Archive for the ‘Fracture Trace’ Category

Well Siting Map

Fracture trace analysis involves several steps, each of them designed to add to the previous one, and culminate in a map of potential drilling locations in a fractured bedrock regime.  Unlike many methods, fracture trace analysis relies upon several well-proven geologic and hydrogeologic practices to create this map, and reduce the risks of drilling a less than adequate water well.

The first steps in conducting a fracture trace analysis involve terrain analysis using aerial photography.  Specifically, discernible linear features are mapped on the photographs, along with indications of recharge areas, and surface expressions of geologic interest relative to groundwater movement and quality.  The second step is called “ground truthing” were in a geologist goes into the field to verify the features identified during the terrain analysis, along with discarding false positives (e.g. fence lines and old roads).  The geologist will also prepare a basic geologic map, including the strike and dip of these features, in preparation of the next step.

Using the structural geology information obtained in the second step, the third step involves plotting the information using stereonets, from which the primary fracture systems orientation and inclination can be determined.  Once these are plotted, a return to the field to focus on these primary fractures occurs.  It is during the second mapping event, that the “Hydro-Potential” values of the fracture system are measured.

Having collected the “Hydro-Potential” value data, a final map is prepared of the potential drilling locations based upon likelihood of water, accessibility, and compatibility with future plans.  With this map, the drilling operations can be directed to begin.

USGS Digital Elevation Model showing fractured bedrock

USGS Digital Elevation Model showing fractured bedrock

Fractured bedrock aquifers, are the most common source of groundwater in mountainous terrain.  Nearly vertical fractures, and systems of fractures, can extend hundreds of meters below the surface, and can be present over great distances.  It is these fractures in which water accumulates, moves, and ultimately can be recovered from, for use as a source of water supply to wells.

Conversely, aquifers comprised of inter-layered strata of sand, gravel, silts and clays that have accumulated in basins as sediment derived by erosion of those same mountains, are generally classified as alluvial aquifers.  Alluvial aquifers represent a vast storage system for groundwater, and are quite prolific in their supply and extent.  The majority of human endeavors that rely upon groundwater, rely upon groundwater from alluvial aquifers.

Finding water in either aquifer type has its own requirements, and challenges.  Alluvial aquifers tend to have large volumes of economically available water, but can have naturally or man-made pollutants present in them.  Fractured bedrock aquifers have fewer quality challenges (they do occur) but have far smaller quantities of water available to them.  As such, locating wells in fractured bedrock aquifers requires an understanding of not only the geology and hydrogeology, but of meteorology so that rainfall can be accounted for and recharge may be improved, along with several other factors ranging from water quality to site engineering issues.

In general, locating wells in fractured bedrock aquifers requires solid geologic an hydrogeologic work, in the form of a fracture trace analysis (FTA).  Completing a FTA significantly reduces the risk of drilling a well with unacceptable low-yield, poor recharge, or impaired water quality.