Basin Delineation

Groundwater is used in many industrial and municipal applications and finding new sources are important for the sustainability and viability for the users.  Exploring for groundwater is similar to exploring for any other resource and HGI uses geological and hydrogeological evidence along with geophysically-based measurements to help define potentially new zones for exploitation.  Some direct uses of our studies have included the placement of wells within hard rock or deep basin aquifers, defining basin geometry for groundwater modeling studies, and finding targeted areas suitable for dewatering of mine pits and underground adits.

Example: A general application of geophysics in the southwest is to map the boundaries of unconsolidated basins in an effort to estimate the volume of groundwater available to sustain new communities.  A common geophysical technique applied to this problem is the gravity method, where small perturbations in the earth’s gravitational field will be related to changes in density across the basin.  For example, dry sediments may have a density of 2-2.5 g/cc compared to crystalline bedrock at 2.67 g/cc.  These differences are measured along profiles from one side of the basin to the other.  Data are collected at discrete points along the transect and below is a depth to bedrock map modeled from a series of gravity points obtained in an area slated for suburban development.  The gravitational low is indicative of deep saturated sediments (sand, gravels, silts, and clays) overlying bedrock.

Gravity Profile and Model Showing Bedrock in Gray, Saturated Sediments in Blue and Dry Sediments in Yellow.

When several transects are acquired through a basin, either along parallel or orthogonal tracts, a picture emerges that provides a three dimensional view of the basin’s geometry.  The figure below shows an example, using several parallel transects to the one shown above.  The figure is limited to show the outline of the bedrock interface with red (higher elevations) representing the piedmont.   The basin is asymmetric with a steeply dipping fault defining the western (left) boundary.  From these data we can advise the best location for new groundwater wells to take advantage of the thick sediment sequences.

Gravity Results Showing 3-Dimensional Modeled Depths to Bedrock.