METHOD OF ESTIMATING A MINERAL CONTENT OF A GEOLOGICAL STRUCTURE

Abstract

A method of estimating a mineral content of a seabed geological structure is provided wherein there is provided at least one geophysical parameter of the geological structure. The method includes inverting the at least one geophysical parameter to estimate the mineral content of the geological structure.

Claims

1. A method of estimating a mineral content of a seabed geological structure, wherein there is provided at least one geophysical parameter of the geological structure, the method comprising, inverting the at least one geophysical parameter to estimate the mineral content of the geological structure; and wherein the at least one geophysical parameter is determined from measured geophysical data by inverting the measured geophysical data to determine the at least one geophysical parameter.

2. As method as claimed in claim 1, wherein the mineral content of the geological structure is determined as a function of horizontal and/or vertical position.

3. A method as claimed in claim 1, wherein the at least one geophysical parameter comprises one or more of: electrical resistivity or conductivity, the induced polarisation coefficient, a magnetic parameter, density, p-wave velocity, and s-wave velocity.

4. A method as claimed in claim 1, wherein at least two geophysical parameters are used.

5. A method as claimed in claim 1, wherein the at least one geophysical parameter comprises at least one of the induced polarisation coefficient, magnetization and density.

6. A method as claimed in claim 1, wherein the geophysical data comprises CSEM data, TEM data, magnetic data, magnetotelluric data, gravity data, and/or seismic data.

7. A method as claimed in claim 1, further comprising obtaining the geophysical data.

8. A method as claimed in claim 1, wherein inverting the at least one geophysical parameter to estimate the mineral content of the geological structure comprising using a Bayesian inversion method and/or a phenomenological model.

9. A method as claimed in claim 8, wherein the method comprises selecting one or more phenomenological models that define the relationship between the at least one geophysical parameter and the mineral content of the geological structure.

10. A method as claimed in claim 1, wherein the mineral content of the geological structure is estimated prior to performing a mining operation of the geological structure and/or during a mining operation of the geological structure and/or after a mining operation of the geological structure.

11. A method as claimed in claim 1, the method comprising inverting the at least one geophysical parameter point-wise to estimate the mineral content of the geological structure for multiple different points/locations/volumes/spaces in the geological structure.

12. A method as claimed in claim 1, the method comprising: (a) obtaining first geophysical data of a first area of the geological structure and processing the first geophysical data to estimate the mineral content of the first area of the geological structure; and then (b) obtaining second geophysical data of a second area of the geological structure and processing the second geophysical data to estimate the mineral content of the second area of the geological structure.

13. A method as claimed in claim 12, wherein step (b) is only performed if the mineral content of the first area of the geological structure is found to be greater than a particular value at any point or location within the first area.

14. A method as claimed in claim 12, wherein the first geophysical data are obtained from a vessel and/or comprise gravity and/or seismic data.

15. A method as claimed in claim 12, wherein the second geophysical data are obtained using an automated underwater vehicle and/or comprise CSEM, TEM, magnetotelluric and/or magnetic data.

16. A method as claimed in claim 12, wherein the second area is a smaller area of the first area.

17. A method as claimed in claim 12, further comprising obtaining one or more geochemical parameters related to the geological structure and processing the one or more geochemical parameters to estimate the mineral content of the geological structure.

18. A method as claimed in claim 1, wherein the mineral content of the geological structure is the metal sulphide content of the geological structure

19. A method as claimed in claim 18, further comprising obtaining a sample of geological structure and/or determining which metal sulphide(s) is(are) present in the geological structure.

20. A method as claimed in claim 1, further comprising making a decision to mine the geological structure if the mineral content is estimated to be above a particular threshold.

21. A method of prospecting for minerals comprising performing the method of claim 1 and using the estimated mineral content in the decision-making process for the mining of a mine.

22. A method as claimed in claim 21, further comprising mining the geological structure.

23. A computer program product comprising computer readable instructions that, when run on a computer, is configured to cause a processer to perform the method of claim 1.

Description

[0084] Preferred embodiments of the invention will now be discussed, by way of example only, with reference to the accompanying drawings, in which:

[0085] FIG. 1 is a general multi-geophysical Bayesian network for estimation of the mineral concentration S; and

[0086] FIG. 2 is a flow chart illustrating a method of estimating a mineral content of a geological structure and mining for minerals.

[0087] As illustrated in FIG. 2, an embodiment of a method of estimating a mineral content of a geological structure and mining for minerals involves six main steps.

