Background of the Invention There is a long established practice to deposit rubbish and excess matter of growing cities in landfills. In an attempt to achieve a beneficial end use for landfills once they are full, it is common to turn those landfills into parks and ovals for use by the public. The rubbish and/or excess matter in such landfills decomposes and gas is created.

This can lead to the emission of offensive odours and the risk of fire and/or explosion.

Gas is primarily generated in landfills by the anaerobic decomposition of organic matter. This includes gas produced by material that decomposes rapidly (such as food, paper and lawn clippings) or slowly (such as rubber, wood and textiles).

Methane is the main gas produced in a landfill and constitutes approximately half of the total landfill gas generated. It is also a potential explosion and/or fire risk and has a substantial calorific value and therefore potential to be utilised as a fuel source.

Methane generated from landfill typically constitutes greater than 90 percent of methane emissions from developed countries'waste management activities. In the European Union and the United States, emissions from landfills are the largest anthropogenic sources of methane.

Whether a landfill gas is captured or emitted will depend upon the compaction rate of waste in the landfill. A typical landfill operating in the 1970's would achieve compactions ranging between 130 kg per cubic metre (uncompacted) to 500 kg per cubic metre (well compacted). In recent years, with a need to maximise landfill volume, waste compaction rates commonly exceed 1000 kg per cubic metre. Also, landfills these days are not simply"filling a hole"with significant use of landfill cells with specialised waste lift heights, filling sequences, designated areas for particular types of waste and techniques to maximise compaction. Accordingly, a denser layer of waste is commonly

landfilled over poorly to moderately compacted waste, creating greater resistance for the upward movement of gas. In addition, improved waste covering regimes contribute significantly to resistance to the virtual migration of landfill gas. In such systems, the landfill gas migrates via the path of least resistance around any barriers. Also, deeper landfills, that have a proportionally larger anaerobic zone and greater insulation are more likely to imprison gas produced within the landfill.

In an attempt to reduce greenhouse gas emission and to utilise the gas created in landfills, a system has been developed to extract gas from landfills. This system referred to as Landfill Gas Extraction Systems (LGES) and, as shown in Figure 1, can be retrofitted to an established landfill and includes installing many extraction wells in an evenly spaced grid pattern consistently across the entire landfill. This method is suitable only for landfill sites where the compaction and the types of waste landfilled are uniform, and mixed thoroughly enough to avoid creating significant anomalies in the fill material.

Also, this method is not effective for landfills that have been established for some time (for example, 20 years). Further disadvantages of the above system, are that the costs are significant, the waste heterogeneity can give rise to drilling hazards (car bodies, tree stumps, concrete etc), additional drilling and installation costs, by the use of a large number of drill sites and that certain zones in the landfill may not produce any gas. Still further, most landfills have established environmental and operational management systems and the addition of the LGES system usually results in significant changes to that system which again increases costs.

Accordingly, there is a need to accurately locate points within any type of landfill where landfill gas can be extracted at a higher efficiency rate and lower cost than current methods. There is also a need to identify landfill gas migration pathways out of landfill sites having occupational health and safety and public risk implications.

Object of the Invention It is an object of the present invention to overcome or ameliorate some of the disadvantages of the prior art, or at least to provide a useful alternative.

Summary of the Invention There is disclosed herein a method for strategically locating gas extraction points within a landfill area; said method including the steps of : conducting a thermal infrared scan of the landfill area to identify zones of elevated surface temperature; recording data indicative of said zones of elevated surface temperature; conducting an electrical resistivity survey of the landfill area to identify zones of low resistivity; recording data indicative of said zones of low resistivity; processing the data so as to identify locations having zones of low resistivity and elevated surface temperature, which locations are to have said extraction points.

Preferably, the electrical resistivity survey is only conducted in said zones of elevated surface temperature.

Preferably, the thermal infrared scan is conducted during a time period just prior to dawn.

Preferably, said method includes further processing the data so as to identify locations having zones of continuous resistivity linking areas of low resistivity and elevated surface temperature indicating a high probability of being a gas migration pathway.

Preferably, said method includes further processing the data so as to identify locations having zones of high resistivity and elevated surface temperature indicating a high probability of being a gas confining layer.

Preferably, said method further includes locating a gas extraction well at an identified location being a high gas yielding area.

Preferably, said method can be applied to any type of landfill area.

Brief Description of the Drawings A preferred form of the present invention will now be described by way of example only with reference to the accompanying drawings wherein: Figure 1 is an example of a known Landfill Gas Extraction System; and Figure 2 is a site map of a landfill area surveyed by the method of an embodiment of the invention.

