Sub-surface detection

Abstract

Disclosed herein is a method of sub-surface characterization, the method comprising: generating seismic data by performing a seismic survey; generating electrical / electromagnetic data by performing an electrical / electromagnetic survey; and characterizing the sub-surface in dependence on both the seismic data and the electrical / electromagnetic data.

Description

[0001] SUB-SURFACE DETECTION

[0002] Field

[0003] The invention relates to the characterization of sub-surface materials. Embodiments include using joint analyses of seismic and electrical / electromagnetic measurements to characterize sub-surface materials, such as rock types, fluids and fluid flow.

[0004] Background

[0005] The characterization of sub-surface materials using remote sensing (geophysical) methods has uses in many industries. For example, sub-surface fluid characterization has been used for decades in oil and gas reservoir detection. Other examples of uses include the determination of a rock type in the mining industry and the tracking of sub-surface fluid flow in geotechnical areas.

[0006] In order to reduce uncertainty in interpretation, there is a general need to improve on known techniques for the characterization of sub-surface materials.

[0007] Summary

[0008] Aspects of the invention are set out in the appended independent claims. Optional features are set out in the dependent claims.

[0009] List of Figures

[0010] Figure 1 schematically shows a metal extraction system according to embodiments.

[0011] Description of Embodiments

[0012] Embodiments characterize sub-surface materials based on joint analyses of seismic and electrical / electromagnetic measurements. Embodiments may determine types of sub-surface rocks and fluids, as well as the properties of sub-surface fluid flow. It is well known that seismic techniques can be used to detect sub-surface materials. Seismic imaging is sensitive to the elasticity and density of sub-surface materials and is a geophysical approach well suited to finding geofluids at depths. Seismic techniques are widely used for exploration in the oil and gas industry and are typically used to image fluids at depths of about 1800m (which is a typical underground depth of an oil well). A problem with seismic techniques is that they may not accurately characterize sub-surface fluids. For example, a sub-surface network of isolated fluids typically has the same seismic velocities as a network of interconnected fluids and so seismic techniques may not be able to distinguish between these different situations due to their similar permeabilities. A lot of detailed and expensive analysis is required of data from seismic techniques in order to distinguish between fluid properties with similar seismic velocities. It may therefore be difficult to accurately detect the difference between salty brines, pure water and gas, for example. The cost of performing known seismic techniques may also be very high and time consuming.

[0013] A known alternative approach to sub-surface imaging is the use of electrical and / or electromagnetic techniques. These are sensitive to sub-surface variations in conductivity. High conductivity is an indication of a conducting material that is well connected throughout the rock mass. A problem with electrical and electromagnetic techniques is that they do not accurately distinguish between materials with similar conductivities. For example, a high conductivity may be caused by the detection of connected veins of metal-rich minerals, or it could alternatively be caused by the detection of connected pores filled with salty water.

[0014] Embodiments improve on known techniques by using both seismic techniques and electrical / electromagnetic techniques for the characterization of sub-surface materials. Embodiments also include performing a plurality of surveys with both seismic techniques and electrical / electromagnetic techniques over time to determine and track the location of sub-surface fluids. In a preferred embodiment, the sub-surface fluid flow is associated with leaching agents (or lixiviants) used in obtaining metals from underground rocks. Determining properties of fluid flow can be used to increase the recovery of the lixiviant and for environmental monitoring.

[0015] Embodiments are described in more detail below.

[0016] The seismic techniques used in embodiments may be active or passive. In active seismic techniques, an artificial source is used to generate seismic waves in the ground. The artificial source may be, for example, a sledgehammer, explosives, weight drop or vibrator. The seismic waves are recorded by at least one seismic sensor (such as a geophone and / or seismometer), and typically an array of seismic sensors. The seismic sensors measure reflected, refracted and / or surface waves that have been generated in response to the operation of the artificial source. The seismic waves measured by the seismic sensors are dependent on the properties of sub-surface materials and structures. Well-established techniques can be used to map sub-surface materials and structures. An advantage of active seismic techniques is that they may be performed quickly. Disadvantages of active seismic techniques include the use of an active source sometimes being disruptive. There are locations where the use of active seismic techniques is not allowed due to the potential adverse effects on the environment. Active seismic techniques are also expensive.

[0017] Passive seismic techniques differ from active seismic techniques by not using an artificial source to generate the seismic waves in the ground. The seismic waves are instead generated by natural noise sources. A common example is to use the ambient noise being recorded by the seismic sensors (e.g., cultural, meteorological, or distant storm and wave noise). Another example is that seismic energy may be released by fluids as they flow through the sub-surface. In passive methods, seismic energy must be recorded by at least two seismic sensors (such as geophones and / or seismometers), and typically an array of seismic sensors. The energy recorded on each sensor is cross-correlated with each of the other sensors. The seismic energy may be recorded over a long period of time, such as for a day, for several days, over a period of a few weeks or even longer. The seismic waves measured by the seismic sensors are dependent on the properties of the subsurface materials and structures. Known techniques may be used to characterise sub-surface materials and structures. Passive seismic techniques require a longer measurement period than active seismic techniques. That said, the actual human power required by passive seismic techniques may be less than active seismic techniques. Passive seismic techniques also do not harm the environment and may be performed at a low cost. Passive seismic techniques are described in at least The electrical / electromagnetic techniques used in embodiments include active electrical and electromagnetic techniques that use a power source to generate electrical and / or magnetic fields in the sub-surface. One or more electrical sensors detect the generated electrical and / or magnetic fields so as to characterize the sub-surface. Embodiments include the use of electrical resistivity tomography techniques that use a series of electrodes that are on the surface or down boreholes. Electrical resistivity tomography techniques are described in at least (as viewed on 30th April 2024).

