A 3D Gravity Investigation of Devils Tower, Wyoming, U.S.A

We have completed a preliminary investigation using surface gravity data and 3D geology to detect one or more magma chambers at Devils Tower, Wyoming, U.S.A. By comparison to the complete Bouguer anomaly, we use a local 3D geologic interpretation to estimate the observed Free-air anomaly. Since the Free-air anomaly is the result of the Earth's geology and its 3D density distribution, an expected Free-air anomaly can be estimated using a causal 3D geology interpretation. The estimated Free-air anomaly is the result of applying a geologic and statistically constrained density inversion of the observed Free-air anomaly data and the expected Free-air anomaly of an expected causal geology interpretation. The difference between the observed and estimated Free-air anomaly values is the residual Free-air anomaly, RFAA. The RFAA shows where the interpreted geology is in error with respect to the Earth's geology. This means that if magma chambers have not been modeled, then the RFAA of the Devils Tower area shows the location, shape and scale of potential source rocks for Devils Tower. The Free-air anomaly estimator requires building a causal 3D geologic interpretation and assigning realistic a priori densities and density uncertainties to the elements in the geologic interpretation. This preliminary 3D geology interpretation of the Devils Tower area was developed using topographic elevation models, the expected geology map, and stratigraphic unit thicknesses. Using the RFAA and a causal 3D geology to estimate the Free-air anomaly, we can isolate the gravity effect of each component of the geologic interpretation and utilize this information to update the geology interpretation. Given the large spatial distribution of similar volcanic outcrops, expanding the 3D geology from a local interpretation to a regional interpretation would be useful for an improved understanding of the complex geologic setting of the region.