Towards a global spectral-geomechanical database of volcanic rocks using VNIR-SWIR spectroscopy

Volcanoes are subject to intense fluid circulation, altering primary rock properties. The rock alteration can be due to “cold” water-driven weathering or “hot” hydrothermal fluids. Such alteration is facilitated by the efficient circulation of fluids through fractures and the connected pore-network. Fluid-driven alteration can manifest as mineral dissolution and precipitation, ultimately changing the properties of the host rock, such as strength and elasticity. Alteration-induced geomechanical changes are complex due to the large range of protolith porosity (e.g. 0.01-0.8) and permeability (e.g. 10^-10 to ≤10^-18 m²). For example, fresh pyroclastic rocks can initially have low strength, which can be ‘reinforced’ by mineral precipitation. In contrast, dense lava rocks can decrease their strength due to pore space enlargement and development of secondary clays, oxides, sulfides and sulfates.

This study analysed lab-tested samples from Ruapehu, Merapi, Whakaari, Chaos Crags, Ohakuri, Styrian Basin and La Soufrière de Guadeloupe volcanoes to provide new insights into the controls on geomechanical properties due to weathering and hydrothermal alteration. The volcaniclastic and lava rocks range from basaltic to rhyolitic in composition and encompass surface weathering and intermediate and advanced argillic alteration styles. The physical, geomechanical, Visible-Near Infrared (VNIR; 350-1000 nm), and Shortwave Infrared (SWIR; 1000-2500 nm) properties of the rocks were measured on core samples. Porosity, P-wave velocity, and uniaxial compressive strength ranged from 0.02-0.67, 88-5800 m/s, and 0.1-312 MPa, respectively. Partial Least Squares Regression (PLSR) was employed to successfully predict physical and mechanical properties using VNIR-SWIR spectroscopy data. The PLSR-based prediction models highlighted a handful of spectral bands around 400-600 nm, 1400 nm and 2200-2300 nm, indicating that hydrated secondary minerals were responsible for the observed geomechanical changes. The proposed method using VNIR-SWIR spectroscopy can lead to a new way of mapping physical and geomechanical properties at outcrop-scale using field spectrometers, and at volcano-scale using airborne and satellite remote sensing.