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Environmental geophysics mapping salinity and water resources
International Journal of Applied Earth Observation and Geoinformation 9 (2007) 130–136 www.elsevier.com/locate/jag
Environmental geophysics mapping salinity and water resources David Dent ISRIC-World Soil Information, P.O. Box 353, 6700 AJ Wageningen, The Netherlands Received 23 June 2006; accepted 11 September 2006
Abstract Salinity and fresh water are two sides of the same coin, most conveniently measured by electrical conductivity; they can now be mapped rapidly in three dimensions using airborne electromagnetics (AEM). Recent developments in the calibration of airborne data against in-field measurements and additional information from radiometrics, magnetics and digital elevation models lend new insights into salinity, groundwater flow systems and water resources. Freshwater resources can be mapped, and salinity risk and the outcome of management interventions may be forecast, on the basis of the specific architecture of complete groundwater flow systems-enabling practical, cost-effective protection and development of water resources. # 2006 Elsevier B.V. All rights reserved. Keywords: Airborne geophysics; Electromagnetics; Salinity; Water resources management
1. Introduction Across the dry regions of the world, fresh water resources are threatened by salinity: salt in the wrong place. In Australia, this is a legacy of a dry climate and sluggish drainage, probably exacerbated by changes in land use since European settlement; soil erosion and replacement of native vegetation with crops and pastures that use less water mean that more water is infiltrating to the groundwater, elsewhere irrigation applies more water than comes naturally, so rising water tables bring salt to the surface and into the rivers. It is argued that salinity can be arrested only by extensive land use changes and, even then, response times will often be 100 years or more (NLWRA, 2001). But it is not all the same out there! To protect water resources, we need to know where the salt lies in the landscape, how it is mobilised, what are the conduits
E-mail address: [email protected]. 0303-2434/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jag.2006.09.005
carrying it to streams and the ground surface, the rate of delivery now and under feasible management options, and if there are alternative water resources that may be exploited. Answers are emerging from a combination of: (1) airborne geophysics, mapping the salt stores, conduits and groundwater resources in three dimensions; (2) drilling to calibrate the patterns revealed by airborne surveys and to establish the nature of the aquifers; (3) modelling water and salt movement on the basis of the architecture of each groundwater flow system, to establish the risk of salinity and the outcomes of possible management interventions. With this information, cost-effective action on the ground can be tailored to specific situations. 2. Where is the salt and how much is there? Salt is held as briny pore fluid in the soil and regolith, especially in clays. Recent advances in airborne electromagnetics (AEM) enable rapid mapping of salt and fresh water to more than 100 m below ground (Dent
D. Dent / International Journal of Applied Earth Observation and Geoinformation 9 (2007) 130–136
et al., 1999). The survey aircraft generates an electromagnetic field that penetrates the ground. This, in turn, induces a secondary current in conductive materials and the current induces a secondary electromagnetic field that is detected by a receiver towed behind the aircraft. The signals are translated into a three-dimensional map by conductivity depth imaging (CDI) or layered earth inversion (LEI). Conventionally, these models are guided towards low conductivity values at the base of investigation, generally fresh rock, between 100 and 200 m depth. Recently, constrained inversion procedures have been developed to account for different field conditions, such as conductive basement (Lane et al., 2004), and field measurements of conductivity (EM39) and water table geometry from test bores. Initial CDIs generated by the EMFLOW model (McNae et al., 1998) exaggerate the near-surface conductivity. A much better representation may be achieved by: first, iteration of the specified transmitter terrain clearance,
131
transmitter-receiver horizontal and vertical separation; and secondly, governing the maximum conductivity within the range actually measured (Christiansen, 2002). Fig. 1 shows part of the catchment of the Broken River between the Strathbogies Range to the south and the Shepparton Irrigation Area to the north; it contrasts modelled conductivity before and after calibration. A perfect match is not possible, because the 150 m radius footprint of the AEM system encompasses much more inherent variation than the 1 m radius footprint of the EM39 instrument, but r2 was improved to 0.33–0.47 for the 0–5 and 5–10 m slices, rising to 0.87 for deeper layers. The significance of these advances cannot be exaggerated; earlier applications of AEM to salt mapping (Duncan et al., 1993) met with scepticism because of the imprecision and inaccuracy of the vertical dimension, especially in the near-surface layer; now, a detailed, accurate, three-dimensional picture of conductivity may be had at a cost of a US$ a hectare.
