Abstract:
Disclosed herein is a technique of prospecting for deep massive sulphide ore bodies, comprising the steps of selecting a geologic region which is substantially acoustically transparent; directing seismic waves at the region and collecting reflected or diffracted waves therefrom; and analyzing the collected waves for the presence of the massive sulphide ore bodies.

Description:
This application is a continuation-in-part of Ser. No. 09/295,378 filed Apr. 21, 1999 now U.S. Pat. No. 6,330,513. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to mineral exploration, and more particularly to methods of detecting deep massive sulphide deposits by the use of seismic reflection surveys. 
     2. Description of the Related Art 
     Multidimensional seismic reflection survey techniques, for example two dimensional (2-D) and three dimensional (3-D) seismic reflection survey techniques, have allowed the petroleum industry to generate remarkably accurate structural and stratigraphic subsurface geologic models. This can be achieved because multiple reflection events are recorded from the highly reflective sedimentary stratigraphy. As a consequence of this non acoustic transparency, the industry has been able to discover oil and gas deposits several thousand meters below the earth&#39;s surface, making deep oil drilling an economically viable procedure. 
     The minerals industry has made a considerable investment in multi-dimensional seismic reflection techniques to adapt the procedure for detecting the presence of deep massive sulphide deposits. However, the use of multi-dimensional seismic technologies by the minerals industry has thus far been met with comparatively less success. In most cases, a massive sulphide is extremely difficult to discern within the complex acoustic wavefield generated by the other stratigraphic contracts of the geological region being explored. 
     To date, mineral exploration using seismic reflection surveys has been typically done on a broad spectrum of geological terrain known to be potential host sites for massive sulphide deposits Unfortunately, many of these regions exhibit poor signal to noise conditions. To date, this relatively poor data has been found to be an inherent feature of multi-dimensional seismic exploration for massive sulphides. Experts in die field have worked diligently to overcome these problems by using state-of-the-art acquisition and processing strategies derived from the petroleum industry, hoping to filter out the noise and to generate an accurate and reliable subsurface geologic model based on the remaining relatively poorly understood acoustic signature of the massive sulphide deposit. With this goal in mind, experts have developed a number of sophisticated procedures which have been published in a number of leading mining journal articles, many of which are listed herein below. 
     However, these investigations have thus far not yielded significant useful results. 
     It is an object of the present invention to provide a novel technique for prospecting for massive sulphides. 
     SUMMARY OF THE INVENTION 
     Briefly stated, the invention involves a technique of prospecting for deep massive sulphide ore bodies, comprising the steps of: 
     selecting a geologic region which is substantially acoustically transparent; 
     directing seismic waves at the region and collecting reflected and diffracted waves therefrom; and 
     analyzing the collected waves for the presence of the massive sulphide ore bodies. 
     In one presently preferred aspect, the present invention provides an exploration process for evaluating a plurality of geological regions that are candidates for massive sulphide deposits. The process includes the steps of obtaining impedance characteristics for corresponding strata located in a selected one of said geological regions; analyzing the impedance characteristics for determining the presence of impedance transitions from one of the strata to another; validating the presence of a host lithology in said strata of the selected region based on said impedance transitions by determining whether said host lithology is substantially acoustically transparent, such that the host lithology is represented by a seismic profile which contains a substantial absence of impedance markers of magnitude greater than that of any survey noise present in said impedance characteristics; and confirming the selected region possesses a host stratigraphy that is substantially acoustically transparent and is therefore a candidate for a subsequent seismic investigation for massive sulphide deposits. 
     In another of its aspects, the invention provides a method of exploration for evaluating a plurality of geological regions for the suitability of direct detection of a massive sulphide deposit. This method includes the steps of obtaining impedance characteristics for corresponding strata located in a selected one of said geological regions; analyzing the impedance characteristics for determining the presence of impedance transitions from one of the strata of the selected region to another; validating the presence of a host lithology in said strata based on said impedance transitions by determining whether said host lithology is substantially acoustically transparent, such that said impedance transitions from said strata primarily contain only survey noise; and confirming the selected region possesses a host stratigraphy that is substantially acoustically transparent and is therefore a candidate for a subsequent seismic exploration for detection of the massive sulphide deposit. 
