Skip to main content

OPTIMIZATION OF ELECTROMAGNETIC AND GRAVITY METHODS OF MINERAL EXPLORATION FOR THE LOCATION AND DELINEATION OF BURIED OREBODIES IN THE IBERIAN PYRITIC BELT

Objective

- DEVELOPMENT OF AN ADVANCED METHODOLOGY FOR THE DELINEATION OF DEEP OR SHALLOW SULPHIDE OREBODIES LEADING TO A MORE EFFECTIVE SELECTION OF DRILL SITES AND A REDUCTION IN THE HIGH COSTS OF DEEP DRILLINGS IN PARTICULAR.
- IF RESULTS ARE POSITIVE, THIS EC KNOW-HOW CAN BE EXPORTED WORLWIDE WITH SELLING OF PATENTED EQUIPMENT AND SOFTWARE. MOREOVER WIDE POSSIBILITIES OF APPLICATION EXIST IN THE EEC AND MAY LEAD TO DEVELOPMENT OF NEW MINES AND EVEN OF NEW MINING DISTRICTS.
The objective was to develop a methodology of measurement and interpretation for the various frequency domain electromagnetic method (FEM) techniques devised at BRGM (the surface multifrequency electromagnetic (EM) system MELIS, the borehole 1-axis EM receiver probe REMI, the borehole 3-axis EM system with receiver probe ARLETT), and to assess their ability to detect and characterise conductive deposits. The study was effected by computer modelling and by field experimentation at Valverde (Spain). For the 2 EM methods studied in detail (surface to surface MELIS, and surface to borehole REMI), the measurement procedures have been optimised and a method of interpretation has been developed by multiple 2-dimensional modelling. Subsequently, the measurements made at Valverde were successfully interpreted. The 2 methods are now operational and ready to be marketed. The surface to borehole 3-axis EM method ARLETT (under development) has been studied only briefly. The modelling of this method has provided the direction for continuing research.

Methodology has been developed for measuring and interpreting results obtained by various frequency domain electromagnetic (FEM) techniques and systems. The ability to detect and characterise conductive deposits has been assessed in terms of location, depth, thickness, conductance and limits.

The objective of research project was to strengthen the effectiveness of electromagnetic methods for mineral exploration.Efforts were concentrated on the use of transient electromagnetics (TEM). 2 goals were considered of main importance:
to test the depth of exploration and the resolving power under the geological conditions in the Iberian pyritic belt;
to develop a new receiver, which allows a wider time range of measurement channels and provides better possibilities for noise suppression and data processing.

The measurements with the TEM system demonstrated the excellent capabilities of the method very clearly.
TEM can detect massive sulphide orebodies below conducting layers with a conductivity of several Siemens down to a depth of 600 m below surface and soundings allow determination of the vertical resistivity distribution beneath the coil array with a high lateral resolution. Borehole measurements can be used to locate ore even if the orebody itself has been missed by the hole. From borehole data a more accurate value for the conductivity of the ore can be found and help to improve the interpretation of other TEM data. Also, TEM measurements, especially fixed transmitter profiling, can be performed quickly and are practically unaffected by topographic features. This is a tremendous advantage in areas with a strong topographic relief. Besides the field tests with commercially available instrumentation, research has been undertaken to overcome some of the problems found in such units. This lead to the construction of a new receiver with a high digitizing frequency. The data set collected provide a strongly improved processing capability.

The research focused on frequency domain electromagnetic (FEM) techniques both for surface measurements and borehole measurements. The objectives were:
to develop a methodology for measuring and interpreting results obtained by the various FEM techniques and systems (MELIS, a surface to surface multifrequency electromagnetic (EM) system, REMI, a surface to borehole 1-axis EM receiver probe, and ARLETT, a surface to borehole 3-axis EM system under development);
to assess their ability to detect and characterize conductive deposits in terms of location, depth, thickness, conductance and limits.

For the 2 EM methods studied (MELIS and REMI), the measurement procedures have been optimized and a method of interpretation has been developed using 2-dimensional modelling. The 2 methods are now operational and ready to be marketed.
Compared to classical CSAMT, measurements of magnetic only components, such as performed with MELIS, are less sensitive to superficial inhomogeneities.
Compared to time domain transient electromagnetic (TEM), FEM is easier in terms of field operation (stronger signal to noise radio, lighter equipment) and of numerical modelling for interpretation. Although its depth of investigation is limited by the transmitter to receiver distance, the MELIS system appears to be one of the most efficient technique for deep massive sulphide exploration.
It has been possible to study the borehole 3-axis EM method only briefly (the ARLETT probe is still being developed). The modelling achieved on this method has shown the direction for continuing research. Such a research should lead to a promising tool, in view of the good results already obtained with the single axis REMI probe.
THE MAIN OBJECTIVE OF THIS JOINT PROJECT IS THE DEVELOPMENT OF AN ADVANCED METHODOLOGY FOR THE LOCATION OF DEEP SULPHIDE OREBODIES (> OR = 500 M)USING IMPROVED GRAVITY MEASUREMENTS AND DIFFERENT EM SYSTEMS. TEST SITE IS VALVERDE AREA, SOUTH SPAIN.
THE AGRUPACION WILL PERFORM A DETAILED GEOLOGICAL AND STRUCTURAL STUDY MAINLY BASED ON DRILL CORES INFORMATION AND WILL IMPROVE THE INTERPRETATION OF GRAVITY MEASUREMENTS.
BRGM WILL BE INVOLVED WITH FREQUENCY DOMAIN EM METHODS BOTH ON THE GROUND USING THE MELIS SYSTEM AND WITHIN BOREHOLES USING ROMULUS AND ERIC PROBES TOGETHER WITH A REMI AXIAL RECEIVER ABLE TO DETECT CONDUCTIVE OREBODIES UP TO 100 M AWAY FROM A HOLE.
BGR WILL PERFORM TRANSIENT EM MEASUREMENTS ON THE SURFACE AND IN BOREHOLES USING DIFFERENT COIL CONFIGURATIONS. SPECIAL ALGORITHMS WILL BE DEVELOPED TO SUPPRESS NOISE.
RESONANCE GEOPHYSICS LTD WILL CARRY OUT FIELDWORK WITH ITS NEWLY RAS-1 EQUIPMENT WHICH PROVIDES ENERGIZATION OF THE GROUND AT FREQUENCIES UP TO 100 KHZ. IN PARTICULAR THE RESONANCE CONDITIONS WILL BE ANALYZED.
THE UNIVERSITY COLLEGE OF WALES WILL DEVELOP NEW MATHEMATICAL DESCRIPTIONS AND COMPUTING METHODS TO GIVE GEOLOGICALLY RELEVANT INFORMATION OF FIELD DATA.

Coordinator

Enadimsa - Empressa Nacional de Investigaciónes Mineras SA
Address
138,Doctor Esquerdo
28007 Madrid
Spain

Participants (4)

Bureau de Recherches Géologiques et Minières (BRGM)
France
Address
39-43 Quai André Citroën
75739 Paris
Federal Institute for Geosciences and Natural Resources
Germany
Address
Stilleweg 2
30655 Hannover
Resonance Geophysics Ltd
United Kingdom
Address

SY23 3QW Aberystwyth
University of Wales, Aberystwyth
United Kingdom
Address
Old College King Street
SY23 2AX Aberystwyth