The present invention relates generally to mining exploration, geophysical exploration, the detection of natural and/or unnatural sub-surface heat anomalies and heat flows. It is, thus, to be appreciated that the invention can relate to the exploration of both minerals and sub-surface heat. The invention has particular application in the context of locating heat sources below the surface of the ground and will herein be generally described in this context. However, it is to be appreciated that the invention may have other applications.

Background to the Invention

Current mining exploration is generally labour intensive and expensive. Exploration usually involves several techniques, one of which includes drilling boreholes in an area to be explored and then analysing the samples retrieved from the boreholes. These holes are often hundreds of metres deep and so take a great deal of time, and often difficulty, to drill. It can also take time and effort to anaylse the boreholes and the samples obtained.

Another potential issue when drilling exploration boreholes is choosing the precise location to drill. The analysis of borehole samples may suggest little in the way of a useful mining site, simply because the borehole location selected revealed little, if any, valuable deposits in an otherwise valuable deposit area. If the borehole was drilled only a small distance from the site chosen then a valuable deposit may have been discovered, but those involved in the exploration would be none the wiser from the samples obtained from the borehole drilled.

Locating a valuable deposit at a borehole location selected requires the hole to be drilled to the required depth to locate the deposit. If the hole isn't drilled to the required depth then the deposit will remain undiscovered, which is clearly undesirable.

Furthermore, beyond mining exploration there may be other commodities that may exist in the subsurface that may wish to be identified. For example, groundwater, buried infrastructure, or hidden nuclear weapons caches. Additionally there may be interest in knowing the net transfer of energy from the atmosphere to the ground, or the thermo-physical properties of the ground.

The main barrier to detecting buried thermal sources from the surface is the thermal disturbance of the diurnal and seasonal temperature cycles at the surface of the earth. At any given moment and location, the temperature and heat flow in the top few metres of the earth is dominated by the periodic ebb and flow of solar energy diffusing into and out of the ground. Some previous work has been undertaken in relation to terrestrial heat flow probes. Previous work relevant to the development of a terrestrial heat flow probe can be divided into two broad categories; techniques designed to obtain terrestrial heat flow (or temperature gradients) and techniques developed to obtain heat flow on astronomical bodies (e.g. the moon, comets, planets etc). A brief summary of the published work to date is provided below.

Heat flow probes for terrestrial application of various forms have been routinely used to determine heat flow in the deep ocean since the 1950's. The first probes were Bullard and Ewing-type probes, essentially thermistor-lined probes with no in situ thermal conductivity measuring capabilities (Bullard, 1954; Gerard et al., 1962). Samples of the ocean floor were required for thermal conductivity analyses at the surface. The oceanic heat flow probe evolved in the early 1970's to the Lister probe, which included an in situ thermal conductivity sensor in the form of a line-source heater (Hyndman et al., 1978).

Christoffel and Calhaem (1969) designed a heat flow probe intended for use in soft sediments. The six-foot long, cylindrical, steel probe incorporated four thermistors, which measured both absolute and relative thermal gradients, as well as a line-source thermal conductivity sensor in the form of a coil of heating wire wrapped around the probe. However they only reported testing the probe in the relatively shallow water of the Wellington Harbour (NZ) and did not report any experiments conducted on land.

Sass et al. (1981 ) constructed a probe to determine heat flow in boreholes while drilling was still progressing. The probe consisted of a two-metre long steel tube, with three thermistors for measuring temperature, and a coiled-heater-wire thermal conductivity sensor (line-source of heat). The methodology involved ceasing drilling temporarily to obtain heat flow 'on the fly'. The probe was lowered down the drill stem and Injected' about 1 .65 m into the formation at the bottom of the hole using hydraulics. Temperature and thermal conductivity measurements were then taken and the entire process was complete after about one hour. Several such measurements throughout the drilling resulted in a final heat flow value closely comparable to that obtained by high-resolution temperature logging (completed months after drilling so that the hole was thermally equilibrated) and thermal conductivity measurements on core.

Shallow temperature surveys are occasionally utilized during exploration for conventional geothermal systems (i.e. relatively high temperature, convecting systems). Such settings are generally associated with particularly high heat flow and high temperature gradients, which make anomalies relatively easy to detect. Most of these surveys require inserting thermistor probes 1 -2 m into the ground, and allowing the temperatures to equilibrate. Some authors have devised methods of correcting for near-surface effects such as the annual solar cycle (e.g. Olmsted and Ingebritsen, 1986).

Experimenters in Norway investigated the thermal structure and seasonal heat transfer patterns in permafrost using a shallow thermistor-lined probe (Putkonen, 1998). From a full annual cycle of temperature data in the top metre of permafrost, collected at intervals between once per hour and once per day, they determined that thermal conduction was the dominant heat transfer mechanism in that environment.

