Incorporating Transient Electromagnetic, Gravity, and Well Log Data for Developing Hydrogeologic Framework and Ground Water Models

Some of the uncertainty in predicting the potential responses of ground-water systems to hydrologic stresses is related to unknown aquifer properties and geology.  Much of this uncertainty is related to sparse well data, especially in deep basin-fill aquifer systems typical of the southwestern United States.  Subsurface data from gravity and transient-electromagnetic (TEM) inversion models provide additional constraints on aquifer stratigraphy and depth to bedrock.  A hydrogeologic framework model (HFM), constructed using well log and geophysical data for portions of the Upper and Lower San Pedro Basins in southeastern Arizona, will provide inputs to a numerical ground-water flow model.  A HFM consists of three-dimensional relations between hydrogeologic units, hydraulic properties of the hydrogeologic units, and geologic structures.  Gravity models using discrete density blocks and Fourier-transform techniques indicate depth-to-bedrock and basin-fill lithology along profiles.  One-dimensional vertical TEM inversion models indicate areas of shallow bedrock having higher electrical resistivity at the basin margins and locally at the southern part of the study area.  TEM inversion models also indicate areas of lower electrical resistivity likely related to thick and relatively impermeable fine-grained silts and clays in the center of the basin at depths of several hundred meters.  Lithologic and borehole geophysical data were used to constrain the TEM and gravity models.  The HFM developed for the study area includes the following hydrogeologic units: impermeable basement, pre-Basin and Range sediments, lower-basin fill, upper-basin fill, and recent alluvium.   Alternate HFMs constructed using several gravity and TEM inversion model results will be used to evaluate and quantify uncertainties in the ground-water flow model.