[0018] In a particularly preferred embodiment, electrical resistivity measurements are obtained and used to characterise the sub-surface. To obtain electrical resistivity measurements, current carrying electrodes are inserted into the ground and a potential difference forms around them. The electrodes may be arranged in a number of different configurations, such as a pole-dipole configuration. Embodiments include using an array of electrodes that comprises a large number of electrodes. The voltage distribution surrounding each electrode depends on the electrical resistivities of the subsurface materials and their spatial variations. The measurable potential difference between electrodes can therefore be used to deduce the resistivities, and / or variations of the resistivities, of the subsurface materials. Electrical resistivity measurements can only be used on land. They are for characterising the sub-surface to relatively shallow depths of up to about 1000m, and are not suitable for characterising the sub-surface at greater depths. A description of how electrical resistivity measurements may be obtained in embodiments can be found here: (as viewed on

[0019] 5th June 2025).

[0020] Embodiments also include the use of passive electrical / electromagnetic techniques in which one or more electrical sensors measure naturally occurring fields, such as telluric currents flowing the sub-surface. An example of a passive electrical / electromagnetic technique that may be used in embodiments is the self-potential method as described in at least (as viewed on 30th April 2024). Another example of a passive electrical / electromagnetic technique that may be used in embodiments is the use of electromagnetic fields generated through the interaction of solar wind with the ionosphere or those generated by lightning strikes in the atmosphere. These are known as magnetotelluric (MT) methods and described in at least method#:~:text=The%20MT%20method%20utilizes%20naturally,wind%20with%20the%20Eart h's%20magnetosphere (as viewed on 30thApril 2024).

[0021] As described above, embodiments include using both seismic techniques and electrical / electromagnetic techniques for the characterization of sub-surface materials. Due to its above-identified advantages, the seismic techniques used in embodiments are preferably passive. However, embodiments also include the use of active seismic techniques. The electrical / electromagnetic techniques may be either active or passive.

[0022] In embodiments, a location of interest is determined for surveying.

[0023] A seismic system is deployed. The seismic system comprises seismic sensors, any active sources and all of the equipment required for the seismic techniques (e.g., recording equipment). The components of the seismic system are arranged in accordance with known techniques for the seismic survey to be performed.

[0024] An electrical / electromagnetic system is deployed. The electrical / electromagnetic system comprises electrical sensors, any active sources and all of the equipment required for the electrical / electromagnetic techniques. The components of the electrical / electromagnetic system are arranged in accordance with known techniques for the electrical / electromagnetic survey to be performed.

[0025] As described in more detail below, in a preferred embodiment the surveyed location is the site of an in-situ leaching process and the area of interest may be about 300m to 400m underground. The arrangement of the seismic and electrical / electromagnetic sensors may be determined as appropriate for surveying at such depths. This is a difference to a seismic survey in the oil and gas industry in which the area of interest may be at a much greater depth (e.g. 1800m).

[0026] Embodiments include the use of separate seismic sensors and electrical / electromagnetic sensors. The arrangement of the seismic sensors may be independent from the arrangement of the electrical / electromagnetic sensors.

[0027] Embodiments alternatively include using sensor units with each sensor unit comprising both a seismic sensor and an electrical / electromagnetic sensor. The sensor units may be arranged so that there is a sensor unit at each location required for obtaining seismic data and each location required for obtaining electrical / electromagnetic data. Each sensor unit may then be used to obtain seismic data and / or electrical / electromagnetic data.

[0028] The seismic and electrical / electromagnetic surveys may be performed concurrently. The surveys may be performed over a time period that is long enough to generate the desired survey data. Preferably, the seismic and electrical / electromagnetic data is recorded over several days, over a period of a few weeks, or even longer. A long survey time may increase the accuracy of the surveys because there is more measured data for determining the nature of sub-surface materials. An extended survey time may also allow the flow of fluids to be accurately determined and monitored (i.e., tracked).

[0029] Embodiments include initially performing seismic and electrical / electromagnetic surveys concurrently, and then continuing with only one of these survey types. The initial survey would identify the sub-surface materials and their properties and the continued survey, which may be either the seismic survey or the electrical / electromagnetic survey, could then be used to monitor the flow of the fluids that were characterized by the initial survey.

[0030] The surveys generate both seismic data and electrical / electromagnetic data. These different types of survey data may be automatically processed to characterize the sub-surface materials and their properties at the location of the survey. By using more than one type of survey data, the accuracy of the characterizations may be improved compared to known techniques.