Coolbaugh et al. (2007) described a recent shallow temperature surveying methodology to detect 'blind' geothermal systems (i.e. those systems that do not have surface features such as hot springs and geysers) by rapid measurement of ground temperature at a depth of two metres. They constructed 2.2 m long, hollow, thin, cylindrical steel probes within which they placed several platinum resistance (RTD) thermometers. A hammer-drill, run by a generator, was used to drive the probes into the ground. The entire system could be transported on the back of a 2-person ATV. Two base stations were set up to monitor the drift of the temperature gradient throughout the survey, due mostly to the annual solar cycle. They used these base stations to 'correct' the other stations by adding to each measurement the average temperature drop of the two base stations between the time the survey commenced and the time of the particular measurement. They found that both base stations declined at a steady rate of ~0.05°C/day for the 9 days that their survey ran. Their method successfully delineated the Desert Queen geothermal aquifer (60 m deep, 90 °C thermal aquifer) and also identified a previously unknown continuation of the aquifer. The authors mentioned above admitted that their correction is not a complete correction for the drift in temperature gradient, as the magnitude and depth to which it penetrates depends on the thermal diffusivity of the soil. They later reported attempts to apply corrections for this effect, and for variations in surface albedo (Coolbaugh et al., 2010). The technique appears useful for detecting temperature anomalies on the order of ±0.5 °C at two metres depth.

Heat flow from astronomical bodies (planets, moons, comets, etc) is of interest to researchers for a number of reasons, arguably the most important of which is to constrain models of planetary evolution and composition (i.e. the amount of radioactive elements) (Hagermann, 2005). A number of heat flow measurements have been made on the lunar surface and measurements are planned in the future for other astronomical bodies (e.g. Mars). The Apollo 13 mission was the first to contain a heat flow probe as part of its payload but was unsuccessful in deploying the probe. The later Apollo 15 mission was the first successful attempt at measuring heat flow.

The Apollo 15 and 17 heat flow probes were essentially identical and consisted of one-metre probes split into two 50 cm sections, each with two differential thermocouples. Thermal conductivity sensors were line-source heaters (coils of heater wire) within the probe. The 'LUNAR-A penetrator', a much bulkier and self- propelled probe, consisted of similar temperature and thermal conductivity sensors. That probe was launched from orbit and penetrated about one metre into the surface. The MUPUS probe, intended to measure heat flow on a comet, is a thin cylindrical carbon fibre probe about 40 cm long, with a bulky head containing electronics. Most of the astronomical heat flow probes contain thermocouples or platinum resistance thermometers (RTD's) for measuring temperature, and coiled-heater wire to generate a line-heat source for thermal conductivity measurements. Banaszkiewicz et al. (2007), however, designed modular thermal conductivity sensors (~2 cm length) of coiled-heater wire, which generate a point source of heat for thermal conductivity measurements.

None of the probes discussed above were specifically designed to be deployed in an array or to detect relative variations in heat flow. None of the probes discussed above measured vertical thermal diffusivity or conductivity.

It would be desirable to provide an alternative exploration method that addresses, at least in part, the disadvantages identified above with drilling boreholes and existing probes.

Moreover, it would be desirable to precisely and accurately record a time series of shallow temperature measurements and thermo-physical ground properties and extract the geothermal conductive heat flow signal from within the solar dominated signal.

Summary of the Invention

According to one aspect of the present invention, there is provided a generally elongate exploration probe. The probe is provided for inserting into the ground to measure heat flow into and out of the ground. The probe includes at least two temperature sensors, with each of the temperature sensors being mounted to or within a probe housing. The sensors are spaced along the probe between a proximal end of the probe and a distal end of the probe. In use, each temperature sensor obtains a separate temperature reading for analysis by a processing unit, which compares the temperature reading obtained from each of the sensors.

Preferably, the probe is provided for inserting into the ground to measure both the temperature and thermo-physical properties of the ground. It is envisaged that each temperature sensor obtains a separate temperature at recorded times for analysis by the processing unit, which can then compare the temperature and time readings obtained from each of the sensors.. In a preferred form, the probe includes five or six temperature sensors, with the sensors equidistantly spaced along the probe. However, it is to be appreciated that the probe may be configured such that it includes any practical number of sensors. Further, while it is desirable that the sensors be equidistantly spaced along the probe, the invention encompasses embodiments in which the sensors are not equidistantly spaced. That said, the distance between the sensors and from an upper end of the probe must be constant and known.

The applicant envisages that each of the sensors may be mounted within a core of the probe housing, although other configurations may be possible. For example, the sensors may be mounted within recesses provided in the outer surface of the housing.

The sensors may be of any suitable type, including thermocouples, thermistors and RTDs. The specific type of sensor adopted may ultimately depend on the necessary precision, number of required components, reliability and cost.

