Application of seismic reflection methods for ore prospecting in crystalline rock

In the period 1971-82, the sulphide-ore-mining companies at Sulitjelma (northern Norway) and Lékken (central Norway) jointly financed a project aimed at adapting the reflection seismic technique for oreprospecting in crystalline rocks. The project was also supported by the Royal Norwegian Council for Scientific and Industrial Research (NTNF). Ore occurrences in Sulitjelma go deep and are stratabound in a flat-lying or slightly dipping sequence of metasediments. Depth to ore bodies from surface is typically 400-1000 m. The ore bodies appear as massive sheets of some hundred metres in length and width and 1-10 m thickness. The ore deposit in Lékken terminates at a depth of about 1000 m. The ore body is massive and ruler shaped, dipping 20-30°, and appears in greenstone in close proximity to an overlying gabbro. The length and thickness of the Lékken deposit are somewhat greater than deposits in Sulitjelma. Field experiments have been performed in the Lékken Mine, shooting at three different levels (down to about 900 m) and recording at the surface. These experiments revealed some basic wave-transmission characteristics which were helpful in determining field parameters for later reflection measurements. A test profile across a known ore body was undertaken in 1979, and good reflections were obtained after processing. Prospecting profiles with various shot-receiver layouts have been shot both in Sulitjelma and Lékken, and reflections have been obtained within the depth range of interest. Diamond-core drilling has been performed at both places. The drilling control indicates that the reflection seismic method with present data-acquisition and data-processing technology is able to detect geological units of different seismic impedance and that the probability of finding ore deposits may be considerably increased by including this technique prior to expensive drilling. As far back as in 1971 the Norwegian mining company A/S Sulitjelma Gruber made their first attempts to apply the reflection seismic method for ore prospecting purposes. Experiments were continued throughout the early 1970s. However, the results of these investigations were not too promising. The data were considerably dominated by noise and no reliable interpretation of the seismic sections was possible. At this stage it was concluded that the recording equipment was not up to the proper standard (too low sampling frequency, etc) and that more efforts should be made in determining favourable values of field parameters, such as geophone coupling, charge type and size, shot-receiver configuration, etc. In 1976, A/S Sulitjelma Gruber, in cooperation with Orkla Industrier A/S -another mining company- established a research project with the following major aims: - to test seismic instrumentation and make it suitable for ore prospecting in crystalline rocks; - to develop methods for data acquisition and subsequent data processing; - to apply the methods to data collected in the ore fields of Sulitjelma and Lékken (Orkla)in an attempt to detect known and unknown ore bodies by seismic means. The project could be realised due to the rapid development of digital recording systems with very fast sampling rate and because geophones with high natural frequency were introduced on the market at that time. Later on, the project work was encouraged by the positive results obtained in the area of seismic high resolution prospecting for coal (Ziolkowski and Lerwill 1979) and by other reflection seismic measurements in crystalline rock (Smithson, Shive and Brown 1977). This paper presents the major results of field experiments, test profiling, prospecting profiling and borehole measurements performed during the project.