[0031] For example, the electrical / electromagnetic survey may provide similar results for both a network of interconnected veins of metal-rich minerals and an interconnected system of salty brines because they both have a high conductivity. However, the data from the seismic survey would allow these different situations to be clearly distinguished because the network of interconnected veins of metal-rich minerals would increase seismic velocities, whereas the interconnected system of salty brines would reduce seismic velocities. Similarly, the electrical / electromagnetic survey may help distinguish between a sub-surface network of isolated fluids and a network of interconnected fluids, even though they have the same, or similar, seismic velocities. Another example of improved characterization is a porous rock filled with gas. This would have low seismic velocities and also low conductivity. Embodiments therefore provide a survey system that comprises a seismic system and an electrical / electromagnetic system.

[0032] The survey system also comprises a processing system that is configured to receive seismic data from the seismic system and electrical / electromagnetic data from the electrical / electromagnetic system. The processing system is configured to perform sub-surface characterization in dependence on both the received seismic data and electrical / electromagnetic data. The survey system of embodiments provides improved sub-surface characterization through the combined use of different survey types.

[0033] A preferred use of embodiments is in the detection and monitoring of rock types and / or fluid flow in the below described metal extraction system.

[0034] Embodiments include using leaching to extract metals from underground rocks. The extracted metals may include valuable metals such as rare earth metals, zirconium, niobium, tantalum, hafnium and zinc.

[0035] Certain types of underground rocks contain valuable metals that may be recovered by leaching.

[0036] Leaching is a chemical process for selectively solubilising elements of interest from solid substances. A rock that contains metals, for example in the form of metal oxides, is a metal carrier. If the rock is porous and permeable, a liquid lixiviant (i.e., a leaching solution) may flow through the rock. The lixiviant dissolves and / or reacts with at least some of the metals in the rock to form a metal- containing solution, that is referred to throughout the present document as a metal solution.

[0037] The form of the metals, or ions of the metals, in the metal solution is dependent on the chemical composition of the lixiviant and the type of metal. If the lixiviant is sulfuric acid and / or contains a high concentration of sulfate ions, some of the metals in the rock, such as the rare earth metals, may form sulfate complexes that can be highly soluble in the metal solution. The rare earth metals may therefore be present in the metal solution as dissolved metal sulfates.

[0038] The metal solution may flow out of the rock to thereby remove at least some of the metals from the rock.

[0039] In a separate process from leaching, the metals, which may be in the form of metal ions or soluble metal complexes, may then be extracted from the metal solution. There are a number of possible techniques for the metal extraction from the metal solution. For example, the metal solution may be mixed with ammonium bicarbonate so that metal carbonates are formed that precipitate out of the solution.

[0040] If substantially pure metals are required, known techniques may be used for the further processes of separating the different types of metal compound present and obtaining substantially pure metals from the metal compounds.

[0041] Each metal may have different forms when it is in the rock before leaching, when the metal is in the metal solution after leaching, and during the processes for obtaining substantially pure metals. Throughout the present document, references to metals include metals that may be any in form, including metal compounds, metal ions and metal elements.

[0042] A particularly suitable rock for such a leaching process is a peralkaline rhyolite volcanic tuff. Table 1 shows typical main metal oxide compositions of peralkaline rhyolite volcanic tuff as a weight percentage (WT%). Table 1 also shows typical other metals that may be present in trace amounts that are given as parts per million by weight (PPM). The information in Table 1 was published in: Hutchison, W., Pyle, D.M., Mather, T.A., Yirgu, G., Biggs, J., Cohen, B.E., Barfod,

[0043] D.N. and Lewi, E., 2016, The eruptive history and magmatic evolution of Aluto volcano: new insights into silicic peralkaline volcanism in the Ethiopian rift, Journal of Volcanology and Geothermal Research, 328, pp.9-33.

[0044] ROCK AIRFALL PORPHYRITIC PORPHYRITIC

[0045] TYPE PUMICE OBSIDIAN OBSIDIAN

[0046] SIO2 72.81 72.35 72.85

[0047] WT%

[0048] TIO2 0.17 0.17 0.18

[0049] WT%

[0050] AL2O3 7.99 8.14 8.46

[0051] WT%

[0052] FE2O3 7.40 7.52 7.14

[0053] WT% MNO 0.333 0.323 0.30

[0054] WT%

[0055] MGO 0.004 0.000 0.000

[0056] WT%

[0057] CAO 0.14 0.18 0.20

[0058] WT%

[0059] NA2O 6.87 6.96 6.53

[0060] WT%

[0061] K2O 4.26 4.35 4.32

[0062] WT%

[0063] P2O5 0.014 0.011 0.010

[0064] WT%

[0065] SC PPM 2.0 1.9 2.5 V PPM 0.1 0.0 0.1 CR PPM 0.5 0.8 0.8 CO PPM 0.0 5.8 11.7 NI PPM 0.5 0.5 0.6 CU PPM 1.1 0.7 1.6 ZN PPM 475 446 401 GA PPM 35 36 35.5 RB PPM 156 141 128 SR PPM 8.5 7.9 7.1 Y PPM 190 161 145.6 ZR PPM 1655 1437 1283 NB PPM 264 229 204.6 CS PPM 1.5 1.4 1.2 BA PPM 397 369 389

[0066] LA PPM 230 196 176 CE PPM 475 403 365 PR PPM 52.3 45.2 41.9 ND PPM 215 185 165 SM PPM 42 38 34.4 EU PPM 8.5 7.4 7.0 GD PPM 40 34 31.4 TB PPM 6.5 5.5 5.0 DY PPM 38 33 31.4 HO PPM 7.6 6.3 6.0 ER PPM 22 19 17.8