The probe would require insertion into the ground prior to use. Insertion may be undertaken by any suitable mechanical and/or manual means. A drill bit may be mounted to the distal end of the probe to facilitate insertion of the probe into the ground.

The probe may be between approximately 1 .0 and 2.0 metres in length, although any other suitable length may be adopted. Preferably, at least one radial thermal conductivity and thermal diffusivity analyser is integrated or otherwise mounted within the probe. A radial thermal conductivity and thermal diffusivity analyser may be provided for obtaining a radial thermal conductivity and thermal diffusivity reading of the ground (or other medium) into which the probe is inserted, with the reading obtained then provided to the processing unit for analysis. A radial thermal conductivity and thermal diffusivity analyser may, of course, be separately provided. However, it may be less convenient to use a separate radial thermal conductivity and thermal diffusivity analyser in the field when compared to a probe having a radial thermal conductivity and thermal diffusivity analyser integrated therein.

It is envisaged that the probe may include a plurality of radial thermal conductivity and thermal diffusivity analysers equidistantly spaced along the probe. A provision of more than one analyser would allow for a mean thermal conductivity and thermal diffusivity calculation to be undertaken by the processing unit. It would also potentially allow for differences of soil composition (or other medium) along the length of the probe to be recognized and taken into consideration.

In a preferred form, the probe will measure vertical thermal diffusivity and vertical thermal conductivity. Vertical thermal diffusivity will be measured from the diffusion rate of the diurnal temperature cycle along the length of the probe. The volumetric heat capacity of the ground will be found from the ratio of radial thermal diffusivity to radial thermal conductivity obtained from a thermal pulse generated from the probe. Vertical thermal conductivity will be calculated as the product of the volumetric heat capacity and the measured vertical thermal diffusivity.

A depth insertion regulator may be provided for regulating the depth of insertion of the probe into the ground. This is desirable to obtain consistently accurate readings between probes, which may otherwise be compromised if one probe is inserted too deeply or not deeply enough into the ground relative to other probes.

The probe may also include an alignment regulator to regulate the inclination of the probe when inserted into the ground. It is desirable that the probe is consistently inserted at the same angle of inclination, preferably vertical, to allow for consistently accurate readings between probes and avoid errors, which would otherwise be introduced if one probe was inserted into the ground at an angle different to other probes. It is envisaged that the alignment regulator would enable the probe to be consistently inserted into the ground in a substantially vertical orientation. Alternatively, the probe may include a tilt-meter to allow for the correction of results back to vertical if necessary.

Preferably, the probe will be robust enough to withstand repeated penetrations into the ground.

Preferably, each temperature sensor and, optionally, each radial thermal conductivity and thermal diffusivity sensor will be sensitive enough to detect very small differences in temperature and thermal conductivity and diffusivity in the ground. Preferably, the temperature sensors will be accurate enough to measure +/- 0.005 °C to a precision +/-0.001 °C. That said, temperature sensors need not be this accurate when studying more intense heat sources, such as active geothermal sites of high heat flow like those found near volcanos. The applicant envisages that thermal conductivity measurements are likely to be the largest source of error in heat flow calculations, and so care will be necessary to ensure that the measuring technique accurately determines the thermal conductivity of the medium. The invention has, so far, been described in the context of a probe. However, the invention also relates to an exploration device, the device including a probe of the type previously described.

The device is configured for measuring thermal gradient, and preferably also vertical and radial thermal diffusivity, and radial thermal conductivity over a desired depth interval. Vertical thermal conductivity will be calculated from the vertical diffusivity and the ratio of the radial thermal diffusivity and thermal conductivity. Derived heat flow variations will be corrected for variations in vertical thermal diffusivity. The device is intended to delineate geothermal heat flow anomalies.

It is envisaged that the device would be used in conjunction with a processing unit having a processing algorithm. The algorithm would be used to interpret data obtained from the data logger of each probe. The processing unit may include a crustal heat flow calculator for calculating the crustal heat flow from the thermal conductivity and temperature sensor readings. The processing unit may include algorithms to utilise diurnal and annual temperature signatures for the purpose of calculating the vertical thermal diffusivity. The processing unit may include algorithms for calculating the ratio of radial thermal conductivity and thermal diffusivity of the medium surrounding the probe at one or more depths.

The thermal gradient will be affected by the vertical thermal diffusivity of the particular soil/rock/material into which the probe is inserted. Therefore, the processing unit is preferably configured to calculate the vertical thermal diffusivity from the readings obtained from the probe.

