Fracture Mapping
Fracture, fault, and fissure mapping is a fairly common application of geophysical surveys. The main component to finding these structural features is the offset in contouring or a large change in geophysical property values (resistivity or velocity) along lineations consistent with independent geologic mapping. Fracture mapping is useful for identifying features that may represent infiltration zones or affect potential groundwater pathways. Fracture mapping is also performed to find transport pathways of Acid Rock Drainage (ARD) which is not easily solved by traditional characterization or monitoring methods such as drilling and groundwater sampling.
Geophysical Methods for Subsurface Geological Fracturing:
Seismic Reflection Method:
- Process: This involves generating seismic waves on the surface that travel through the subsurface, get reflected at different layer boundaries, and then are detected by geophones or seismometers on the surface.
- Application: It helps identify the depths and orientations of fractures and faults in the subsurface.
Electrical Resistivity Tomography (ERT):
- Process: ERT involves injecting electrical current into the ground through electrodes and measuring potential differences. The measurements are used to create a resistivity distribution model of the subsurface.
- Application: It helps identify areas of different resistivities, which can indicate the presence of fractures or faults.
Magnetic Method:
- Process: This involves measuring the Earth’s magnetic field variations at the surface.
- Application: Changes in magnetic properties can indicate the presence of fractures or faults.
Gravity Method:
- Process: This involves measuring the Earth’s gravitational field variations at the surface.
- Application: Density variations in the subsurface, often associated with fractures and faults, can be identified using gravity measurements.
Each geophysical method provides unique insights into the Earth’s subsurface, helping comprehensively understand and map geological fracturing. The appropriate method or combination of methods is chosen based on the site’s specific characteristics and the survey’s objectives.
By employing these geophysical methods, it becomes possible to understand the subsurface geological fracturing better, which is essential for identifying infiltration zones, potential groundwater pathways, and transport pathways of Acid Rock Drainage (ARD).
Example:
U.S. Department of Energy (DOE) Los Alamos National Laboratory, Los Alamos, NM
-Identification of possible infiltration zones and groundwater pathways of Upper Sandia Canyon
HGI performed a fracture mapping investigation in the wetland area near the headwaters of Sandia Canyon at the Los Alamos National Laboratory. The objective for this geophysical investigation was to collect electrical resistivity data to identify low resistivity regions that could be indicative of increased moisture content, changes in geologic lithologies, geological structure, or an increased concentration of electrolytes compared to background conditions. The survey area is known to include splays of the Rendija Canyon fault zone supporting the theory that faults and fractures in this area may be secondary pathways for deep infiltration. HGI’s data acquisition included four parallel longitudinal lines on strike with the Sandia Canyon and five supplementary lines placed orthogonally to the longitudinal lines.
Differences in surface conditions during Sandia Canyon resistivity surveying
The figure below displays the modeled resistivity results for one of the resistivity profiles running parallel to the canyon. The depth of the profile is almost 900ft. The near-surface conductive layer relates to elevated moisture in alluvium present in the canyon-floor flood plain and wetland area. Moving across the profile, the near-surface rocks become more resistive as the canyon becomes incised and the profile crosses bedrock exposures of welded tuff. Offsets in the resistivity contours indicate potential fault structures (as indicated by dashed lines on the profile). Other conductive features not associated with significant offsets in the model resistivity contours may represent enhanced water or clay content.
Modeled resistivity results for a select profile across upper Sandia Canyon.