[0067] TM PPM 3.1 2.8 2.5

[0068] YB PPM 20 18 16.8

[0069] LU PPM 2.9 2.6 2.4

[0070] HF PPM 42 37 34.9

[0071] TA PPM 16 14 13.3

[0072] PB PPM 30 26 23.5

[0073] TH PPM 31 27 24.2

[0074] U PPM 8.2 7.3 6.5

[0075] CD PPM 2.3 2.0 1.8

[0076] LI PPM 40 44 38.7

[0077] MO PPM 10 6 10.6 9.8

[0078] SB PPM 0.3 0.4 0.3

[0079] SN PPM 12 6 11.1 10

[0080] TE PPM 0.2 0.3 0.2

[0081] W PPM 3.6 94.5

[0082] TL PPM 0.4 0.3

[0083] GE PPM 2.8 2.3

[0084] AS PPM 3.1 2.3

[0085] Table 1

[0086] The three columns in Table 1 respectively show the specific compositions of three different samples of peralkaline rhyolite volcanic tuff. The two samples of porphyritic obsidian rock were obtained from the same location. The sample of airfall pumice was obtained from a different but nearby location.

[0087] The types of metals that may potentially be obtained in substantial quantities from these specific examples of peralkaline rhyolite volcanic tuffs therefore include all of the above listed metals in either their compound or elemental form. Embodiments are not restricted to only leaching the peralkaline varieties of volcanic tuff and other types of rock may be leached. In particular, embodiments include leaching all types of glassy volcanic tuffs. Rocks with different compositions may be leached to obtain different metals. Further metals that may be obtained by leaching glassy volcanic tuffs include lithium, boron, rubidium, cesium and tin. An advantageous property of peralkaline rhyolite volcanic tuffs is that their phosphorous content is very low. This results in little, or no, rare earth phosphates forming and precipitating during the leaching process.

[0088] Known locations of peralkaline rhyolite volcanic tuffs include regions in Ethiopia, Kenya, Eritrea, Yemen, Turkey, Mongolia, Western USA, North Korea, China and Italy.

[0089] Peralkaline rhyolite volcanic tuffs are suitable for leaching because they are typically glassy, porous and permeable.

[0090] Figure 1 schematically shows a metal extraction system according to embodiments.

[0091] The metal extraction system performs two main processes for obtaining metals from an underground rock. A first main process pumps a lixiviant underground to extract metals from the underground rock. This generates a metal solution that flows through the sub-surface. A second main process, that is performed at the ground surface, then produces metals by removing the metals from the metal solution. The first main process is performed by a leaching system that is comprised by the metal extraction system. The leaching system injects a lixiviant (i.e. a leaching solution) into underground rock. The second main process is performed by a metal production system comprised by the metal extraction system. The metal production system obtains metal solution from the underground rock and extracts metals from the metal solution.

[0092] Figure 1 shows in cross-section three different layers of rock at the location of the metal extraction system. A first layer 101, that provides the ground surface, is the overlying strata and may be, for example, about 300m deep. The first layer 101 preferably comprises substantially impermeable rock such as rhyolite, trachyte or basalt lava flows. A second layer 102, that is immediately below the first layer 101, may comprise the rock that is leached. The second layer 102 may have an internal depth of, for example, about 10m to 100m, and so it may start at about 300m below the ground surface and extend to about 310m to 400m below the ground surface. The rock in the second layer 102 may be the above- described peralkaline rhyolite volcanic tuff, or any other type of leachable rock. A third layer 103, that is immediately below the second layer 102, is the underlying strata. The third layer 103 preferably comprises substantially impermeable rock such as rhyolite, trachyte or basalt lava flows. The rock in the second layer 102 preferably has a substantially higher porosity and higher permeability than the rocks in the first and third layers 101, 103. The second layer 102 may be an aquifer that is bounded by aquicludes above and below it.

[0093] The above-described properties of the underground layers make it a particularly suitable location for the metal extraction system according to embodiments.

[0094] The leaching system of the metal extraction system comprises a lixiviant source 104, an injection wellhead 110 and an injection well 109. The metal production system of the metal extraction system comprises a production well 108, a production wellhead 107 and a metal extraction plant 105. The metal extraction system may also optionally comprise a heat exchanger 106, as will be described later.

[0095] The lixiviant source 104 comprises lixiviant for extracting metals from the rock in the second layer 102. The lixiviant is supplied to the injection well via the injection wellhead 110.

[0096] The injection wellhead 110 comprises a valve arrangement for controlling the fluid flow into the injection well 109 as well as the pressure of the injected fluid into the injection well 109. The injection well 109 comprises a fluid inlet 109a at the ground surface. The fluid inlet 109a may receive a fluid flow from the lixiviant source 104 and / or the metal extraction plant 105. The injection well 109 comprises a fluid outlet 109b that is underground and, as shown in Figure 1, the fluid outlet may be located in the second layer 102. The injection well 109 may comprise a pipe system for supporting the flow of fluid from its fluid inlet 109a to its fluid outlet 109b.