The invention has been generally described in the context of only one probe. However, in practice, it is envisaged that a plurality of probes will be used to explore an area of ground, with the probes inserted into the ground at spaced intervals over the exploration area. Preferably, the probes will be equidistantly spaced in a matrix arrangement over the exploration area, although other probe layouts over an exploration area may be possible. Generally, the greater the area to be explored, the greater the number of probes required.

The temperature sensors of each probe preferably provide separate temperature readings for analysis by the processing unit. This allows for the processing unit to separately calculate a thermal gradient for the medium at the site of each probe within an exploration area.

The invention also relates to a method of use of both the probe and the exploration device. Broadly, the method of use of the probe includes inserting the probe into the ground, and then obtaining a temperature reading from each temperature sensor of the probe. The method also includes providing the temperature readings to a processing unit for calculating a thermal gradient at the location of the probe. Preferably, the step of simultaneously obtaining temperature readings is repeated at pre-defined intervals. More preferably, the step of obtaining temperature readings is repeated multiple times at predefined equal time intervals. It is to be appreciated that the greater the number of temperature readings obtained the more likely that extraneous temperature reading components will be identified and removed from the data, resulting in potentially more accurate exploration data being obtained from the probe and exploration device. Also, the greater the number of temperature reading obtained the greater will be the precision and accuracy of the interpreted results. The exploration device preferably includes a plurality of probes, and so the method of use of the device includes obtaining temperature readings from each probe for processing by the processing unit to determine the thermal gradient of the medium at the site of each probe. Preferably, the time of each temperature reading is also recorded.

The method preferably also includes the processing unit filtering any undesired temperature reading components from each of the temperature readings obtained from each probe. This allows for removal of 'noise' from the readings, but not the diurnal signal.

The method, in a preferred form, also includes obtaining a radial thermal conductivity reading of the soil surrounding each probe from at least one radial thermal conductivity analyser provided in each probe. The radial thermal conductivity readings could, of course, be obtained from an analyser(s) which is separate to the probes.

The processing unit may also be configured to enable calculation of the vertical thermal diffusivity and crustal heat flow of the soil surrounding each probe from the temperature and/or thermal conductivity readings obtained from each probe. Brief Description of the Figures

It will be convenient to hereinafter describe a preferred embodiment of the invention with reference to the accompanying figures. The particularity of the figures is to be understood as not limiting the preceding broad description of the invention. Figure 1 is an exploration device according to one embodiment of the present invention.

Figure 2 is a magnified portion of the device illustrated in Fig. 1 .

Figure 3 is a schematic diagram of a sensor board that forms part of the device of Fig. 1 .

Figure 4 is a schematic diagram of a data logger that forms part of the device of Fig.1 . Detailed Description of the Figures

Referring to the Figures, there is shown an exploration device 10.

The device 10 includes a hollow, elongated probe 12 having a stainless steel housing 13. It is to be appreciated that the probe 12 shown is merely a proto-type. It is envisaged that the probe 12 could be approximately 1 .6m in length, although any suitable length may be selected. A drill bit 14 is mounted on the distal, lower end 16 of the probe 12 to assist in inserting the probe 12 into the ground (or other medium). The drill bit 14 will add to the overall length of the probe 12.

The illustrated probe 12 includes two temperature sensors 18,20, although it is anticipated that a final form will include more than two sensors. It is currently envisaged that five or six sensors may be included, although this number may vary. The sensors 18,20 are thermistors mounted alongside analog-digital converters and other electronic components, although any other suitable temperature sensor components may be used. The temperature sensors 18,20 form part of an assembly 22, which is mounted within the probe 12. The assembly 22 has been removed from the probe 12 in Figure 1 , and a portion of the assembly 22 has been magnified in Figure 2. The assembly 22 is receivable through the proximal, upper end 24 of the probe 12, which may be opened for receiving and removing the assembly 22 for deployment, maintenance and repair.

It can be seen that, with the assembly 22 fitted inside the probe 12, the sensors 18,20 would be spaced along the probe 12 between the proximal end 24 and distal end 16 of the probe 12. In use, the temperature sensors 18,20 obtain separate temperature readings at one or multiple times for analysis by a processing unit (not shown). The processing unit then compares the temperature and time readings obtained from the sensors 18,20 to determine the thermal gradient in the surrounding ground at the time(s) of measurement.

The device 10 and associated method of using the device 10 has been specifically developed by the applicant to rapidly and precisely measure the thermal gradient and potentially also the heat flow over a specified depth interval from 0.1 m to 1 .1 m beneath the ground surface. Of course, the device 10 and method may be reconfigured for a different depth interval and for a different depth if desired by reconfiguring the probe.

The device 10 and method are intended to precisely, accurately and reliably measure the thermal diffusivity of the medium over the depth interval. Although the illustrated embodiment isn't configured for measuring the thermal conductivity of the ground, other embodiments may allow this. It is to be appreciated that the thermal conductivity may be measured by a thermal conductivity and thermal diffusivity analyser (not shown), which may be separate from the device 10, or may be integral to the device 10.

