investigation on the groundwater resources of the south eyre peninsula, south australia, determined...
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EXTENDED ABSTRACT SUBMISSION
20th International Geophysical Conference and Exhibition Adelaide, South Australia
22-25 February 2009
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Complete name and mailing address of the presenting author. This author will be the key person for all contact about the paper.
Presenter Esben Auken
Organisation Full address: Department of Earth Sciences, University of Aarhus
State, Country and Postcode: 8000 Aarhus C, Denmark
email: [email protected]
Second Author Ander Vest Christiansen Organisation Full address: Department of Earth Sciences, University of Aarhus email: [email protected] Third Author Andrea Viezzoli Organisation Full address: Department of Earth Sciences, University of Aarhus email: [email protected] Fourth Author Andrew Fitzpatrick Organisation Full address: CSIRO Exploration & Mining State, Country and Postcode: PO Box 1130, BENTLEY WA 6102 email: [email protected] Fifth Author Kevin Cahill Organisation Full address: CSIRO Exploration & Mining email: [email protected] Sixth Author Tim Munday Organisation Full address: CSIRO Exploration & Mining email: [email protected] Seventh Author Volmer Berens Organisation Full address: DWLBC State, Country and Postcode: Adelaide, SA email: [email protected]
INVESTIGATION ON THE GROUNDWATER RESOURCES OF THE SOUTH EYRE PENINSULA, SOUTH AUSTRALIA, DETERMINED FROM
LATERALLY CONSTRAINED INVERSION OF TEMPEST DATA.
Esben Auken1, Anders Vest Christiansen1, Andrea Viezzoli1, Andrew Fitzpatrick2, Kevin Cahill2, Tim Munday2 ,Volmer Berens3.
1. Department of Earth Sciences, University of Aarhus, Denmark, [email protected] 2. CSIRO, 26 Dick Perry Avenue, Kensington, WA 6151, Australia
[email protected], [email protected], [email protected] 3. DWLBC, [email protected]
Key words: TEMPEST, constrained inversion, geometry, groundwater, saline intrusion INTRODUCTION Groundwater in the Eyre Peninsula of South Australia is scarce with potable resources limited to the western coastal margin and the southern tip of the peninsula. Consequently an understanding of their extent has become increasingly important particularly with demand being close to current extraction limits. In September 2006, about 1000 line km of TEMPEST AEM data were acquired over the Southern Eyre Peninsula, in order to assist in the definition of freshwater lens systems and in particular aquifer bounds associated with them as part of a resource definition project. Following their acquisition, the TEMPEST data set was analysed for data quality and then transformed into conductivity depth images (CDI) using EMFLOW (see Fitzpatrick and Munday, 2007). In an effort to better define to better define the geometry of specific bounding surfaces of hydrogeological relevance the TEMPEST data were inverted through the application of the laterally constrained inversion (LCI) technique. This paper describes the initial results from the first application of the LCI to data from a fixed wing AEM system. HYDROGEOLOGY The survey area is located in the southern tip of the Eyre Peninsula in South Australia (Figure 1). The principal groundwater resources lie within the Quaternary limestone of the Bridgewater Formation. The basement forms a topographically undulating surface expressed as a series of north-south trending ridges and troughs which reflect differences in lithotype. In the regional airborne magnetic data, these differences are manifest as marked changes in magnetic intensity. Available drillhole data indicates that the Quaternary limestone and the underlying Tertiary sediments thin over the basement highs and thicken in the adjacent troughs or basins. The consequence is a marked litho-structural control on the distribution and thickness of the Quaternary limestone aquifer (Evans, 2002). There are three main groundwater “lenses” associated with this aquifer which are characterized by high yields and low salinity (<1000mg/L). They occur in a series of sub-basins known as the Coffin Bay, Uley and Lincoln Basins, and are separated by local topographic highs coincident with basement highs. INVERSION METHODOLGY TEMPEST data are voltage data corrected with the high altitude system transfer function and deconvolved thus obtaining the B field from a vertical magnetic dipole with a 100 % duty cycle (Lane et al., 2000). The raw B field data were basically unprocessed in the sense that gates with noisy data had not be culled. A rough processing of the data were made, which, in the high-resistive areas, often removed more than half of the gates in the soundings – more
in the X component data then in the Z component data. Data were inverted using a modified version of the Laterally Constrained Inversion (LCI) (Auken and Christiansen, 2004) algorithm. The LCI algorithm implements a full nonlinear inversion scheme with constraints between parameters in neighbouring soundings. During the inversion there is an option to solve for layer parameters, transmitter altitude, receiver pitch and roll, tow cable length and receiver altitude. The model is either discrete with 3 – 4 layers or smooth with up to 20 layers. In the later case layer thicknesses are kept constant and the algorithm only solve for layer resistivities subject to a smoothness constraint. The basic assumption when using the LCI algorithm to solve for both layer and geometry parameters is that the geometry parameters are smoothly varying along the profile, because the aircraft movements are relatively slow. Therefore, the geometry parameters were constrained laterally very tight. This means that the number of unknowns for each model is greatly reduced. We have done quite a number of experiments and unexpected to us the most reliable results were obtained when we allowed only very small deviations for the transmitter altitude, receiver roll and receiver position, leaving only receiver pitch for the inversion. It seems that the inversion problem becomes to equivalent when all of these parameters are allowed to change. Though, the receiver pitch is by far the most important parameter to invert for as the X-component and Z-component data are nearly uninterpretable if the forward response is not corrected for by the pitch of the bird.
