Location of sub-fresnel scale mineral targets in the subsurface

Moffat, Lucky (2004) Location of sub-fresnel scale mineral targets in the subsurface. Masters thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (13MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

With the decline of already discovered and exploited shallow mineral deposits, the mining industry is intent on finding new techniques that could explore deeper but still be cost-effective. One method that has been considered is the seismic reflection technique which has been very successful in the petroleum industry. This thesis examines one approach, seismic beamforming, which can be used in conjunction with the seismic reflection technique for the direct location of massive sulfide bodies. Implementation of the technique could prove successful in reducing exploration costs. -- Beamforming can be used to extract a variety of information about a target (e.g. range, angle of emergence, temporal frequency content, power) from a propagating signal. This thesis studies the use of the angle of emergence of wavefronts propagating across a linear array of seismic receivers to assist in location of the wavefront source. The angles are back-projected to a common point which represents the point source or scatterer located at the far-field of the receiver array. -- A simple conventional beamformer is designed and tested on data generated from four different geometries of a linear array of receivers and a single source of seismic energy. The intersection of back-projected rays is not tight enough for all four cases but on using a distance-weighting scheme the location of the scatterers is satisfactorily estimated. A number of performance measures are carried out on the beamformer to determine its effectiveness. The beamformer is tested on data subjected to different noise fields, on data decimated in the spatial domain and on data obtained from a geometry that includes the presence of two scatterers which are more than a Fresnel zone apart. The results indicate that the beamformer is able to localize a scatterer when the data has a signal-to-noise ratio of at least 5 dB. The results also show that spatial decimation of the data has no adverse effect on location estimates. However, to achieve desired beam steering resolution, the data need to be interpolated with more spatial points in the beamformer. The beamformer is not able to distinguish between two scatterers in its vicinity when the source is at a point directly between them. But it is able to detect the two scatterers when the source is moved to a point on the other side of either scatterer. -- One main issue that affects the reliability of location estimates is the way the angle of emergence is picked. The angles are handpicked and this has the potential of introducing bias in the estimates which could then affect the overall performance of the beamformer. A computer code can be written to automatically pick these angles. This would improve location estimates and may eliminate the need to use the distance-weighting scheme used in the investigations. -- Overall, the results obtained show that if correctly implemented, seismic beamforming together with reflection seismic can serve as a cost-effective method for the location of mineral deposits.

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