[0097] The production well 108 comprises an underground fluid inlet 108b. The fluid inlet 108b of the production well 108 may be located in the same underground layer as the fluid outlet 109b of the injection well 109. The fluid inlet 108b of the production well 108 may therefore be located in the second layer 102. The fluid inlet 108b may receive a fluid flow from the second layer 102. The production well 108 comprises a fluid outlet 108a that is at the ground surface. The production well 108 may comprise a pipe system for supporting the flow of fluid from its fluid inlet 108b to its fluid outlet 108a.

[0098] The production wellhead 107 may comprise a pressure valve system for controlling the flow of fluids out of the production well 108 and the pressure of the fluid flow. The metal extraction plant 105 receives fluid from the production well 108 via the production wellhead 107. The metal extraction plant 105 comprises apparatuses that are configured to perform one or more processes that extract metal from the received fluid. This may be performed by a number of known processes, such as by using ammonium bicarbonate to precipitate out the metals as metal carbonates. The metal extraction plant 105 may also perform a process for separating the different types of metal compound and obtaining substantially pure metals.

[0099] The metal extraction plant 105 outputs the fluid that has been processed by the metal extraction plant 105. This output fluid may be supplied to the injection wellhead for re-injection into the injection well 109. The output of the metal extraction plant also includes extracted metals by the process performed on the received fluid by the metal extraction plant 105.

[0100] The leaching process of embodiments comprises supplying lixiviant, in a lixiviant solution, to the rock in the second layer 102. The leaching process generates a metal solution in the second layer 102 that is extracted through the production well 108. Metal in the metal solution is then extracted by the metal extraction plant 105.

[0101] The lixiviant source 104 comprises lixiviant that is supplied to the fluid inlet 109a of the injection well 109 via the injection wellhead 110. At, or before, the injection wellhead 110 the lixiviant may be mixed with the output fluid from the metal extraction plant 105 and / or other fluids, such as water. This allows the composition of the leaching solution supplied to the second layer 102 by the injection well 109 to be controlled so that it has the desired composition and concentration of lixiviant, and other properties, for the leaching process.

[0102] The injection well 109 supplies the leaching solution to the second layer 102. Within the second layer 102, the leaching solution flows out of the fluid outlet 109b of the injection well 109 and through the rock in the second layer 102. As the leaching solution flows through the rock in the second layer 102, it dissolves and / or reacts with the rock in the second layer 102 to form a metal solution.

[0103] At least some of the metal solution flows to the fluid inlet 108b of the production well 108. The time required for fluids to flow from the fluid outlet 109b of the injection well 109 and to the fluid inlet 108b of the production well is dependent on how permeable and porous the rock in the second layer 102 is to the leaching solution, as well as the separation of the fluid outlet 109b and the fluid inlet 108b. The typical time for fluid to flow from the fluid outlet 109b to the fluid inlet 108b may be in the range of about 1 day to 1 month.

[0104] The fluid inlet 108b of the production well 108 receives fluids that are present in the second layer. The received fluids may include the metal solution. The received fluids may flow through the production well 108 and to the metal extraction plant 105.

[0105] The metal extraction plant 105 performs processes for extracting at least some of the metal from the metal solution. After the metal extraction processes have been completed, the remaining fluid may be output from the metal extraction plant 105 and supplied back to the injection well 109. Before the output fluid is supplied to the injection well 109, it may be processed, such as filtered or cleaned of other solutes such as silica.

[0106] Embodiments also include the fluid output from the metal extraction plant 105 not being supplied to the injection well 109. The fluid may alternatively be stored or, if it is environmentally safe, allowed to flow away as waste fluid.

[0107] Accordingly, embodiments provide a metal extraction system for extracting metals from underground rocks. The lixiviant source 104, injection wellhead 110 and injection well 109 provide a leaching system that injects a leaching solution into underground rock. The production well 108, production wellhead 107 and metal extraction plant 105 provide a metal production system that obtains metal solution from the rock and extracts metal from the metal solution.

[0108] As described above, the rock in the second layer 102 may be peralkaline rhyolite volcanic tuff. This type of rock is a preferable target for the leaching process of embodiments. It may be a metal containing rock that is permeable and porous to the leaching solution and the metal solution. Due to naturally occurring geothermal heat, the typical temperature of the rock in the second layer may be 150°C to 200°C and this may be the temperature range within which the leaching process occurs. The pressure in the second layer may be about 30 bar to 300 bar.

[0109] The lixiviant source 104 comprises lixiviant that is the main active chemical in the leaching process. Embodiments include the actual leaching solution that is injected into the second layer 102 being only the lixiviant as supplied by the lixiviant source. Embodiments alternatively include the actual leaching solution that is injected into the second layer 102 being the fluid output from the metal extraction plant 105. This may be appropriate if the fluid output from the metal extraction plant 105 comprises a sufficient concentration of lixiviant.

[0110] Embodiments also include the actual leaching solution that is injected into the second layer 102 being a mixture of fluids. In particular, the actual leaching solution may be a mixture of lixiviant supplied directly from the lixiviant source 104, the fluid output from the metal extraction plant 105 and / or other fluids such as water.

[0111] The composition of the leaching solution that is injected into the second layer 102 may be controlled in an attempt to maximize the effectiveness of the leaching process at extracting metal from the rock. The specific composition of the leaching solution may be determined in dependence on the composition of the rock being leached, the temperature of the rock being leached, the targeted metal(s) for recovery and the pressures at the leaching location.