Figure 1. Flight line diagram for the TEMPEST survey over Coffin bay. The results shown in following figures belong to the northern-most tie-line (Line 17010) running in a NE-SW direction.
RESULTS Figure 2 shows the results of the northernmost NE_SW tie line, as indicated on Figure 1. Results are plotted SW to NE. All of the LCI sections shown have summarized RMS data fits of less than 4%. Figure 2a shows the full line inverted with a LCI using a smooth multi-layer model. In Figure 2b is a close-up on the second half of the profile marked by the thin black lines in Figure 2a. In Figure 2a and Figure 2b the green line is the transmitter altitude and the red line is the receiver position. The panel above shows the inverted pitch angle in blue. The pitch angle shows a smooth behaviour with a period of 8-15 seconds assuming a flying speed of 65 m/s. The inverted pitch is very much alike for all sections (not shown) no matter the underlying model. This suggests that the pitch angle is a very well resolved parameter. Figure 2c shows the result of an LCI using a 3-layer model and finally, Figure 2d shows the result obtained with EMFLOW (Macnae et al. 1998) . Sea water intrudes into Tertiary aquifers that extend across the NE part of the survey area. The intrusion extends under the Coffin Bay National Park, which lies south of the Coffin Bay township. A depression in the conductive feature between 540000 and 544000 is believed to be linked to a freshwater groundwater mound developed above the saltwater wedge. A sharp transition between saline and freshwater is clearly visible at around location 545000, and this is interpreted to represent the point where the Quaternary limestone aquifer of the Bridgewater Formation cuts the tie line section. Fresh groundwater is also believed to be present in this part of the section, linked to an extension of the Coffin Bay and Uley Lens systems. Figure 2b and 2c provide a close up view of the results from this part of the line for the multi and few layers model respectively. The results of the outcome from an EMFLOW transformation of same line are plotted in Figure 2d for comparison. Overall, the results are comparable, with the exception of the deeper part of the model, where EMFLOW is known to underestimate conductivities, especially underneath conductive layers. Both the few and multilayer LCI models are more continuous than the EMFLOW ones. This is true both laterally and vertically. Finally, the extent of the freshwater resource appears to be well determined in the few layer LCI, whereas EMFLOW results are difficult to interpret. CONCLUSIONS The Laterally Constrained Inversion was successfully applied for the first time to data from a fixed wing system time domain EM system (TEMPEST). The observed conductivity structure accords well with the known hydrogeology of the area, but provides some additional insights into the complex interplay between the seawater intrusion and the freshwater in the overlying freshwater lens systems. Inversion of the entire dataset with LCI is recommended to produce a clearer hydrological model at regional scale.
Figure 2. LCI for line 17010. Smooth multi-layer in (a) with transmitter (green) and receiver positions (red) as well together with pitch angle in the panel on top. In (b)a close up view to right-central part of the line, and a 3-layer inversion in (c). Comparison with EMFLOW results (d). REFERENCES Auken, E. and Christiansen, A. V., 2004, Layered and laterally constrained 2D inversion of resistivity data: Geophysics, 69, 752-761. Evans, S.L, 2002. Southern Basins Prescribed Wells Area Groundwater Monitoring Status Report 2002, South Australia. Department of Water, Land and Biodiversity Conservation Report DWLBC 2002/13. Fitzpatrick, A, and Munday T.J., 2007, Coffin Bay TEMPEST AEM Survey: Final Report. CRC LEME Restricted Report 255R / E&M REPORT P2006/754, 50pp. Lane R., A. Green, C. Golding, M. Owers, P. Pik, C. Plunkett, D. Sattel and B. Thorn, 2000, An example of 3D conductivity mapping using the TEMPEST airborne electromagnetic system, Exploration Geophysics 31(2) 162 - 172 Macnae, J.C., King, A., Stolz, N., Osmakoff, A., Blaha, A., 1998, Fast AEM data processing and inversion: Expl. Geophys., 29, 163-169.