Westerhoff, R.S.; Herpe, M.; Rawlinson, Z.J.; Santamaria Cerrutti, M.E.; Brakenrig, T.; Cameron, S.G. 2022 Feasibility of aquifer investigation with the SkyTEM method in the Aupōuri Peninsula, Northland. Aqua Intel Aotearoa report 2021/06; (GNS Science consultancy report 2021/99). 85 p. https://doi.org/10.21420/ZRKT-FT95
Abstract
This report has been co-developed by Aqua Intel Aotearoa (a collaboration between Kānoa Regional Economic Development & Investment Unit [Kānoa and GNS Science]) and Northland Regional Council as part of the third phase of work of the investigations of water storage and aquifer mapping in Northland. The report assesses the feasibility of airborne electromagnetic (AEM) geophysical surveying for aquifer characterisation in the Aupōuri Peninsula, Northland, specifically utilising the SkyTEM method.The objective of this study was to assess the potential of the SkyTEM method (a geophysical airborne time-domain electromagnetic [TEM] method) to characterise the Aupōuri Aquifer. This report describes the SkyTEM method in terms of cost, quality, target depth and other practical considerations, with comparison to other potential mapping methods where appropriate. The SkyTEM method was assessed based on its likely efficacy to map aquifer structure, geometry and properties; in particular, four key features of the Aupōuri hydrogeological setting: depth to basement; shell beds; groundwater–surface water connectivity, focusing on shallow dune lakes and perched water tables caused by iron pans; and saline intrusion. Our assessment included a literature review on the SkyTEM method and its applications, both internationally and in New Zealand; ground-based TEM measurements on the peninsula; and synthetic modelling of SkyTEM data and derived resistivity models from hydrogeological scenarios that incorporated the four key features. The literature review indicated that the SkyTEM method is widely and successfully used throughout the world, more recently in New Zealand, to investigate a diverse range of aquifer properties (including some of our key features) using high-quality data from the ground surface to several hundred metres depth. The SkyTEM method measures material properties (electrical resistivity) with a spatial resolution and depth penetration that cannot typically be provided by other geophysical methods within reasonable cost. The SkyTEM method yields a consistent 3D data grid over large areas, including areas that are inaccessible for most ground-based measurements, e.g. wetlands, forests and near-shore for mapping the freshwater–saline groundwater interface. The cost of SkyTEM to map areas such as the Aupōuri Peninsula is orders of magnitude less than ground-investigation methods, including drilling and geophysics. Ground-based TEM (groundTEM) undertaken at 10 locations identified basement, saline intrusion and saturated layers – some above iron pans – well. The method was less effective at identifying shell beds less than approximately 10 m thick at depths greater than 60 m. GroundTEM was not able to identify the iron pans directly. Synthetic modelling of 21 characteristic scenarios of the Kānoa Peninsula hydrogeologic setting indicated that SkyTEM will be most accurate at mapping top of basement and salinity and the presence of perched water tables. It also showed that there is more uncertainty in the mapping of thin shell beds and formations under saline water. (auth)