[0088] At step 1, geophysical data related to a geological structure is obtained over a relatively large subsea area, which has possibly been identified as being of potential interest, e.g. due to the presence of one or more black smokers.

[0089] As step 2, the geophysical data is processed to obtain an estimate of the mineral content, specifically the metal sulphide concentration, of the geological structure. The metal sulphide concentration is estimated as a function of horizontal position.

[0090] At step 3, if the metal sulphide concentration estimate is above a certain threshold (e.g. as 2.5%, 3.0%, 3.5%, 4.0% or 4.5%) at any locations in the geological structure, then more geophysical data is obtained for those locations. The threshold which is used is determined based on various factors including the economic viability of exploring and/or mining in that location, e.g. as discussed above.

[0091] At step 4, the new geophysical data obtained at step 3, possibly in combination with the geophysical data obtained at step 1, is processed to obtain a further (improved) estimate of the metal sulphide concentration of the geological structure as a function of horizontal position.

[0092] At step 5, if the further estimate of the metal sulphide concentration (determined at step 4) is above a certain threshold (e.g. as 2.5%, 3.0%, 3.5%, 4.0% or 4.5%) at any locations in the geological structure, then a sample of the geological structure at that/those locations is taken, by drilling, and analysed (e.g. in a laboratory) to determine which metal sulphides are present and at what concentration(s). Again, the threshold which is used is determined based on various factors including the economic viability of exploring and/or mining in that location, e.g. as discussed above.

[0093] At step 6, if a/any metal sulphide(s) of interest, e.g. copper, zinc, silver or gold metal sulphide(s), is (are) found to be present at sufficiently high concentration(s), e.g. above 2.5%, 3.0%, 3.5%, 4.0% or 4.5%, then a decision is taken to perform a mining operation for that/those metal sulphide(s) and the mining operation is subsequently performed.

[0094] Each of the above steps 1-6 will now be described in more detail.

[0095] At step 1, geophysical data related to a geological structure is obtained over a relatively large subsea area, such as up to 10,000 km.sup.2. In some embodiments, the geophysical data collected at this step consists of only gravity, (possibly) magnetic and seismic data. This data can be collected using apparatus on board a survey vessel and there is no need, for example, to send an autonomous underwater vehicle (AUV) down to the seabed to collect other kinds of geophysical data.

[0096] However, in other embodiments, one or more of TEM data, magnetic data, CSEM data and magnetotelluric data are also or alternatively collected at this step, for example with EM receivers dropped from a vessel.

[0097] As step 2, the geophysical data is processed to obtain an estimate of the mineral content, specifically the metal sulphide concentration, of the geological structure. Step 2 actually contains two stages: at stage (i), the geophysical data collected at step 1 is inverted to determine geophysical parameters; and at stage (ii), the determined geophysical parameters are inverted to estimate the metal sulphide concentration of the geological structure.

[0098] This processing step is now explained in more detail with reference to FIG. 1.

[0099] Dependencies between physical quantities can conveniently be represented by Bayesian networks. FIG. 1 shows a general multi-geophysical Bayesian network for estimation of a mineral (e.g. metal sulphide) concentration S from geophysical parameters {σ, η, M, ρ, v.sub.p, v.sub.s}. As shown in FIG. 1, the geophysical parameters {σ, η, M, ρ, v.sub.p, v.sub.s} in turn depend on geophysical data, such as controlled source electromagnetic data (CSEM), transient electromagnetic data (TEM), magnetic data (mag), gravity data (gray), and seismic data (seismic), which can be included in an extended Bayesian network. In this figure, σ is resistivity (or conductivity), η is the induced polarisation coefficient, M is total magnetization (including both induced and remnant magnetization), ρ is density, v.sub.p is p-wave velocity, and v.sub.s is s-wave velocity.

[0100] Geochemistry parameter(s) Y may also be used to estimate the mineral concentration S. The geochemical parameters can be obtained by making laboratory measurements of rock samples and/or water samples. For example, the presence or concentration of certain signature minerals from hydrothermal alteration or gases (e.g. Helium 3He) in the sea water may be measured.

[0101] In principle, all of the variables in the Bayesian network of FIG. 1 can be regarded as stochastic. However, here, a simplified approach is used, taking only S and the actual measured geophysical data and the geophysical parameters on which they depend, as stochastic variables. The other variables are treated as deterministic hyperparameters or as having delta-function distributions.