     Rather than simply applying the latest multi-dimensional seismic techniques on all geologic regions suspected of bearing massive sulphide deposits, the present technique involves selecting only those geologic regions which have a particular range of characteristics, primarily those suspected of having a local host stratigraphy which is essentially acoustically transparent, and then applying seismic survey techniques only on those selected geologic regions. 
     The term ‘acoustically transparent’ as a characteristic of a local host stratigraphy, refers to the ability for seismic waves to pass through the statigraphy while producing minimal or otherwise substantially non-interfering seismic reflections from geological boundaries which are not the boundaries of massive sulphide contacts. For example, an acoustically transparent local host stratigraphy may be one in which a massive sulphide deposit generates a recognizable peak (positive amplitude) on stacked seismic refection trace that exceeds the amplitude of noise from the survey, for example the noise originating from the local host rock itself, that is geologic noise, and the ambient noise originating from the seismic survey source and environment, The local host stratigraphy, in this case, is that which is in the vicinity of he massive sulphide itself, That is above and below the massive sulphide and not necessarily the entire depth investigation range of the geological terrain. In this case, a major ‘marker’ may be above or below the local host stratigraphy and could present a significant peak in an impedance trace but not otherwise substantially interfere with The acoustic transparency of the adjacent local host stratigraphy itself. This geologic marker could be used to help focus the search for anomalous seismic amplitudes to that zone of the stratigraphic column most likely to host deep massive sulphide deposits. For example, in some cases, a local host statigraphy may be essentially acoustically transparent if the seismic amplitude anomaly, resulting from a sulphide orebody, is approximately twice that of contacts within the local host stratigraphy. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Several preferred embodiments of the present invention will now be described, by way of example only, with reference to the appended drawing in which: 
     FIG. 1 a  is a representation of a geological region; 
     FIG. 1 b  is a representation of an impedance log for the geological region of FIG. 1 a;    
     FIG. 1 c  is a representation of a synthetic seismogram of the geological region of FIG. 1 a;    
     FIG. 1 d  is a representation of another geological region; and 
     FIG. 1 e  is a representation of a synthetic seismogram for the geological region of FIG. 1 d.    
     FIG. 2 is a flow chart of a seismic exploration. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present technique is, at least in pan, based on the proposition that these selected geologic regions with an essentially acoustically transparent local host stratigraphy should be the source for a relatively easily detected seismic reflection, namely one generated by a massive sulphide deposit of sufficient size and thickness. This is due to the relatively strong impedance contrast that usuly exists between the massive sulphide deposit and their local host rocks. This means that established seismic techniques can be used to image the approximate size and particular location of the massive sulphide deposit, without first having to separate the deposit signal from the contaminating signals otherwise generated by the strata of the geologic regions not essentially acoustically transparent. 
     Desirably, a candidate geological region can be identified, in some cases, by the use of a relatively straightforward 1-D seismic analysis of a borehole, which can indicate, at least on a preliminary basis, the likelihood that the stratigraphy therein has the correct combination of velocity and density throughout its depth to have what may be a host lithology which presents a sufficiently acoustically transparent profile to make further seismic investigations worthwhile. A 1-D seismic analysis of this type can yield a useful plot (known in the field as a synthetic ‘seismogram’) based on The continuous velocity and density measurements along the borehole, as is illustrated. for example, in FIGS. 1 a  to  1   e.    
     FIG. 1 a  shows the cross section of one example of a desirable geological region, with the vertical dimension corresponding to the depth thereof. A massive sulphide deposit is shown by the block MS within a local host lithology which is below a major stratigraphic marker. FIG. 1 b  shows a view of an acoustic impedance log (velocity×density) indicating the host lithology shows, in this particular example, essentially no impedance contrasts, it being understood that there may in fact be some residual impedance contrast depending on the nature of the geological region in question. FIG. 1 c  shows a synthetic seismogram trace indicating the seismic impulses at the leading and trailing edges of the impedance at the marker as well as at the massive sulphide deposit, while the local host lithology presents relatively little amplitude disturbance apart from the geological and seismic noise, illustrated, for example at the region identified at ‘N’. FIG. 1 d  in turn illustrates a similar geological regional lithology to that shown in FIG. 1 a , but in this case without the massive sulphide deposit of FIG. 1 a . The synthetic seismogram for the geological region of FIG. 1 d  is shown in FIG. 1 e  which in this case has a similar curve to FIG. 1 c , but without the two seismic impulses generated by the massive sulphide deposit. 
