Inversion models, Iliamna Volcano, Alaska June 2012

Airborne electromagnetic (AEM) and magnetic survey data were collected during June 2012 along 556 line-kilometers over Iliamna Volcano, Alaska. These data were collected in support of alteration and volcano flank instability mapping as part of the USGS Volcano Hazards Program. Data were acquired by SkyTEM Survey ApS SkyTEM304 system with the Soloy Helicopters Eurocopter Astar 350 B3 and Bell 407 dual-moment, time-domain helicopter-borne electromagnetic system together with a Geometrics G822A cesium vapor magnetometer with Kroum KMAG4 counter. The survey was flown at a nominal flight height of 30 m above terrain along block-style lines with a nominal spacing of 250 m. The survey was designed to cover the summit and flanks of Iliamna volcano and includes transit lines east of the volcano which provide additional information on nearby structures and glacial thickness and resistivity. There are data gaps near Iliamna’s summit due to poor flight visibility.
Deterministic laterally and spatially constrained inversions of the processed airborne electromagnetic data were developed using the AarhusINV code (Auken et al. 2014, https://doi.org/10.1071/EG13097) implemented in Aarhus Workbench software. Spatially constrained inversion was used over the main survey grid area and laterally constrained inversions were used for transit lines east of the main grid. Inversion parameters were selected by running a series of test models with varying starting model resistivity values, layer discretization, horizontal/lateral constraints, and other inversion parameters. A smooth 40-layer fixed-depth inversion model was developed with layer thickness increasing with depth (see model data for exact thicknesses). Moderate vertical and lateral constraints on resistivity were used with values of 2.0 and 1.5, respectively. An automatically determined starting model was used. Sensor altitude was treated as a free parameter after the 5th iteration. The final model parameters described above were selected because they best represented the physical understanding of the system and minimized data misfit. The data provided include spatially constrained and laterally constrained inverted resistivity models and plotted depth sections along all flight lines. Digital data are described in the data dictionary; additional details regarding data inversion are described in the metadata processing steps.