Fig. 1. Mid-Broken Catchment, Victoria. Initial and calibrated CDIs: blue indicates resistive materials and red indicates conductive. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
132
D. Dent / International Journal of Applied Earth Observation and Geoinformation 9 (2007) 130–136
Fig. 2. Mid-Broken Catchment, salt load, after Cresswell et al. (2004).
Conductivity may be expressed in terms of salt load, either using pore fluid from undisturbed samples or a water extract such as EC1:5. The direct relationship enables translation of maps of conductance into maps of salt load, Fig. 2 (Cresswell et al., 2004; Mullen et al., 2007). 3. Where is it going? Airborne magnetics reveals geological structure including magnetic gravels that may serve as conduits, dykes that may be barriers to groundwater flow, and faults that may act either way. Sharp-edged and typically shallow features, such as prior-stream gravels, are picked out by the First Vertical Derivative (1VD) image that measures the rate of change in magnetic intensity. Combining the 1VD with a CDI for the MidBroken (Fig. 3), we see: - Salt draining north from the Strathbogies Range ends up in a graben delineated by a fault trending WSWENE. - Salt in buried channels north of the fault drains towards the Shepparton irrigation area.
4. How soon will it be there? This is the big issue. If water supplies, farmland or freshwater habitats will be contaminated within a few years, then something must be done now; if the situation is stable or the time scale is centuries, then it is not so pressing. Groundwater flow systems may extend over a few km2, closely coupled with local rainfall and land use, or over thousands of km2 and responding slowly to regional changes. Once the architecture of the system is mapped, groundwater simulation models may be used to estimate the movement of water and salt. In the case of the Mid-Broken, the rate of delivery of salt northwards from the Honeysuckle Creek catchment to the Broken River and Shepparton Irrigation Area may be estimated from the transmissivity of the deep leads and the salt content of the groundwater; it is of the order of 40,000 t/year. For the country round St. George in SE Queensland, AEM (Fig. 4) shows the opposite of the salinity situation presumed before the airborne survey: salt accumulation in the bottomlands being brought to the surface by rising groundwater. Instead, we see the present river system superimposed on a much older landscape; salt in the upland regolith, 15–50 m below
D. Dent / International Journal of Applied Earth Observation and Geoinformation 9 (2007) 130–136
133
Fig. 3. Mid-Broken Catchment, CDI 30–40 m overlaid on 1VD image.
the surface; fresh groundwater in the alluvium of the floodplains, flushed periodically by fresh water spate flows from the upper catchments to the north and east. In this issue, Macaulay and Mullen (2007) combine groundwater modelling with the regional AEM data to assess the likelihood of mobilization of the salt under different land use scenarios. Hydraulic conductivity data needed for the model were measured in bores drilled in the unsaturated zone along presumed flow lines, values were low in the upper 6 m (0.1–0.15 m/ day). Simulation of accessions to the groundwater table by the FLOWTUBE model suggests that mobilisation of salt to the floodplains is unlikely; localised groundwater mounds develop under leaking surface water storages and continuous irrigation (Fig. 5) but the head is dissipated over a short distance. 4.1. Groundwater prospecting Large AEM surveys can reveal a three-dimensional picture of sub-artesian groundwater resources. In the St.