In use, the thermal gradient and thermal conductivity readings would be combined to provide a relative (rather than an absolute) variation in crustal heat flow between probe locations. Conductive heat flow cannot be measured directly. Rather, it is found from the product of geothermal gradient and thermal conductivity. Geothermal gradients require the measurement of temperature at a number of discrete depth intervals.

In practice, the device 10 will include a number of probes 12 inserted into the ground over an area to be explored. This enables the device 10 to identify regions of the ground with a relatively high heat flow. It is envisaged that the device 10 will be particularly useful in the exploration for geothermal resources, high heat-producing uranium deposits and uranium-enriched mineralization (e.g. lOCG-type ore bodies). In this regard, relatively large, conductive heat flow anomalies in Australia are known to exist above bodies enriched in radioactive elements (U, Th and K) due to the production of radiogenic heat.

Crustal heat flow is a measure of the rate of heat loss through a unit area of the surface of the earth. Conduction is the dominant heat transfer mechanism in a solid, relatively impermeable, medium. With decreasing viscosity (e.g. the mantle) or increasing permeability (e.g. the vertical fracture systems), advection plays an increasing role through movement of heat energy with the medium itself or fluids within the medium. Conduction is generally assumed to be the dominant heat transfer mechanism within the Earth's crust, although the localized influence of advection may be revealed during measurements.

Thermodynamic principles dictate that all heat generated within the Earth's crust must flow to the surface, thus leading to anomalous heat flow in the vicinity of active heat sources.

It is noted that temperature disturbances due to the daily surface temperature variations can be significant at depths less than 1 m. The variation due to the annual temperature cycle can be expected to affect the gradient to depths greater than 20m. The present invention aims to take into account this temperature variation by obtaining data over an extended period of time, which will allow the daily temperature cycle component of the data obtained to be identified and removed from the data. The annual temperature cycle component is expected to be largely constant across a survey area, and its effect on thermal gradient will be related to the vertical thermal diffusivity of the ground. The probe will measure the vertical thermal diffusivity of the ground, allowing corrections to be applied to the data. Any heat flow variations observed after removal of the daily signal and correction for vertical thermal diffusivity could be due to underground heat sources. The observed rate at which the daily diurnal pulses of heat from the atmosphere diffuse into the near surface will be used to calculate the vertical thermal diffusivity. The probe 10 (or, alternatively a separate tool used in conjunction with the probe) will generate its own heat signature for determining the volumetric heat capacity of the ground from the radial thermal conductivity and thermal diffusivity. The vertical thermal conductivity is the product of vertical thermal diffusivity and volumetric heat capacity.

Preferably, the thermal conductance of each probe is minimized in the vicinity of any thermal conductivity and thermal diffusivity sensors provided in the probe (not included in the illustrated embodiment), so that the thermo-physical properties of the ground dominates the thermal conductivity and thermal diffusivity and temperature measurements obtained. A depth insertion regulator (not shown) is provided for regulating the depth of insertion of the probe into the ground. This is desirable to obtain consistently accurate readings, which may otherwise be compromised if different probes are inserted to different depths into the ground. The probe might also include an alignment regulator (not shown) to regulate the inclination of the probe when inserted into the ground. It is envisaged that the alignment regulator would enable the probe to be consistently inserted into the ground in a substantially vertical orientation. The basic embodiment shown in the figures includes a data logger 26 for logging the temperature readings obtained from the sensors 18,20. In more sophisticated versions, the data logger 26 may be replaced with a telemetric data transmitter or integrated with the processing unit. It is envisaged that the processing unit would contain software allowing for analysis of the temperature readings obtained from the sensors 18,20. The processing unit may take the form of a PC or laptop. If the probe 12 was also configured for obtaining thermal conductivity and thermal diffusivity readings then the processing unit would preferably be configured to also analyse those readings. The software would also include a filter for filtering unwanted temperature reading components from the temperature sensor readings. The device 10 might also include an external atmospheric thermometer (not shown), in which case ambient atmospheric temperature would also be recorded by the data logger 26.

