetic remanence constraints on magnetic inversion models

A magnetic anomaly is produced when a rock unit has a magnetic contrast with a laterally adjacent rock unit. The cause of this magnetic contrast might be produced by a change in the magnetic susceptibility and/or magnetic remanence of the source bodies. After Vine and Matthews (1963) demonstrated that magnetic anomalies observed over ocean basins record a chronology of ocean floor generation, it was readily apparent that in some instances the genesis of a magnetic anomaly can be dominated by the remanence component. Surprisingly, few investigators have acknowledged that magnetic remanence can have a similar influence on continental magnetic anomalies. The incorporation of magnetic remanence data into a magnetic inversion scheme continues to present a major problem. In this note, we outline the problem and present some approaches that might be used to derive relevant remanence information.

Effect of signal amplitude on magnetic depth estimations

The magnetic signal generated over any source can be analyzed in terms of the two basic components of any wave: frequency (λ) and amplitude (A). With respect to magnetics, the frequency will be a function of the magnetic source body depth and geometry, while the amplitude is a function of magnetization intensity (magnetic susceptibility and natural remanent magnetization, NRM, if present). Many processing and interpretation methods developed over the last 50 years take advantage of the intrinsic relationship between frequency and depth to generate a variety of depth-estimation routines. Furthermore, many methods are independent of magnetic susceptibility (and therefore amplitude) contrasts since the methods incorporate some sort of mathematical expression that nulls the effect of varying susceptibilities.

Integration of lithological, geophysical, and remote sensing information: a basis for remote predictive geological mapping of the Baie Verte Peninsula, Newfoundland

The Baie Verte Peninsula is a geologically complex area on the northwest coast of Newfoundland. The region is composed of multiple geological domains and has undergone extensive tectonic activity, uplift, and deformation, producing multiple unconformity surfaces. In addition, multiple phases of intrusion, some with volcanic cover sequences, add to the complexity of the geological model. This study concentrates on establishing a basic framework of the spatial relationships between individual lithological units and their contained mineral deposits from remotely sensed data. Of primary interest is providing basic lithological discrimination, which could form the basis of future field mapping. Multiple thematic maps were produced from compilations of satellite imagery, aerial photography, high-resolution digital elevation maps (DEMs), airborne radiometrics, and aeromagnetics. Common edge boundaries within the geophysical and spectral data were used to discriminate regions having similar physical and mineralogical signatures. The resulting patterns provide a proxy, or remote predictive method, for recognition of the distinct geological features within the study area. Furthermore, the techniques and methods introduced in this paper can be applied to other areas for producing presurvey geological templates.

An assessment of topographic effects on airborne and ground magnetic data

Recent advances in magnetic surveying have focused on achieving higher levels of instrument sensitivity and better definition of the morphology of the magnetic field through the use of measured magnetic field gradients. Images derived from these high-resolution magnetic surveys are widely used as a direct proxy for geologic mapping, especially in areas of limited surface exposure. Commonly, this involves the application of skeletonization (e.g., multiscale edges, or “worms”), Euler, and/or wavelet-based processing routines to generate estimates of the location, and morphology of the edges of anomalous source bodies. The primary assumption for all of these image- (map-) based data processing routines is that the observed magnetic data set provides an unbiased representation of the magnetic mineral variation in the surface and subsurface geology. This assumption may be valid when the observed magnetic anomalies are greater than 5000 nT and the topography is relatively flat, but it is certainly not valid whe...

Predictive Ore Deposit Targeting Using Neural Network Analysis

Summary New multispectral, hyperspectral and radar sensors provide an opportunity to improve integrated interpretation of geophysical and remote sensing data. A methodology has been developed to extract geological signatures from these data, and predict ore deposit targets using neural network simulation. It has been successfully tested on a number of deposit types.

Deriving geological contact geometry from potential field data

The building process of any geological map involves linking sparse lithological outcrop information with equally sparse geometrical measurements, all in a single entity which is the preferred interpretation of the field geologist. The actual veracity of this interpretative map is partially dependent upon the frequency and distribution of geological outcrops compounded by the complexity of the local geology. Geophysics is commonly used as a tool to augment the distribution of data points, however it normally does not have sufficient geometrical constraints due to: a) all geophysical inversion models being inherently non-unique; and b) the lack of knowledge of the physical property contrasts associated with specific lithologies. This contribution proposes the combined use of geophysical edge detection routines and ‘three point’ solutions from topographic data as a possible approach to obtaining geological contact geometry information (strike and dip), which can be used in the construction of a preliminary geological model. This derived geological information should first be assessed for its compatibility with the scale of the problem, and any directly observed geological data. Once verified it can be used to help constrain the preferred geological map interpretation being developed by the field geologist. The method models the contacts as planar surfaces. Therefore, it must be ensured that this assumption fits the scale and geometry of the problem. Two examples are shown from folded sequences at the Bathurst Mining Camp, New Brunswick, Canada.

Remote predictive mapping of Cretaceous sediments in the Cypress Hills area, Alberta, using terrain data acquired from aeromagnetic surveys.

The use of radar altimeter and global positioning system (GPS) data combined with levelling techniques is a viable, effective, and cost-efficient method for the production of digital terrain models. Here we show the result of merging two different topographic datasets derived from two separate airborne magnetic surveys with varying control and sample density. The topographic data were then examined using slope analysis and differential gridding algorithms for lineament and contact edge mapping extraction. The contacts derived were subsequently used to derive a three-dimensional (3D) geological model of the area of study. The final model agrees with previous geological maps of the area and shows the potential of the utilization of topographic data for rapid production of a predictive geological map that could be tested by a number of preselected field verification points.

Cluster Analysis of Euler Deconvolution Solutions: New Filtering Techniques And Actual Link to Geological Structure

With the latest advances in airborne geophysical data acquisition, the search for fast and semi-automatic data interpretation tools has become critical. Euler deconvolution is a rapid and reliable method for gravity and magnetic source mapping. However, the large number of solutions that it usually generates makes it impractical. The current work details the application of clustering and kernel density distribution techniques to a standard Euler generated dataset. This approach allows the discrimination of statistically valid solutions and the extraction geologically relevant structural (strike and dip) information from the solution database. The output of this algorithm can be used as an input to any 3D geological modeling package.