George area (Fig. 4), the resistive lobe to the NW is interpreted as a fresh groundwater in a deep palaeo valley. However, resistive signatures may be generated by bedrock of low porosity as well as by fresh groundwater; a reliable interpretation has to be built up from several lines of evidence. In this issue, Mullen and Kellett (2007) assess this groundwater resource by careful calibration of the AEM survey – both to the map the alluvium-bedrock boundary and to estimate water quality. 4.2. Applications Environmental geophysics provides new insights into salinity, water resources and groundwater flow systems and the scales at which they operate. Quite different management interventions and development strategies may arise from this knowledge: Mid-Broken: The previous management plan envisaged extensive afforestation of the ranges to protect
134
D. Dent / International Journal of Applied Earth Observation and Geoinformation 9 (2007) 130–136
Fig. 4. St. George, Queensland, LEI 50–55 m.
irrigation streams. We now see that these systems are not connected: saline seepage from the ranges intersects (and breaks up) the Hume Freeway but, once it reaches the sump, this salt will be a problem only if the groundwater rises close to the surface. Road damage may be arrested by drainage and by local tree planting. Salt transport to the Shepparton Irrigation Area may be contained by improved irrigation efficiency, to control the water table, and by pumping saline groundwater from the prior stream gravels to evaporation basins.
St. George: The sub-artesian aquifers identified by airborne survey are already being developed in areas of good water quality. Modelling indicates that seepage from existing irrigation areas is not a hazard to surface waters; impacts on groundwater will be restricted to the area under irrigation. In this issue, Cresswell et al. (2007) explore applications in both irrigation areas and more extensive rain-fed farming areas in Australia. At farm scale, at St. George, airborne geophysical data have proved
D. Dent / International Journal of Applied Earth Observation and Geoinformation 9 (2007) 130–136
135
Fig. 5. St. George, Queensland, LEI section with FLOWTUBE simulation of 100 years irrigated cotton.
Fig. 6. Ternary radiometric image of part of the Southern Tablelands, to the east, draining to the Murray–Darling Basin. Spatial patterns of soil parent materials are depicted by combining gamma signals from potassium (coloured red), ethorium (green) and euranium (blue). Areas of thick clay soils rich in salt, derived from interpretation of the image, are overlaid in scarlet. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
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D. Dent / International Journal of Applied Earth Observation and Geoinformation 9 (2007) 130–136
invaluable for property planning: the digital elevation model (DEM) from radar altimetry provides detailed contours; the 0–5 m LEI slice provides a qualitative indication of root zone salinity and soil texture, the latter needed for sitting on-farm water storages; deeper slices in the unsaturated zone indicate the likely magnitude of deep drainage below the root zone. The salt load transform provides a baseline of current conditions for subsequent geophysical surveys of hot spots at 10-year intervals planned by the community. Although not featured in this issue, airborne radiometrics provides invaluable information from the uppermost 30 cm of the soil (Fig. 6) – through which nearly all salinity and water resources management must act (Wilford et al., 2001). Combined with a digital elevation model, it is a powerful tool for soil landscape mapping at any scale. AEM is employed most effectively where high-value assets and infrastructure may be under threat from salinity. Mobilisation costs dictate that large areas must be flown to bring costs down to US$ ha 1. The knowledge won can be projected over larger areas using information from DEMs, magnetics and radiometrics at one-tenth of the cost of AEM per line km, and by use of landscape analogues. Minerals exploration companies already hold geophysical data for regions worldwide that are short of water and plagued by salinity; these data might be reinterpreted for water resources at marginal cost. 4.3. Methods Fugro Airborne Surveys flew surveys at a height of 120 m and line spacing of 200 m (Mid-Broken) and 200 and 400 m (St. George) using the TEMPEST AEM system (Lane et al., 2000). Magnetics and radiometrics were flown at 60 m with line spacing of 100 m (Mid-Broken and Southern Tablelands) and 100 and 200 m (St. George). I am indebted to Ross C. Brodie for their quality control and assistance with physical calculations.