The sensors 18,20 are provided as part of respective sensor boards which, in turn, form part of an insert assembly for receiving within the housing 13. The insert assembly is designed to hold the sensors at known depths and intervals within the probe, for example at 10, 30, 50, 70, 90 and 1 10cm beneath the surface. The insert assembly is designed for inserting into an upper open end of the housing 13 after the housing is inserted into the ground. A cap then closes the upper open end of the housing 13 providing a watertight seal. A cable extends through the cap to provide power to the sensor boards and to transmit data from the sensor boards. The sensor boards are designed to operate in a paraffin oil environment, although they would also operate within other electrically insulating fluids. The data logger 26 is connected to the probe by wire 28. The logger and/or processing unit may instead be wirelessly connected to the probe 12 using telemetry apparatus. Wireless connection may be particularly practical for connecting multiple probes 12 spread out over a large area. Each probe may have a separate data logger at the surface, although it would be possible to connect several probes to a single data logger. The data logger controls the timing of data collection and stores the collected data, although it is possible that the data logger could immediately transmit the data to another location. The data logger contains a power source and clock for the thermistors (which are mounted on 'sensor boards'). The data logger contains an internal clock that can be synchronized to the clock on the PC that initializes the data collection. The data logger is initialized by an operator, but is then self-contained during a measurement.

The temperature sensors 18,20 of each probe 12 provide separate temperature readings for analysis by the processing unit, whereby the processing unit calculates a thermal gradient for the medium at the site of each probe within an exploration area.

A controller (not shown) is integrated with the logging unit, and controls the operation of the probe. Suitable software would be provided for the controller to obtain a timed temperature reading from each temperature sensor of each probe for processing by the processing unit. This would be repeated a large number of times, possibly over many days, weeks or months.

An operator input (not shown) is provided on the data logger 26 or processing unit, allowing for the manual input of device operating parameters. In use, each of the probes is inserted into the ground to the required depth and at the required inclination. Once the probes are in place, the processing/control unit (or data logger 26) can be connected by wires or wirelessly to the probes to enable temperature readings from each temperature sensor of each probe to be obtained and fed to the processing/control unit. This step is repeated multiple times over an extended period of many days, weeks or months.

The inventive method also includes obtaining a vertical thermal conductivity and thermal diffusivity reading of the ground at the site of each probe 12, which may be done using the device 10 (if fitted with a thermal conductivity and thermal diffusivity analyser) or using a separate device (if not fitted with a thermal conductivity and thermal diffusivity analyser).

With this information, the processing unit can then calculate the thermal gradient and vertical thermal conductivity of the ground at the site of each probe. Specific details of the exemplary components, including the calibration, deployment and use of the components is provided below. Comment pertaining to the interpretation of the data obtained from the exemplary probes is also provided.

Thermal gradient measurement Sensitivity, accuracy and stability of temperature sensitive components of the probe 12

Individual temperature sensitive components are responsive to temperature changes less than one milli-kelvin (<0.001 K).

Individual temperature sensitive components are accurate (after calibration) to better than five milli-kelvin (±0.005 K).

Individual temperature sensitive components are stable to within the sensitivity of the recording equipment. Accuracy, precision and stability of temperature sensitive components are maximised by carrying out analog-to-digital conversion of the component responses on the sensor boards.

Precision of sensor boards is about ±0.0003 K over the range 0-50°C, although improvements in digital component technology could provide for greater precision.

Calibration

Each assembled sensor board (rather than the individual temperature sensitive component) is calibrated for its digital response versus temperature.

- The output of each sensor board is a unique digital integer at any given

temperature.

Calibration involves mapping the precise digital response of the sensor board against precisely and accurately known temperature

Each sensor board is calibrated over the temperature range 0-50 °C, although it could be calibrated over another temperature range.

Measurement

A single thermal gradient measurement involves simultaneously recording the digital response of all sensor boards in a deployed probe, then using the calibration data for each individual sensor board to convert the digital responses to precise and accurate temperatures.

It is anticipated that a time-series of thermal gradient measurements will be carried out during any given survey, at regular time intervals for an indefinite period of time.

The logging box controls measurement time intervals.

- Measurement depth intervals are controlled by the position of the sensor

boards on the probe insert within the casing.

It is anticipated that only data from the shallowest and deepest sensor boards will be used to measure thermal gradient, although data from other sensor boards could also be used.

- It is anticipated that any individual thermal gradient measurement will be

dominated by the daily solar heating and nightly cooling of the ground.

It is anticipated that the effect of diurnal (daily) heating and cooling can be recognised and negated by collecting a series of individual thermal gradient measurements at regular time intervals over an extended period of time, and applying mathematical processes to the data.

It is anticipated that there are at least two possible approaches to processing the time-series data to negate the diurnal temperature cycle, but other approaches could also be possible.

The first anticipated approach is to apply a frequency-domain low pass' filter to the time series temperature data from each individual sensor, with a cut-off period much greater than 24 hours.

It is anticipated that a time-shift might also be applied to the time-series data of all sensors relative to the shallowest sensor to account for the diffusion of the seasonal temperature cycle into the ground.

The temperature gradient between any two sensors at any given time with the diurnal signal dampened or removed is then the difference in the temperatures of the two sensors (after filtering and time-shift) divided by the difference in depth between the sensors.