     Thus, the present process provides a method by which massive sulphide deposits can be detected by first selecting only those geologic regions whose stratigraphy are, for example, crystalline and which have a local host presenting an essentially acoustically parent seismic profile. Conveniently, the present process may employ the 2-D and 3-D seismic technologies which are common to the petroleum industry. 
     Thus, the present process provides an exploration process which involves evaluating or surveying a plurality of geological regions which are candidates for massive sulphide deposits, identifying a site which has a host lithology is essentially acoustically transparent and carrying out a seismic investigation on the site. 
     One example of the present procedure follows. 
     Rock Properties 
     The first step is to select a number of geological regions tat are suspected of having the desired essentially acoustically transparent local host statigraphy and of hosting deep massive sulphide deposits. Next representative rock samples, both core and surface rocks if desired, may be collected and analyzed, for example at a laboratory bench, for velocity and density If desired, the data may be analyzed in the form of graphs of density versus velocity for the purpose of identifying impedance markers, that is those impedance transitions in the strata that are likely to produce acoustic reflections, the greater the impedance changes from one strata to another, the greater the acoustic reflection as the wave passes through the transition. If there are a number of these impedance markers in the local host lithology, the lithology may not be sufficiently acoustically transparent On the other hand, the absence of these impedance markers in the local host lithology make the geologic region bearing the local host a possible candidate for further seismic analysis. 
     Borehole Logging 
     Once a candidate region has been identified, borehole data may be acquired, either by accessing already recorded data, if such exists for the candidate region, or by having dedicated boreholes sufficiently deep to pierce a significant portion of the stratigraphy of the terrain, in order to measure the continuous ranges in density and velocity This step can use a number of different borehole techniques including that referred to in the field as “slim hole” technology which uses bores less than 10 cm in diameter. 
     Numerical Modeling 
     A series of 1-D synthetic seismograms may be generated from the borehole data to evaluate reflectivity, resolution and signal-to-noise ratio, which are important to determine if the local host lithology is likely to be sufficiently acoustically transparent. The synthetic seismogram is a plot of the expected wavefield response from a local hosting stratigraphy defined by the acoustic impedance log. A substantially acoustically transparent lithology should present a seismogram which is relatively featureless except for ambient environmental and instrument noise, one example is shown in FIG. 1 e.  Additional 2D and 3D synthetic modeling may also be employed to investigate issues such as dip, shape and volume on the expected wavefield response of various hypothetical massive sulphide deposits. 
     Field Test 
     Given the high cost of 3-D surveys, a 2-D test survey line may, if desired, be acquired across the proposed 3-D survey area to generate initial seismic data in an attempt to verify whether the candidate region in fact does possess a local host lithology with an acoustic transparency indicated by the earlier tests. The 2-D analysis results are also used to evaluate the proper locations and other parameters of both the seismic sources and receivers, offsets, fold, frequency content, signal-to-noise and other data acquisition and processing issues, which are well known to the petroleum industry and described in more detail in the references below. 
     3-D Exploration 
     Given the need to properly locate targets in 3-D space and the relative size of the potential targets, 2-D surveys may not be adequate in many cases. It is from the 3-D survey and the subsequent analysis thereof that massive sulphide deposits, if they exist in a local host lithology of the candidate geological region may be detected and, if so, at what size, depth and so on. These studies, per se, are straightforward and well known to those skilled in the art. 
     A technique of prospecting  10  for massive sulphide ore bodies starts by selecting a prospecting region  12  and then determining acoustic properties of the region at step  14  is shown in FIG.  2 . The acoustic properties are evaluated at step  16  to determine whether a substantially acoustically transparent host lithology is present at step  18 . If the transparent host lithology is present, then the seismic investigation is carried out by directing seismic waves at the transparent lithology at step  20  and collecting the reflected waves from the lithology at step  22 . The reflected waves are then analyzed by creating a multidimensional data set that can contain two dimensional or three dimensional data at step  24 . The presence of sulphides is determined through analysis of the data set at step  26 , thereby either signaling the start of a mining evaluation program or stopping investigation in the selected region. 
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