[0112] The leaching solution may be, for example, an acid with a pH of about 4 or less. The leaching solution may alternatively be an alkali with a pH of about 10 or higher.

[0113] When the leached rock is peralkaline rhyolite volcanic tuff, the temperature within the leached rock is about 150°C to 200°C, and the pressure in the leached rock is about 30 bar to 300 bar, a suitable leaching solution is sulfuric acid with a pH of about 4 or less that preferably has a high sulfate content. In particular, the leaching solution may comprise sulfate-rich solutions that may be combined with other acids such as hydrochloric acid.

[0114] There may be a first mass flow controller at the injection wellhead 110 for controlling the pressure of the leaching solution flowing into the injection well 109. There may be a second mass flow controller at the production wellhead 107 for controlling the pressure of the metal solution flowing out of the production well 108. The first and second mass flow controllers may be controlled independently of each other. The first and second mass flow controllers may allow the injection and / or production pressures to be adjusted so as to regulate the flow of the leaching solution through the rock and thereby control the leaching rate. The metal extraction system of embodiments may be a stand-alone metal extraction system. That is to say, the only intended output product of the metal extraction system may be the metal extracted by the metal extraction plant 105.

[0115] It is known to use underground heat sources to generate geothermal energy. A geothermal energy production plant receives a hot fluid that has been heated underground. The hot fluid may be supplied to a heat exchanger where it heats a liquid, such as water or a low-boiling point fluid such as isopentane, or a refrigerant. The heated liquid may be used to drive a turbine, such as a steam turbine, and thereby generate electricity.

[0116] Embodiments also include the provision of a metal extraction system that is combined with a geothermal energy production system.

[0117] As shown in Figure 1, the metal extraction system may comprise a heat exchanger 106 upstream of the metal extraction plant 105. The output fluid by the production well 108 may flow through the heat exchanger 106 before flowing to the metal extraction plant 105. The heat exchanger 106 may be substantially the same as the heat exchangers used in known geothermal energy product plants. Accordingly, the heat exchanger 106 may heat a liquid, such as water. The heated water, that may be steam, may be used to drive a turbine, such as a steam turbine, and thereby generate electricity. Alternatively, the hot fluid may be used for direct use heating purposes such as domestic heating or greenhouses.

[0118] Embodiments alternatively include the heat exchanger 106 being located downstream of the metal extraction plant 105. This may be advantageous if the preferred temperatures of the metal extraction processes are substantially the same as the temperature of fluid that flows out of the production well 108.

[0119] Embodiments alternatively include a first heat exchanger 106 being located upstream of the metal extraction plant 105 and a second heat exchanger 106 being located downstream of the metal extraction plant 105. This may increase the amount of usable geothermal heat that is obtained.

[0120] A combined geothermal energy and metal extraction system provides a number of advantages. In particular, the generated geothermal energy may be sufficient to power the metal extraction system and provide other local energy needs. The combined economic value of the obtained metal and Y1 energy may also be greater than that achievable by only a metal extraction system or only a geothermal energy production system.

[0121] The above-described survey system of embodiments may be used alongside the above described metal extraction system of embodiments. In particular, the survey system may be used to characterize the sub-surface in order to detect the presence of peralkaline rhyolite volcanic tuffs and thereby determine an appropriate location of the metal extraction system. Embodiments also include using the survey system to detect and track the flow of the leaching solution and metal solution in the sub-surface. By detecting and tracking the underground fluid flow, the most appropriate location of each production well 108 may be determined. The occurrence of high risk situations may also be detected, such as if there is a flow of metal solution towards an underground aquifer. Preventative action may then be taken.

[0122] Embodiments include a number of modifications and variations to the above-described techniques.

[0123] The survey system of embodiments may be used in a number of different applications and is not restricted to use with a metal extraction system. For example, the characterization of sub-surface materials and monitoring of sub-surface fluid flow according to embodiments may be used in the mining industry, such as the uranium mining industry, the oil and gas industry, and for general monitoring for environmental purposes (such as the monitoring of ground water flow). In particular, the survey system of embodiments may be generally used with a product extraction system that uses leaching to extract a product from underground rock. The product is not restricted to being metallic and the rock is not restricted to being volcanic.

[0124] The leaching process of embodiments is not restricted to use in peralkaline rhyolite volcanic tuffs. Embodiments include the leaching process being performed in other types of metal containing rocks, such as other types of glassy volcanic tuffs and any other type of leachable rock.

[0125] Embodiments may be used to leach underground rocks at any depth. Embodiments are therefore not restricted to operations at the specific depths described with reference to Figure 1.

[0126] Embodiments also include the metal extraction plant 105 being remote from the leaching system. The metal solution from the production well 108 may be transported to remote metal extraction plant 105 where the metals are extracted. In Figure 1, one injection well 109 and one production well 108 of the metal extraction system are shown. Embodiments include the metal extraction system comprising a plurality of injection wells 109 and / or a plurality of production wells 108. By providing a plurality of production wells 108 around each injection well 109, the amount of metal solution that is collected may be increased.