The second anticipated approach is to forward model the diffusion of shallow temperature changes into the ground over a time much greater than 24 hours. Derive the starting surface temperature and gradient that would result in the convergence of the predicted and observed deepest temperature time-series after a nominal time, given the diffusion of the observed temperature changes at the shallowest depth.

It is anticipated that the nominal time might be 14 days, but other nominal times might also be considered.

The 'starting surface temperature and gradient' is effectively the thermal state of the ground at the start time, excluding the effect of the diurnal temperature cycle.

It is recognised that vertical thermal diffusivity must first be precisely known in order to follow this approach.

It is recognised that the thermal gradient after processing the diurnal temperature cycle from the data by any approach might still be influenced by longer period temperature cycles such as weather patterns and seasons. Thermal conductivity Hardware

Heater wire of known and constant length and thermal resistance in each probe.

- Power regulator to maintain constant and known electrical power to better than ±1 % through heater coil for duration of each measurement.

Current to be maintained over a set period of time to be defined before each measurement.

Calibration

- Each probe to be constructed so that all probes have approximately identical thermal properties, so that thermal property calibration of a small number of probes is sufficient to characterise all probes.

Thermal properties include the size, shape and volumetric heat capacity of the steel casing, insert, sensors and oil assembly.

- Thermal properties include the potential for thermal coupling with the

surrounding medium.

Thermal properties include the length and thermal resistance of the heater wire Calibration includes defining the 'self-heating time' of the probe once the heating circuit is activated.

- Self-heating time is defined as the time (seconds) at which recorded

temperature-time curves diverge from each other after the heating circuit is activated.

Repeated measurements in at least three media of different volumetric heat capacity can be used to derive self-heating time.

- Following the method of Waite et al. (2006), the self-heating time becomes the effective start time of each subsequent measurement.

Vertical thermal diffusivity

Simultaneously record precise temperature at each depth at constant pre-set time intervals not greater than 15 minutes over a period of time (same data set as for thermal gradient measurement).

Transform the discrete time-series temperature data into the frequency domain. Derive the phase shift of different temperature-cycle frequencies with depth relative to the shallowest sensor.

Derive the average vertical thermal diffusivity of the surrounding medium between the shallowest sensor and each successive deeper sensor. Volumetric heat capacity

Operate the heater circuit as a line source needle probe' as first described by DeVries and Peck (1958)

Record precise temperature at all depths at short time periods for a preset length of time after heater is turned on; eg 2 second intervals for 30 seconds. - Continue recording temperature at all depths at a longer time period for a

preset length of time; eg 30 second intervals for 30 minutes.

Apply method as described by Waite et al. (2006) to derive individual radial thermal diffusivity and conductivity estimates from each set of sensor data except the top and bottom sensors of the probe assembly.

- Estimate volumetric heat capacity (radial thermal conductivity / radial thermal diffusivity) for each set of sensor data except the top and bottom sensors of the probe assembly.

Calculate the average volumetric heat capacity of the surrounding medium from the arithmetic mean of the estimates in the previous step. Vertical thermal conductivity

Calculate vertical thermal conductivity as the product of vertical thermal diffusivity and volumetric heat capacity.

Alternatively, calculate vertical thermal conductivity as the harmonic mean of the radial thermal conductivity measurements Deployment

Minimal surface disturbance

Optimal operation of the probe requires that the physical surface of the ground in the area in which the probe is to be deployed has been undisturbed for a significant period of time (years).

- Optimal operation of the probe requires that the surface albedo in the area in which the probe is to be deployed has remained constant for a significant period of time (years). Optimal operation of the probe requires that the surface albedo around the probe remains constant during deployment and measurement

Therefore every effort should be made to minimise disturbance of the ground surface during probe deployment. Known and constant depth

Individual probes are designed for insertion into the ground to a known and constant depth.

A line etched around the top of the probe is intended to lie level with the ground surface when the probe is correctly inserted into the ground.

- The probes are designed to hold the sensor boards at constant positions

relative to the probe casing, and hence at constant depths underground when the probe is correctly inserted into the ground.

Known inclination

The probes are designed for deployment in a vertical orientation.

- It is recognised that it might not be possible to always insert the probes exactly vertically in the ground.

It is anticipated that the inclination of the probe casing after insertion into the ground will be measured to an accuracy of 0.1 ° or better.

Inserting the sensors

- After insertion into the ground and measurement of inclination, the probe

casing should be filled with paraffin oil or other electronically inert fluid.

The inserts with sensor boards and heating wires should then be carefully inserted into the casing and screwed securely into place.

Tests should be carried out to ensure there is no electrical short between the heating wire and the probe casing.

Air temperature

It is anticipated that surface air temperature will be recorded at the same times that underground temperatures are recorded.