References Christiansen, A., 2002. Calibration of electromagnetic data. in: Dent, D.L. (Ed.), MDBC Airborne Geophysics Project, final report. Bureau of Rural Sciences, Canberra, pp. 20–38. Cresswell, R.G., Dent, D.L., Jones, G.L., Galloway, D.S., 2004. Three-dimensional mapping of salt stores in the SE MurrayDarling Basin, Australia. 1 Steps in calibration of airborne electromagnetic surveys. Soil Use Manage. 20, 133–143. Cresswell, R.G., Mullen, I., Kingham, R., et al., 2007. Airborne electromagnetics supporting salinity and natural resource management decisions at field scale in Australia. Int. J. Appl. Earth Observ. Geoinform. 9, 91–102. Dent, D.L., Lawrie, K.C., Munday, T.M., 1999. Running down the salt in Australia 1, a multidisciplinary approach. Land (Gent) 3, 179– 199. Duncan, A.C., Roberts, G.P., Buselli, G., et al., 1993. SALTMAP— airborne EM for the environment. Explorat. Geophys. 26, 138– 143. Lane, R., Brodie, R.C., Fitzpatrick, A., 2004. Constrained inversion of AEM data from the Lower Balonne area, Southern Queensland, Australia. Cooperative Research Centre—Landscape Environments and Mineral Exploration Open File Report 163, Canberra. Lane, R., Green, A., Golding, C., et al., 2000. An example of D conductivity mapping using the TEMPEST airborne electromagnetic system. Explorat. Geophys. 31, 162–172. Macaulay, S., Mullen, I., 2007. Predicting salinity impacts of land use change: groundwater modelling with AEM and field data, SE Queensland, Australia. Int. J. Appl. Earth Observ. Geoinform. 9, 124–129. McNae, J.C., King, A., Stolz, N., Omakoff, A., Blaha, A., 1998. Fast AEM data processing and inversion. Explorat. Geophys. 29, 163– 169. Mullen, I., Kellett, J., 2007. Groundwater salinity mapping using airborne electromagnetics and borehole data. Int. J. Appl. Earth Observ. Geoinform. 9, 116–123. Mullen, I., Wilkinson, K., Cresswell, R.G., Kellett, J., 2007. Threedimensional mapping of salt stores in the Murray-Darling Basin, Australia: calculating landscape salt loads from airborne electromagnetics and laboratory data. Int. J. Appl. Earth Observ. Geoinform. 9, 103–115. NLWRA, 2001. Australian dryland salinity assessment 2000: extent, impacts, processes, monitoring and management options. National Land and Water Resources Audit, Commonwealth of Australia, Canberra. Wilford, J., Dent, D.J., Braaten, R., Dowling, T.I., 2001. Running down the salt in Australia II: smart interpretation of airborne radiometrics and digital elevation models. The Land (Ghent) 5 (2), 79–100.
Environmental geophysics mapping salinity and water resources David Dent ISRIC-World Soil Information, P.O. Box 353, 6700 AJ Wageningen, The Netherlands Received 23 June 2006; accepted 11 September 2006
Abstract Salinity and fresh water are two sides of the same coin, most conveniently measured by electrical conductivity; they can now be mapped rapidly in three dimensions using airborne electromagnetics (AEM). Recent developments in the calibration of airborne data against in-field measurements and additional information from radiometrics, magnetics and digital elevation models lend new insights into salinity, groundwater flow systems and water resources. Freshwater resources can be mapped, and salinity risk and the outcome of management interventions may be forecast, on the basis of the specific architecture of complete groundwater flow systems-enabling practical, cost-effective protection and development of water resources. # 2006 Elsevier B.V. All rights reserved. Keywords: Airborne geophysics; Electromagnetics; Salinity; Water resources management
1. Introduction Across the dry regions of the world, fresh water resources are threatened by salinity: salt in the wrong place. In Australia, this is a legacy of a dry climate and sluggish drainage, probably exacerbated by changes in land use since European settlement; soil erosion and replacement of native vegetation with crops and pastures that use less water mean that more water is infiltrating to the groundwater, elsewhere irrigation applies more water than comes naturally, so rising water tables bring salt to the surface and into the rivers. It is argued that salinity can be arrested only by extensive land use changes and, even then, response times will often be 100 years or more (NLWRA, 2001). But it is not all the same out there! To protect water resources, we need to know where the salt lies in the landscape, how it is mobilised, what are the conduits
E-mail address: [email protected]. 0303-2434/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jag.2006.09.005
carrying it to streams and the ground surface, the rate of delivery now and under feasible management options, and if there are alternative water resources that may be exploited. Answers are emerging from a combination of: (1) airborne geophysics, mapping the salt stores, conduits and groundwater resources in three dimensions; (2) drilling to calibrate the patterns revealed by airborne surveys and to establish the nature of the aquifers; (3) modelling water and salt movement on the basis of the architecture of each groundwater flow system, to establish the risk of salinity and the outcomes of possible management interventions. With this information, cost-effective action on the ground can be tailored to specific situations. 2. Where is the salt and how much is there? Salt is held as briny pore fluid in the soil and regolith, especially in clays. Recent advances in airborne electromagnetics (AEM) enable rapid mapping of salt and fresh water to more than 100 m below ground (Dent
