Kellett, R.L.; Rosser, B.J.; McColl, S.T. 2025 Potential of helicopter-based SkyTEM technology for imaging large landslides: case study of Te Puia and Mangahauini landslides, Tairāwhiti Gisborne. Lower Hutt, NZ: Earth Sciences New Zealand. GNS Science report 2025/04. 42 p.; doi: 10.21420/XVJM-0Q68
Abstract
The Te Puia Springs and Mangahauini Gorge landslides are large features in the landscape of Tairāwhiti Gisborne. The movement of these landslides is recognised as a risk to the main highway around the East Cape (State Highway 35) and, in the case of Te Puia Springs, a direct risk to buildings and people in the village, which includes a regional hospital. The movement of Te Puia Landslide has been monitored for many decades. Landslides in Mangahauini Gorge have blocked the river and caused damage to the road on several occasions. The most recent movements during Cyclone Gabrielle in 2023 resulted in complete closure of the road through the gorge, and remedial engineering work is ongoing at a cost of millions of dollars. The landslides are large and complex, so re-aligning the highway to avoid unstable ground is not practical. An airborne electromagnetic (AEM) survey was undertaken to research the utility of this method for investigating large complex landslides and placing the landslides in a regional context. The technology has been used elsewhere in New Zealand to map aquifers in the top 10–200 m and identify depth to bedrock in alluvial basins. In the investigation of landslides, geophysical data can provide some information on the physical properties of the material in the landslide, as well as the geometry of the landslide, provided that there is a contrast in properties with the underlying bedrock. The rock property derived from the AEM survey is electrical resistivity. Geologic materials demonstrate electrical-resistivity variations in relation to changes primarily in mineral composition, porosity, permeability and fluid saturation. Seven lines of AEM data were collected over Te Puia Springs, and five lines were collected over the Mangahauini Gorge. The resulting resistivity models identified the deeper (50–250 m) bedrock geology, including layering in the Cretaceous and Cenozoic sediments of the East Coast Allochthon. The morphology of the bedrock units may have some influence on the geometry of the landslides but, to a first order, Te Puia Landslide is occurring within the thin Quaternary cover and/or weathered near-surface bedrock, mostly within the top 50 m. In some places, this layer is less than 10 m thick and the AEM method cannot image changes with any confidence. In contrast, Mangahauini Landslide has a deep-seated detachment surface (up to 150 m deep), and the block of high-resistivity sandstone in the main landslide may have undergone significant rotation. Slumping of the over-steepened toe of the rotated block produces secondary landslides into Mangahauini River. The resistivity variations within both landslides are significant, so resolution of the mapping in future surveys could be increased by using closer-spaced lines and being closer to the ground. A different airborne system would be needed for this approach. Deploying ground-based systems in some of the terrain is also an option. While it was not a primary objective of this work, the highly saline thermal waters at Te Puia Springs were imaged to depths of 150 m. There appears to be some influence of faulting and sedimentary layering on the distribution of low-resistivity zones associated with the saline fluids. The lines that crossedthe prominent sinter at the surface showed that it was highly resistive and mappable. The results of this work provide motivation for considering AEM as a technique for mapping low-temperature geothermal systems (auths)