[0127] Embodiments also include using each well as either an injection well 109 or a production well 108. That is say, an injection well 109 may be converted into a production well 108 by reconfiguring the arrangements at the ground surface for controlling fluid flow into and out of the well. Similarly, a production well 108 may be converted into an injection well 109 by reconfiguring the arrangements at the ground surface for controlling fluid flow into and out of the well.

[0128] When there is more than one production well 108, all of the metal solution received by each production well 108 may be transported for processing at the same metal extraction plant 105. Embodiments alternatively include the metal extraction system comprising a plurality of metal extraction plants 105. This may be appropriate if transporting the metal solution from all of the production wells 108 to the same metal extraction plant 105 is difficult.

[0129] Embodiments include the following numbered clauses:

[0130] 1. A method of sub-surface characterization, the method comprising: generating seismic data by performing a seismic survey; generating electrical / electromagnetic data by performing an electrical / electromagnetic survey; and characterizing the sub-surface in dependence on both the seismic data and the electrical / electromagnetic data.

[0131] 2. The method according to clause 1 , wherein the seismic survey is performed concurrently with the electrical / electromagnetic survey.

[0132] 3. The method according to clause 1 or 2, wherein the seismic survey is a passive seismic survey.

[0133] 4. The method according to clause 1 or 2, wherein the seismic survey is an active seismic survey. 5. The method according to any preceding clause, wherein the electrical / electromagnetic survey is a passive electrical / electromagnetic survey.

[0134] 6. The method according to any of clauses 1 to 4, wherein the electrical / electromagnetic survey is an active electrical / electromagnetic survey.

[0135] 7. The method according to any preceding clause, wherein characterizing the sub-surface includes one or more of determining a sub-surface rock type, determining a sub-surface fluid type, determining the presence of a sub-surface fluid flow, determining the direction of a sub-surface fluid flow, and monitoring a sub-surface fluid flow.

[0136] 8. The method according to any preceding clause, wherein the seismic survey and electrical / electromagnetic survey characterize the sub-surface to depths of up to 1000m, preferably to depths of up to 500m, and more preferably to depths of up to 400m.

[0137] 9. The method according to any preceding clause, further comprising repeatedly performing the seismic survey and / or the electrical / electromagnetic survey so as to monitor the flow of a sub-surface fluid.

[0138] 10. An exploration and product extraction method comprising: performing one or more sub-surface characterizations according to the method of any of clauses 1 to 9; determining, in dependence on the performed one or more sub-surface characterizations, at least one location for performing a leaching process for extracting a product from an underground location; performing an in situ leaching process at the determined least one location by injecting leaching solution into underground rock, wherein the leaching solution is for leaching the underground rock; receiving a product solution from leached underground rock; and performing one or more extraction processes so as to obtain the product from the received product solution.

[0139] 11. The method according to clause 10, further comprising repeatedly performing sub-surface characterizations according to the method of any of clauses 1 to 9 so as to monitor the underground fluid flow. A metal extraction method comprising: an in-situ leaching method comprising injecting leaching solution into underground glassy volcanic rock, wherein the leaching solution is for leaching the underground glassy volcanic rock; receiving a metal solution from leached underground glassy volcanic rock; and performing metal extraction processes that extract metals, that may be in compound or other form, from the metal solution; wherein the method comprises generating a sub-surface characterization of at least the location of the underground glassy volcanic rock by performing the method according to any of clauses 1 to 9. The method according to clause 12, further comprising using the sub-surface characterization to monitor the flow of the leaching solution and / or the metal solution. A survey system for sub-surface characterization, the survey system comprising: a seismic system configured to generate seismic data by performing a seismic survey; an electrical / electromagnetic system configured to generate electrical / electromagnetic data by performing an electrical / electromagnetic survey; and a processing system configured to characterize the sub-surface in dependence on both the seismic data and the electrical / electromagnetic data. The survey system according to clause 14, wherein the survey system is configured to perform the method of any of clauses 1 to 9. A product extraction system comprising: an in-situ leaching system; a product production system; and a survey system; wherein the in-situ leaching system comprises: a lixiviant source; and an injection well arranged to receive a leaching solution at ground level and to inject the received leaching solution into underground rock, wherein the leaching solution comprises lixiviant from the lixiviant source and the leaching solution is for leaching the underground rock; wherein the product production system comprises: a production well arranged to receive a product solution from the leached underground rock and to provide a flow of the product solution to ground level; and a product extraction plant arranged to receive the product solution from the production well and to perform extraction processes for extracting a product from the product solution; wherein the survey system is according to clause 14 or 15, and the survey system is configured to generate a sub-surface characterization of at least the location of the underground rock. The system according to clause 16, wherein the survey system is configured to: determine suitable locations for the injection well and production well; and / or monitor the flow of the leaching solution and / or the product solution. A metal extraction system comprising: an in-situ leaching system; a metal production system; and a survey system; wherein the in-situ leaching system comprises: a lixiviant source; and an injection well arranged to receive a leaching solution at ground level and to inject the received leaching solution into underground glassy volcanic rock, wherein the leaching solution comprises lixiviant from the lixiviant source and the leaching solution is for leaching the underground glassy volcanic rock; wherein the metal production system comprises: a production well arranged to receive a metal solution from the leached underground glassy volcanic rock and to provide a flow of the metal solution to ground level; and a metal extraction plant arranged to receive the metal solution from the production well and to perform metal extraction processes for extracting metals, that may be in compound or other form, from the metal solution; wherein the survey system is according to clause 14 or 15, and the survey system is configured to generate a sub-surface characterization of at least the location of the underground glassy volcanic rock.