[0102] Thus, in the case where just gravity and seismic data are collected, the main parameters of interest here are gravity data (gray), density (ρ), seismic data (seismic), p-wave velocity (v.sub.p), s-wave velocity (v.sub.s), and metal sulphide concentration (S).

[0103] The Bayesian network can be applied to obtain the joint distribution for a set of parameters, incorporating the principle of conditional independence. The joint probability of a set of stochastic nodes {x.sub.1, . . . , x.sub.n} can be written as

[00001]$\begin{array}{cc}p\left(x_1,\mathrm{.Math.},x_n\right)=\underset{i}{\Pi }p\left(x_i.Math.x_i^{\mathrm{pa}}\right),& \left(1\right)\end{array}$

where x.sup.pa.sub.i denotes the parents of x.sub.i, i.e. nodes on the level above in the network.

[0104] Using equation (1), and marginalizing hidden variables, the posterior distribution for metal sulphide concentration S given gravity and seismic data can be written as

p(S|d)=C∫p(S|m)p(m|d)dm  (2)

where C is the normalization factor, m=(m.sub.1, m.sub.2, . . . , m.sub.n) is a vector of geophysical model parameters and d=(d.sub.1, d.sub.2, . . . , d.sub.k) is a vector of different geophysical data types, as discussed above.

[0105] The integral marginalizes the model parameters.

[0106] The geophysical model parameters m.sub.i may be density, magnetization (induced and remnant), electric resistivity or conductivity, polarization coefficient, seismic P- and S-wave velocity.

[0107] The data d.sub.i may be gravity data, magnetic data, electromagnetic data and seismic data.

[0108] As explained above, in practice, the inversion is performed in two separate steps: [0109] (i) the (e.g. gravity and seismic) geophysical data are inverted to calculate the geophysical parameter(s) on which they depend (e.g. density ρ, p-wave velocity v.sub.p, and s-wave velocity v.sub.s); then [0110] (ii) the geophysical parameters are inverted to determine the metal sulphide concentration S.

[0111] At step (i), the geophysical parameters density ρ, p-wave velocity v.sub.p, and s-wave velocity v.sub.s are computed by inversion of gravity and seismic data. Using Bayes rule, the following is obtained

p(m|d)=p(d|m)p(m)  (3)

[0112] At step (ii), metal sulphide concentration S is computed by inversion of the geophysical parameters, e.g. density ρ, p-wave velocity v.sub.p, and s-wave velocity v.sub.s. Again using Bayes rule, the following is obtained

p(S|m)=p(m|S)p(S)  (4).

[0113] This involves a non-linear phenomenological relationship between the geophysical parameters, e.g. density ρ, p-wave velocity v.sub.p, and s-wave velocity v.sub.s, and metal sulphide concentration S, which is discussed below.

[0114] Finally, the posterior distribution p(S|d) is obtained by means of equation (2). The marginalization of S can be written (in some cases) on a convolution form, which allows for fast and efficient computation using the fast Fourier transform (FFT).

[0115] Step (i) can be performed using standard, well-known geophysical inversion methods.

[0116] In some embodiments, step (i) is performed using a map inversion method, e.g. a Marquardt-Levenberg type map inversion method, to determine laterally varying geophysical parameters (density ρ, p-wave velocity v.sub.p, and s-wave velocity v.sub.s), each averaged over a relevant depth interval.

[0117] Alternatively, step (i) can be performed using any other (e.g. 3D) inversion method serving the same purpose.

[0118] Step (ii) uses a Bayesian inversion method involving a phenomenological model relating the geophysical parameter(s) (e.g. density ρ, p-wave velocity v.sub.p, and s-wave velocity v.sub.s) to the metal sulphide concentration, to calculate the metal sulphide concentration value from the geophysical parameter(s).

[0119] In order to obtain a spatially dependent 3D metal sulphide concentration function, the metal sulphide concentration of the geological structure is calculated point-wise for multiple different points/locations/volumes/spaces in the geological structure. As can be appreciated, the geophysical parameter(s) may vary over the space of the geological structure, and this may correspond to a spatially varying metal sulphide concentration.

[0120] Phenomenological models which can be used to relate geophysical properties to underlying rock properties are illustrated in the charts in FIG. 3.

[0121] Charts (a) and (b) illustrates the relationship between conductivity σ and metal sulphide fraction. The logarithm of conductivity increases linearly with conductivity.

