Tracking Shallow Hydrological Changes in Micro-Fractured Siliceous Sinter During Pre-eruptive Cycles Of Old Faithful Geyser Vent, Yellowstone National Park, USA

摘要

In April 2015, Ground Penetrating Radar (GPR) was used to characterize the shallow subsurface (0-5 m depth) of the western sinter slope, immediately adjacent to Old Faithful Geyser and directly above an inferred, deep, geyser chamber. A series of time-sequence images were collected between two eruptive events. Each set of time-sequence GPR recordings consisted of four transects aligned at approximately 90° to each other, to enable coverage over the location of the inferred deep geyser chamber. Seven time-sequence events were collected over a 48-minute interval to image changes in the near-surface, as well as pre- and post-eruptive cycles. Few fractures were visible at the surface but the time-sequence GPR images revealed a series of micro-fractures at 0-5 m depth that fill and drain repetitively, immediately after an eruption and prior to the main eruptive event. No large cavity was observed, but this could be present at depths greater than those possible to image using GPR. Thermal infrared imaging revealed heat at the surface, on dry areas of the sinter terrace. One surface sample of geyserite sinter representing sinter along the GPR transect lines was analysed by Computerized Tomography (CT) and Scanning Electron Microscopy (SEM). CT and SEM analyses confirmed the presence of micro-fractures and voids in the sinter. SEM images also show the on-going deposition of newly-formed opal-A spheres. This provides evidence that hydrofhermal fluid moves within the geyserite sinter terrace in our study area.