D. Dent / International Journal of Applied Earth Observation and Geoinformation 9 (2007) 130–136
et al., 1999). The survey aircraft generates an electromagnetic field that penetrates the ground. This, in turn, induces a secondary current in conductive materials and the current induces a secondary electromagnetic field that is detected by a receiver towed behind the aircraft. The signals are translated into a three-dimensional map by conductivity depth imaging (CDI) or layered earth inversion (LEI). Conventionally, these models are guided towards low conductivity values at the base of investigation, generally fresh rock, between 100 and 200 m depth. Recently, constrained inversion procedures have been developed to account for different field conditions, such as conductive basement (Lane et al., 2004), and field measurements of conductivity (EM39) and water table geometry from test bores. Initial CDIs generated by the EMFLOW model (McNae et al., 1998) exaggerate the near-surface conductivity. A much better representation may be achieved by: first, iteration of the specified transmitter terrain clearance,
131
transmitter-receiver horizontal and vertical separation; and secondly, governing the maximum conductivity within the range actually measured (Christiansen, 2002). Fig. 1 shows part of the catchment of the Broken River between the Strathbogies Range to the south and the Shepparton Irrigation Area to the north; it contrasts modelled conductivity before and after calibration. A perfect match is not possible, because the 150 m radius footprint of the AEM system encompasses much more inherent variation than the 1 m radius footprint of the EM39 instrument, but r2 was improved to 0.33–0.47 for the 0–5 and 5–10 m slices, rising to 0.87 for deeper layers. The significance of these advances cannot be exaggerated; earlier applications of AEM to salt mapping (Duncan et al., 1993) met with scepticism because of the imprecision and inaccuracy of the vertical dimension, especially in the near-surface layer; now, a detailed, accurate, three-dimensional picture of conductivity may be had at a cost of a US$ a hectare.
Fig. 1. Mid-Broken Catchment, Victoria. Initial and calibrated CDIs: blue indicates resistive materials and red indicates conductive. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
132
D. Dent / International Journal of Applied Earth Observation and Geoinformation 9 (2007) 130–136
Fig. 2. Mid-Broken Catchment, salt load, after Cresswell et al. (2004).
Conductivity may be expressed in terms of salt load, either using pore fluid from undisturbed samples or a water extract such as EC1:5. The direct relationship enables translation of maps of conductance into maps of salt load, Fig. 2 (Cresswell et al., 2004; Mullen et al., 2007). 3. Where is it going? Airborne magnetics reveals geological structure including magnetic gravels that may serve as conduits, dykes that may be barriers to groundwater flow, and faults that may act either way. Sharp-edged and typically shallow features, such as prior-stream gravels, are picked out by the First Vertical Derivative (1VD) image that measures the rate of change in magnetic intensity. Combining the 1VD with a CDI for the MidBroken (Fig. 3), we see: - Salt draining north from the Strathbogies Range ends up in a graben delineated by a fault trending WSWENE. - Salt in buried channels north of the fault drains towards the Shepparton irrigation area.
4. How soon will it be there? This is the big issue. If water supplies, farmland or freshwater habitats will be contaminated within a few years, then something must be done now; if the situation is stable or the time scale is centuries, then it is not so pressing. Groundwater flow systems may extend over a few km2, closely coupled with local rainfall and land use, or over thousands of km2 and responding slowly to regional changes. Once the architecture of the system is mapped, groundwater simulation models may be used to estimate the movement of water and salt. In the case of the Mid-Broken, the rate of delivery of salt northwards from the Honeysuckle Creek catchment to the Broken River and Shepparton Irrigation Area may be estimated from the transmissivity of the deep leads and the salt content of the groundwater; it is of the order of 40,000 t/year. For the country round St. George in SE Queensland, AEM (Fig. 4) shows the opposite of the salinity situation presumed before the airborne survey: salt accumulation in the bottomlands being brought to the surface by rising groundwater. Instead, we see the present river system superimposed on a much older landscape; salt in the upland regolith, 15–50 m below
D. Dent / International Journal of Applied Earth Observation and Geoinformation 9 (2007) 130–136
133
Fig. 3. Mid-Broken Catchment, CDI 30–40 m overlaid on 1VD image.
