Three‐Dimensional Mapping of Mt. Ruapehu Volcano, New Zealand, From Aeromagnetic Data Inversion and Hyperspectral Imaging

Abstract

Combining 3‐D inversion of high‐resolution aeromagnetic data with airborne hyperspectral imaging creates a new method to map buried structure and hydrothermal alteration, applied to Mt. Ruapehu volcano, New Zealand. Hyperspectral imaging is sensitive to surface mineralogy including alteration minerals, while magnetic vector inversion reveals the volumetric distribution of magnetic susceptibility from which we interpret buried geology. Probability assessment from multiple model regularizations provides an important model uncertainty estimate. At Ruapehu, hyperspectral imaging highlights two main regions of surface alteration: the Pinnacle Ridge and the southeast flanks. The magnetic model of Pinnacle Ridge shows that alteration seen at surface continues to depth, but strongly magnetic, unaltered dikes form the core of the ridge. On the southeast flanks, the magnetic model also shows alteration imaged on the surface continues to depth; however, a previously unknown, magnetized sill intrudes part of the flank. Several smaller demagnetized regions are modeled, unlike at neighboring Mt. Tongariro where the hydrothermal system created a large demagnetized core. We propose that these differences relate to spatially focused (Ruapehu) vs distributed (Tongariro) eruption vents, the degree of faulting of the edifice and its glaciation history. Lava‐ice interaction produces fine‐grained lavas with measured magnetic susceptibilities similar to some moderately altered lavas, illustrating that care must be taken in the interpretation of magnetic data in the absence of geological information. The combination of hyperspectral imaging and aeromagnetic data inversion distinguishes shallow surface weathering from deeper‐seated hydrothermally altered rock masses, with implications for the magnitude and probability of collapse events.

Hydrothermal Alteration on Composite Volcanoes: Mineralogy, Hyperspectral Imaging, and Aeromagnetic Study of Mt Ruapehu, New Zealand

“…sediments and thin intercalated lava flows (Conway et al, 2016;Townsend et al, 2017), with some that might also be hydrothermally altered (Miller, Schaefer, et al, 2020). The demagnetized zones beneath Summit Plateau are patchy (Figure 12b), in agreement with electrical resistivity highs observed in the magnetotelluric model, which suggest the dominance of chlorite (Jones et al, 2008).…”

“…Sampling the magnetic field at 10 Hz results in a magnetic field value every 2 m. We corrected for diurnal variations using a local base magnetometer outside the survey region and subtracted the International Geomagnetic Reference Field (IGRF) at each data point location using the IGRF 2005 model. The data were leveled using widely spaced tie lies to minimize crossover difference and were gridded for visualization at 20% of the line spacing using a minimum curvature algorithm (see Miller, Schaefer, et al, 2020, for full details).…”

“…The inversion model uses a mesh with minimum dimensions of 50 × 50 × 25 or 25 × 25 × 10 m over the Pinnacle Ridge and Summit Plateau areas. The typical apparent susceptibility values recorded from the model range from 0 to 0.05 SI, which can be interpreted as a degree of hydrothermal alteration, from fully demagnetized/altered to magnetic/fresh rocks, respectively (Miller, Schaefer, et al, 2020).…”

“…Chlorite is a typical indicator mineral of propylitic alteration (Neal et al, 2018). A broader region around this zone also appears to be demagnetized (Figure 13b; Miller, Schaefer, et al, 2020), which can correspond to the development of an enlarged alteration halo (Figure 13b).…”

“…The 3D depth of hydrothermal alteration can be mapped using aeromagnetic data and subsequent inversion models (Finn et al, 2018(Finn et al, , 2001Miller, Schaefer, et al, 2020), if the lack of the magnetic susceptibility is due to hydrothermal alteration which dissolves (Ti-) magnetite crystals. Based on the SEM imaging, there is a systematic dissolution of (Ti-) magnetite with increasingly pervasive hydrothermal alteration (Figures 11a-11c).…”