[0140] 19. The system according to clause 18, wherein the survey system is configured to: determine suitable locations for the injection well and production well; and / or monitor the flow of the leaching solution and / or the metal solution.

[0141] The foregoing embodiments are only to illustrate the technical ideas and features of the present invention, and their purpose is to enable people familiar with the technology to understand the content of the present invention and implement them accordingly, and cannot limit the protection scope of the present invention. All equivalent changes or modifications made according to the spirit of the present invention should be covered within the protection scope of the present invention.

Claims

Claims1. A method of characterizing the sub-surface of land, the method comprising: generating seismic data by performing a land-based passive seismic survey; generating electrical resistivity data by performing a land-based electrical resistivity survey; and characterizing the sub-surface of the land in dependence on both the seismic data and the electrical resistivity data.

2. The method according to claim 1, wherein the seismic survey is performed concurrently with the electrical resistivity survey.

3. The method according to any preceding claim, wherein characterizing the sub-surface of the land includes one or more of determining a sub-surface rock type, determining a subsurface fluid type, determining the presence of a sub-surface fluid flow, determining the direction of a sub-surface fluid flow, and monitoring a sub-surface fluid flow.

4. The method according to any preceding claim, wherein the seismic survey and electrical resistivity survey characterize the sub-surface to depths of up to 1000m, preferably to depths of up to 500m, and more preferably to depths of up to 400m.

5. The method according to any preceding claim, further comprising repeatedly performing the seismic survey and / or the electrical resistivity survey so as to monitor the flow of a sub-surface fluid.

6. A land-based exploration and product extraction method comprising:performing one or more sub-surface characterizations of land according to the method of any of claims 1 to 5; determining, in dependence on the performed one or more sub-surface characterizations, at least one location for performing a leaching process for extracting a product from an underground location; performing an in situ leaching process at the determined least one location by injecting leaching solution into underground rock, wherein the leaching solution is for leaching the underground rock; receiving a product solution from leached underground rock; and performing one or more extraction processes so as to obtain the product from the received product solution.

7. The method according to claim 6, further comprising repeatedly performing sub-surface characterizations according to the method of any of claims 1 to 5 so as to monitor the underground fluid flows.

8. A metal extraction method comprising: an in-situ leaching method comprising injecting leaching solution into underground glassy volcanic rock, wherein the leaching solution is for leaching the underground glassy volcanic rock; receiving a metal solution from leached underground glassy volcanic rock; and performing metal extraction processes that extract metals, that may be in compound or other form, from the metal solution; wherein the method comprises generating a sub-surface characterization of at least the location of the underground glassy volcanic rock by performing the method according to any of claims 1 to 5.

9. The method according to claim 8, further comprising using the sub-surface characterization to monitor the flow of the leaching solution and / or the metal solution.

10. A survey system for characterizing the sub-surface of land, the survey system comprising: a seismic system configured to generate seismic data by performing a land-based passive seismic survey; an electrical resistivity survey system configured to generate electrical resistivity data by performing a land-based electrical resistivity survey; and a processing system configured to characterize the sub-surface of the land in dependence on both the seismic data and the electrical resistivity data.

11. The survey system according to claim 10, wherein the survey system is configured to perform the method of any of claims 1 to 5.

12. A product extraction system comprising: an in-situ leaching system; a product production system; and a survey system; wherein the in-situ leaching system comprises: a lixiviant source; and an injection well arranged to receive a leaching solution at ground level and to inject the received leaching solution into underground rock, wherein the leaching solution comprises lixiviant from the lixiviant source and the leaching solution is for leaching the underground rock; wherein the product production system comprises:a production well arranged to receive a product solution from the leached underground rock and to provide a flow of the product solution to ground level; and a product extraction plant arranged to receive the product solution from the production well and to perform extraction processes for extracting a product from the product solution; wherein the survey system is according to claim 10 or 11, and the survey system is configured to generate a sub-surface characterization of at least the location of the underground rock.

13. The system according to claim 12, wherein the survey system is configured to: determine suitable locations for the injection well and production well; and / or monitor the flow of the leaching solution and / or the product solution.

14. A metal extraction system comprising: an in-situ leaching system; a metal production system; and a survey system; wherein the in-situ leaching system comprises: a lixiviant source; and an injection well arranged to receive a leaching solution at ground level and to inject the received leaching solution into underground glassy volcanic rock, wherein the leaching solution comprises lixiviant from the lixiviant source and the leaching solution is for leaching the underground glassy volcanic rock; wherein the metal production system comprises: a production well arranged to receive a metal solution from the leached underground glassy volcanic rock and to provide a flow of the metal solution to ground level; and a metal extraction plant arranged to receive the metal solution from the production well and to perform metal extraction processes for extracting metals, that may be in compound or other form, from the metal solution;wherein the survey system is according to claim 10 or 11, and the survey system is configured to generate a sub-surface characterization of at least the location of the underground glassy volcanic rock.

15. The system according to claim 14, wherein the survey system is configured to: determine suitable locations for the injection well and production well; and / or monitor the flow of the leaching solution and / or the metal solution.

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