the surface; fresh groundwater in the alluvium of the floodplains, flushed periodically by fresh water spate flows from the upper catchments to the north and east. In this issue, Macaulay and Mullen (2007) combine groundwater modelling with the regional AEM data to assess the likelihood of mobilization of the salt under different land use scenarios. Hydraulic conductivity data needed for the model were measured in bores drilled in the unsaturated zone along presumed flow lines, values were low in the upper 6 m (0.1–0.15 m/ day). Simulation of accessions to the groundwater table by the FLOWTUBE model suggests that mobilisation of salt to the floodplains is unlikely; localised groundwater mounds develop under leaking surface water storages and continuous irrigation (Fig. 5) but the head is dissipated over a short distance. 4.1. Groundwater prospecting Large AEM surveys can reveal a three-dimensional picture of sub-artesian groundwater resources. In the St.
George area (Fig. 4), the resistive lobe to the NW is interpreted as a fresh groundwater in a deep palaeo valley. However, resistive signatures may be generated by bedrock of low porosity as well as by fresh groundwater; a reliable interpretation has to be built up from several lines of evidence. In this issue, Mullen and Kellett (2007) assess this groundwater resource by careful calibration of the AEM survey – both to the map the alluvium-bedrock boundary and to estimate water quality. 4.2. Applications Environmental geophysics provides new insights into salinity, water resources and groundwater flow systems and the scales at which they operate. Quite different management interventions and development strategies may arise from this knowledge: Mid-Broken: The previous management plan envisaged extensive afforestation of the ranges to protect
134
D. Dent / International Journal of Applied Earth Observation and Geoinformation 9 (2007) 130–136
Fig. 4. St. George, Queensland, LEI 50–55 m.
irrigation streams. We now see that these systems are not connected: saline seepage from the ranges intersects (and breaks up) the Hume Freeway but, once it reaches the sump, this salt will be a problem only if the groundwater rises close to the surface. Road damage may be arrested by drainage and by local tree planting. Salt transport to the Shepparton Irrigation Area may be contained by improved irrigation efficiency, to control the water table, and by pumping saline groundwater from the prior stream gravels to evaporation basins.
St. George: The sub-artesian aquifers identified by airborne survey are already being developed in areas of good water quality. Modelling indicates that seepage from existing irrigation areas is not a hazard to surface waters; impacts on groundwater will be restricted to the area under irrigation. In this issue, Cresswell et al. (2007) explore applications in both irrigation areas and more extensive rain-fed farming areas in Australia. At farm scale, at St. George, airborne geophysical data have proved
D. Dent / International Journal of Applied Earth Observation and Geoinformation 9 (2007) 130–136
135
Fig. 5. St. George, Queensland, LEI section with FLOWTUBE simulation of 100 years irrigated cotton.