Detailed Description of the Preferred Embodiments As previously mentioned, Figure 1 shows a known landfill gas extraction system (LGES). An LGES system is retrofitted to an established landfill, and a systematic method of installing the extraction wells is used. For example, a grid pattern with spacing between wells ranging from 45 metres to 60 metres. The system will generally cover the majority of the landfill area and this system is only acceptable for landfill areas where the compaction and types of waste landfill are uniform, and mixed thoroughly enough to avoid creating significant anomalies in the fill material. This system is also not applicable to landfills that have been established for sometime. As these wells are placed in a grid like arrangement, many wells usually produce no or very little gas. Further, as the drilling of each of the holes is drilled"blind"through the landfill, significant risks mainly from a safety and drilling equipment damage perspective are present.

Figure 2, shows an example landfill surveyed using the method of the present invention to locate the optimum gas extraction points and thus high gas yielding areas.

The site map shown is of a known existing landfill having a landfill area of approximately 125,000 square metres, an average depth of waste fill of 16 metres and approximately 20 to 25 years old. Of the five borehole locations, BH1, BH2 and BH3 were identified by the method and drilled in the locations identified. BH4 was identified by the method, but the actual location was 5m west of the identified location due to access constraints. The location of BH5 was selected at random. Testing at atmospheric pressure indicated gas flows 3.5 to 5.5 times higher in BH1, BH2, and BH3 when compared to BH5. BH4 yielded 20 times less gas than BH5. If the LGES was used on such a landfill up to 35 drill extraction wells would be required to service the site as a systematic method would

be used and as indicated, the majority would have been unproductive. Accordingly, significant cost benefits can be seen from use of the method of the invention.

The general method used to obtain the results of Figure 2 for strategically locating gas extraction points within a landfill area includes the following steps: 1. Conducting a thermal infrared scan of the landfill area to identify zones of elevated surface temperature.

2. Recording data indicative of the zones of elevated surface temperature.

3. Conducting an electrical resistivity survey of the landfill area to identify zones of low, continuous and/or high resistivity.

4. Recording data indicative of the zones of low, continuous and/or high resistivity.

5. Processing the data and identifying locations having a high probability of being high gas yielding areas.

Further testing such as metal detection, electromagnetic, seismic, gravity, ground penetrating radar or magnetic could also be used to add further data to the method. Direct measurement of the surface temperature could also be conducted.

To undertake a resistivity investigation, an electrical current must be injected into the ground by a pair of electrodes. The potential field (or voltage) is then measured at the surface between a second pair of electrodes. If the distance and geometry between the electrode positions is known, along with the applied current and the measured voltage, the sub-surface resistivity can be calculated.

Resistivity was used because it gives the ability to determine the depth of waste fill, the ability to identify wet/perched leachate areas (moisture is required for the production of landfill gas), the potential to locate low density areas in the waste fill, the potential to locate large, homogenous units, and the relatively large body of data produced. Resistivity methods have the added advantages of being relatively inexpensive, having a high resolution and ease of interpretation of the data.

In a preferred method, the electrical resistivity survey would only be conducted in the zones of elevated surface temperature. Resistivity surveys could be used using two-dimensional or three-dimensional methods.

It was found by the applicant that the thermal infrared scans should be taken at the period pre-dawn to dawn because the expected variations in surface temperature were expected to be minor (few degrees Celsius). The addition of a solar load could potentially mask the small variations and reduce the possibility of detecting"hot spots"on objects near the surface of the landfill that store or conduct heat.

The generation of landfill gas from organic matter generates heat as one of the products. Deeper landfills have proportionally greater insulation than shallow landfills.

Thus, areas of elevated surface temperature indicate a near surface area of landfill gas generation, a deeper and more active area of landfill gas generation, a vent or near surface reservoir of landfill gas, some other heat storing/conducting at or near the surface.

The method further includes locating a gas extraction well at each of the locations of high gas yield.

It has been found in the testing conducted by the Applicant that if gas extraction wells are not located accurately, a significant impact on the possibility of finding a high or very poor yielding point results.

Accordingly, the present invention at least in a preferred embodiment has significant advantages/features over the LGES system. These advantages are for example, (a) improves the overall efficiency of a landfill gas extraction system; (b) reduces the total number of landfill gas extraction points required to service a landfill; (c) considering (a) and (b), the installation of a landfill gas extraction system becomes financially viable at smaller landfills; (d) considering (c), the generation of increased quantities of renewable energy becomes possible; (e) also considering (c), the combustion of landfill gas reduces overall greenhouse gas emissions from a landfill site; (f) again considering (c), smaller landfills can pro-actively manage risks posed by landfill gas; and (g) this invention can improve the safety and efficiency of the drilling process to install landfill gas extraction points.

Although the invention has been described with reference to specific examples, it would be appreciated by those skilled in the art that the invention may be embodied in many other forms.