Although it has no direct influence on the ground heat flow measurement, it is a useful dataset during subsequent validation and interpretation of the

subsurface temperature data. Logger boxes

It is anticipated that logger boxes will be directly connected to the probes at the surface by one metre of cable, although other lengths of cable might also be possible.

- During measurement, logger boxes should be protected from direct sunlight, precipitation and other environmental impacts (including fauna).

Logger boxes are designed to contain an air temperature measurement component, although this component could be separate to the logger box. Ideally logger boxes should be placed south of the probe in the southern hemisphere and north of the probe in the northern hemisphere to minimise the possibility of overshadowing the probe during measurement.

Array of probes Deployment Array or profile

- The probes are designed for deployment as a group to record data

simultaneously over the same period of time across an area or along a line.

The line does not have to be a straight line.

The location of each probe should be accurately recorded.

The spacing between probes, either in a line or across an area, will reflect the target that the survey is designed to detect.

The spacing does not have to be constant.

Synchronisation of clocks

The internal clocks of each logger box should be synchronised to the clock on the PC used to initialise data collection.

- The same PC should be used to initialise all the data collection in a single survey, or else the individual clocks of all PC's used to initialise the data loggers should be synchronised with each other prior to initialising the data collection.

In this way all clocks in all logger boxes attached to all probes in a survey should be synchronised with each other Deployment process

The process of deploying probes, initialising data collection, and recovering collected data should follow the steps below.

The steps can be carried out concurrently across a number of probes if desired.

It is anticipated that the steps will be carried out in the same order for every survey, but it is possible that the order of the steps could be altered in some circumstances.

First; insert the probe casing into the ground.

- Second; measure the inclination of probe casing.

Third; fill probe casing with paraffin oil or other electronically insulating fluid.

Fourth; install probe insert into casing and screw securely into place.

Fifth; test for electrical isolation between probe casing and probe insert.

Sixth; mount data logger box in an appropriate and secure location.

- Seventh; attach probe to data logger box.

Eighth; initialise temperature data collection for at least 12 hours.

Ninth; stop temperature data collection after the desired time and check data to confirm that the logger box is functioning properly and that the heat generated by drilling the probe casing into the ground has largely dissipated (temperature drifting by less than 0.1 °C per hour on all but the shallowest sensor).

Tenth; carry out a radial thermal conductivity and diffusivity test.

Eleventh; reinitialise temperature data collection.

Twelfth; return to the probe after a pre-determined time.

Thirteenth; download the collected temperature data and assess the data for integrity before reinitialising data collection or proceeding to the next step.

Fourteenth; optionally perform another radial conductivity and diffusivity test.

Fifteenth; remove the insert from the casing.

Sixteenth; remove the casing from the ground.

Data processing and interpretation Order of processing

Convert all binary logging data from all probes to temperature-depth-time records. Clip all data from all probes in a survey so that the data records all cover the same time period to within one measurement time interval (typically 15 minutes).

Determine vertical thermal diffusivity at the location of each probe following the process given above

Determine volumetric heat capacity at the location of each probe following the process given above.

Determine vertical thermal conductivity at the location of each probe following one of the processes given above

Determine the 'starting surface temperature and gradient' or the 'thermal gradient through time with the diurnal signal dampened or removed' at the location of each probe following one of the processes given above.

Determine 'starting surface heat flow' or 'heat flow through time with the diurnal signal dampened or removed' at the location of each probe by multiplying thermal gradient by vertical thermal conductivity.

Determine 'corrected vertical heat flow' at the location of each probe by dividing observed heat flow by the cosine of the probe inclination (where the inclination of a vertical probe is zero).

It is anticipated that additional processing might be applied to the heat flow results related to variations in vertical thermal diffusivity across the survey area or line.

Interpretation of results

It is recognised that corrected vertical heat flow at individual probe locations determined in this way will be influenced by surface temperature cycles of relatively long period.

It is assumed that the long period surface temperature cycles are relatively constant across the survey area or along the survey line during any given survey time interval.

Heat flow results for all probes in a survey will be interpreted relative to each other as a distribution of observed surface heat flow over a survey area or along a survey line. It is anticipated that corrected vertical heat flow will be calculated and its distribution compared across the survey area or line for a number of discrete times during a full survey interval.

Variations in corrected vertical heat flow across the survey area or line will be attributed to variations in underground heat sources.

Advantageously, once the necessary readings have been obtained the probes can be removed from the ground and used again at another exploration site. Advantageously, the present invention potentially minimises the risk of drilling unsuccessful exploration boreholes, which can be time consuming and costly. The present invention also potentially provides a greater likelihood of identifying an in- ground heat source, even where a probe 12 is inserted in the ground only proximate to the horizontal location immediately above a heat source.

Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the construction and arrangement of the parts previously described without departing from the spirit or ambit of this invention.