The utility of gravity and magnetic methods for understanding subsurface hydrogeology in large alluvial watersheds: Examples from urbanized basins of the Western United States

Population continues to grow rapidly within the large alluvial watersheds associated with structural basins of the Basin and Range Province and the Rio Grande rift of the western United States. Increasing demands on ground‐water resources in these basins, combined with water‐rights disputes, have amplified the need for improved understanding of subsurface hydrogeology. Gravity and magnetic methods provide cost‐effective information critical to the understanding of the subsurface geology that controls hydrology at watershed scales. Gravity models are used to estimate the variations in the overall thickness of basin‐fill aquifers and to define major subbasin boundaries that partition flow systems. High‐resolution aeromagnetic surveys can be used to map the distribution of volcanic and other crystalline rocks in the shallow subsurface that impede flow. In certain geologic settings, the aeromagnetic data can be used to infer the base of basin aquifers or reveal buried, shallow paleotopography. In addition, the utility of high‐resolution aeromagnetic data to locate partially or wholly concealed faults within basin sediments is a non‐conventional application that has gained prominence in recent years. Examples of these uses of gravity and magnetic methods come from studies of basins within the Albuquerque‐Santa Fe, NM, urban corridor, the Virgin Valley in the tristate area of NV, AZ, and UT, the upper Verde River watershed near Prescott, AZ, and the San Luis Valley surrounding Alamosa, CO.