MAGNETIC AND VLF-EM INVESTIGATIONS AT THE OLD FAITHFUL GEYSER, YELLOWSTONE NATIONAL PARK

The Old Faithful geyser is within the Upper Geyser Basin within Yellowstone National Park and is arguably the most famous geyser on Earth. The Old Faithful geyser has one of the most regular eruption cycles (actually two cycles) and thus has been actively studied using geological, geochemical and geophysical methods to determine the cause of this regularity and the path of the hydrothermal water. The most important geophysical study to date has been a 2017 seismic surface wave study that imaged a high velocity region northeast of the Old Faithful vent and a low velocity region southwest of it. The velocity structure to southwest was interpreted to be caused by a porous hydrothermal reservoir that was 10 to 60 meters below the surface, that may be the hydrothermal reservoir for the eruptions at Old Faithful. A detailed gravity study found similar results but suggested that that the actual reservoir may be further to the southwest. In order to better constrain the seismic surface wave and gravity results, a detailed magnetic and VLF-EM investigation was conducted where data were collected along several northeast-trending profiles that crossed the Old Faithful geyser area and the southwest low gravity/velocity region. Over 5000 magnetic (data collected every 1 second while walking) and 600 VLF-EM data points were collected. The magnetic data were processed into reduced to the north magnetic pole (RTP) data and the VLF-EM in-phase and out-of-phase data were used in the analysis. The RTP map indicates that the southwest area is associated with a magnetic minimum constraining the gravity results but the Old Faithful geyser region has a large minimum near the vent but has several magnetic maxima that surround the vent that might indicate a narrow region for the circulation of the hydrothermal water. The VLF-EM results indicate that the southwest region is associated with a electrical conductive region adding additional constraints on the hydrothermal nature of this region. The Old Faithful area is marked by high electrical conductivities except at and immediately surrounding the vent region. Additional analysis will include the construction of magnetic residual and derivative maps and VLF-EM transformations including Fraser filters and Hilbert transforms.