Introduction The use of geophysical methods for hydrogeological investigations has, during the last two decades, attracted much attention. The goal of a hydrogeological investigation is to construct hydraulic models of the areas of interest. The models need to be calibrated against traditional hydrogeological data such as hydraulic head, precipitation, infiltration, base flow, hydraulic conductivities inferred from pumping tests, and geological conditions estimated from drillhole information. The models can provide predictions of groundwater movement under specified natural variations and human extraction rates. An obstacle to information on geological conditions is that drillhole information is spatially sparse. This, combined with the high degree of equivalence in inverse hydraulic modeling, makes it necessary to find alternative methods for reducing the number of possible solutions. One way is to make use of improved geological models derived from hydrogeophysical investigations.