DETAILED GRAVITY STUDIES AT OLD FAITHFUL GEYSER, YELLOWSTONE NATIONAL PARK

The Old Faithful geyser is the most famous geyser within the Yellowstone caldera and has been actively studied to determine why it has been so regularly active and to improve the predictably of eruptions. Old Faithful is in the Upper Geyser basin which contains the most geysers in the world and lies on the southern end of the geyser basin. There have been numerous geological, geochemical and geophysical studies of the Old Faithful region but the most important study was a 2017 seismic surface wave study that showed that there was a higher velocity region northeast of Old Faithful and a lower velocity region southwest of it. The velocity structure of the southwestern was interpreted to be caused by a highly porous hydrothermal reservoir that was 10 to 60 meters below the surface. Thus, this region is thought to be the hydrothermal reservoir for the eruptions at Old Faithful. In order to better constrain the geometry of the reservoir, a detailed gravity study was conducted where data were collected on a five to fifteen meter interval in the Old Faithful and surrounding regions, including next to the vent of the geyser. Approximately 560 points were collected and data were processed into complete Bouguer gravity anomalies using differential GPS techniques to obtain ellipsoidal heights and DEMs to determine terrain corrections. Complete Bouguer gravity maps indicate a large amplitude gravity minimum southwest of Old Faithful but is further southwest than the seismic results. Similar to the seismic results, a high density region occurs to the northeast of Old Faithful but the region next to the geyser’s vent has additional high density regions that suggests these areas might be the conduit for the water. Additionally, there is a gravity minimum that leads from the gravity minimum southwest of Old Faithful to the Old Faithful geyser that might be the path of the water that feeds the geyser. Additional analyzes will include the construction of residual and derivative anomaly maps, and a 3D inverse density model.