[0122] Chart (c) illustrates the relationship between the induced polarisation (IP) coefficient and the metal sulphide fraction. The induced polarisation (IP) coefficient increases with the metal sulphide fraction.

[0123] Chart (d) illustrates the relationship between the total magnetisation M (remnant and induced magnetisation) and the metal sulphide fraction. The total magnetisation decreases with the metal sulphide fraction.

[0124] Thus, following the above method, the metal sulphide concentration is estimated as a function of horizontal position.

[0125] Next, at step 3, if the metal sulphide concentration estimate is above a certain threshold such as described above at any locations in the geological structure (e.g. forming an area or region of interest), then more geophysical data is obtained for those locations (e.g. at the area or region of interest).

[0126] As discussed above, in some embodiments, the geophysical data collected at step 1 consists of only gravity and seismic data, which is collected from a survey vessel over an area of up to or around 10,000 km.sup.2.

[0127] If step 2 indicates that there may be areas within that area which have sufficiently high metal sulphide concentrations to warrant further investigation, at step 3, more geophysical data is obtained for that (those) locations, e.g. within the area over which geophysical data was obtained at step 1.

[0128] In some embodiments, the geophysical data obtained at step 3 includes one or more of TEM data, magnetic data, CSEM data and magnetotelluric data. These kinds of data can be collected by sending an AUV down to the seabed at the area of interest.

[0129] The area or region of interest over which geophysical data is collected at step 3 is smaller than the area or region over which geophysical data is collected at step 1. For example, the area or region of interest over which geophysical data is collected at step 3 could be around 50 km.sup.2.

[0130] At step 4, the new geophysical data obtained at step 3, possibly in combination with the geophysical data obtained at step 1 (e.g. for that area or region of interest), is processed to obtain a further (improved) estimate of the metal sulphide concentration of the geological structure as a function of horizontal position at the smaller area of interest.

[0131] The processing performed at step 4 follows the same stages (i) and (ii) as set out above in relation to step 2, the only difference being that more geophysical data and parameters are included in the calculations. Thus, the equations given above in relation to step 2 can be suitably modified to account for the geophysical data and parameters which are included at step 4.

[0132] As more geophysical data and parameters are included in the processing step to estimate the metal sulphide concentration, the better (more accurate) the estimate of the metal sulphide concentration becomes.

[0133] In some embodiments, steps 3 and 4 are omitted and all of the geophysical data that is used is obtained and then processed together in steps 1 and 2.

[0134] Next, at step 5, if the further estimate of the metal sulphide concentration (determined at step 4, or step 2 in some embodiments where steps 3 and 4 are not performed) is above a certain threshold such as described above at any locations in the geological structure, then a sample of the geological structure at that/those locations is taken, by drilling, and analysed (e.g. in a laboratory) to determine which metal sulphides are present and at what concentration(s).

[0135] At steps 2 and 4, only the total metal sulphide concentration is determined but not the concentration of (a) particular metal sulphide(s). Some metal sulphides are more valuable than others so it is important to check which metal sulphide(s) is(are) present in the geological structure, and at what concentration(s), before deciding whether or not to mine for it (them).

[0136] Thus, at step 5, a sample is taken from the geological structure from an area which has been determined to have a sufficiently high metal sulphide concentration to warrant further investigation. This sample is then tested in a laboratory to determine exactly which metal sulphides are present and at what concentration.

[0137] In some embodiments, step 5 involves determining whether any of all of copper, zinc, silver and/or gold metal sulphide(s) are present in the geological structure and at what concentration.

[0138] Finally, at step 6, if a/any metal sulphide(s) of interest, e.g. copper, zinc, silver or gold metal sulphide(s), is (are) found to be present at sufficiently high concentration(s), e.g. above 2.5-4%, then a decision is taken to perform a mining operation for that/those metal sulphide(s) and the mining operation is subsequently performed.

[0139] The above method can be used when prospecting for minerals (e.g. metal sulphides), e.g. when planning and performing mineral mining operations.

[0140] In one embodiment, the calculated mineral content (e.g. metal sulphide concentration) is used prior to mining, when deciding where to mine the mine and/or how deep to mine the mine.

[0141] In the same or other embodiments, the calculated mineral content is used during or after the mining of the mine, e.g. when deciding in which direction or to which depth to continue mining.

[0142] The mineral content estimate can be updated during mining based on new measured geophysical data.