Fig. 6. Ternary radiometric image of part of the Southern Tablelands, to the east, draining to the Murray–Darling Basin. Spatial patterns of soil parent materials are depicted by combining gamma signals from potassium (coloured red), ethorium (green) and euranium (blue). Areas of thick clay soils rich in salt, derived from interpretation of the image, are overlaid in scarlet. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
136
D. Dent / International Journal of Applied Earth Observation and Geoinformation 9 (2007) 130–136
invaluable for property planning: the digital elevation model (DEM) from radar altimetry provides detailed contours; the 0–5 m LEI slice provides a qualitative indication of root zone salinity and soil texture, the latter needed for sitting on-farm water storages; deeper slices in the unsaturated zone indicate the likely magnitude of deep drainage below the root zone. The salt load transform provides a baseline of current conditions for subsequent geophysical surveys of hot spots at 10-year intervals planned by the community. Although not featured in this issue, airborne radiometrics provides invaluable information from the uppermost 30 cm of the soil (Fig. 6) – through which nearly all salinity and water resources management must act (Wilford et al., 2001). Combined with a digital elevation model, it is a powerful tool for soil landscape mapping at any scale. AEM is employed most effectively where high-value assets and infrastructure may be under threat from salinity. Mobilisation costs dictate that large areas must be flown to bring costs down to US$ ha 1. The knowledge won can be projected over larger areas using information from DEMs, magnetics and radiometrics at one-tenth of the cost of AEM per line km, and by use of landscape analogues. Minerals exploration companies already hold geophysical data for regions worldwide that are short of water and plagued by salinity; these data might be reinterpreted for water resources at marginal cost. 4.3. Methods Fugro Airborne Surveys flew surveys at a height of 120 m and line spacing of 200 m (Mid-Broken) and 200 and 400 m (St. George) using the TEMPEST AEM system (Lane et al., 2000). Magnetics and radiometrics were flown at 60 m with line spacing of 100 m (Mid-Broken and Southern Tablelands) and 100 and 200 m (St. George). I am indebted to Ross C. Brodie for their quality control and assistance with physical calculations.
References Christiansen, A., 2002. Calibration of electromagnetic data. in: Dent, D.L. (Ed.), MDBC Airborne Geophysics Project, final report. Bureau of Rural Sciences, Canberra, pp. 20–38. Cresswell, R.G., Dent, D.L., Jones, G.L., Galloway, D.S., 2004. Three-dimensional mapping of salt stores in the SE MurrayDarling Basin, Australia. 1 Steps in calibration of airborne electromagnetic surveys. Soil Use Manage. 20, 133–143. Cresswell, R.G., Mullen, I., Kingham, R., et al., 2007. Airborne electromagnetics supporting salinity and natural resource management decisions at field scale in Australia. Int. J. Appl. Earth Observ. Geoinform. 9, 91–102. Dent, D.L., Lawrie, K.C., Munday, T.M., 1999. Running down the salt in Australia 1, a multidisciplinary approach. Land (Gent) 3, 179– 199. Duncan, A.C., Roberts, G.P., Buselli, G., et al., 1993. SALTMAP— airborne EM for the environment. Explorat. Geophys. 26, 138– 143. Lane, R., Brodie, R.C., Fitzpatrick, A., 2004. Constrained inversion of AEM data from the Lower Balonne area, Southern Queensland, Australia. Cooperative Research Centre—Landscape Environments and Mineral Exploration Open File Report 163, Canberra. Lane, R., Green, A., Golding, C., et al., 2000. An example of D conductivity mapping using the TEMPEST airborne electromagnetic system. Explorat. Geophys. 31, 162–172. Macaulay, S., Mullen, I., 2007. Predicting salinity impacts of land use change: groundwater modelling with AEM and field data, SE Queensland, Australia. Int. J. Appl. Earth Observ. Geoinform. 9, 124–129. McNae, J.C., King, A., Stolz, N., Omakoff, A., Blaha, A., 1998. Fast AEM data processing and inversion. Explorat. Geophys. 29, 163– 169. Mullen, I., Kellett, J., 2007. Groundwater salinity mapping using airborne electromagnetics and borehole data. Int. J. Appl. Earth Observ. Geoinform. 9, 116–123. Mullen, I., Wilkinson, K., Cresswell, R.G., Kellett, J., 2007. Threedimensional mapping of salt stores in the Murray-Darling Basin, Australia: calculating landscape salt loads from airborne electromagnetics and laboratory data. Int. J. Appl. Earth Observ. Geoinform. 9, 103–115. NLWRA, 2001. Australian dryland salinity assessment 2000: extent, impacts, processes, monitoring and management options. National Land and Water Resources Audit, Commonwealth of Australia, Canberra. Wilford, J., Dent, D.J., Braaten, R., Dowling, T.I., 2001. Running down the salt in Australia II: smart interpretation of airborne radiometrics and digital elevation models. The Land (Ghent) 5 (2), 79–100.