Groundwater Mapping

Geological fracture and fault zones may influence local and regional groundwater passages and flows, which makes these important features to understand and locate. The geological material surrounding the fault contact may generally be more porous and contain a higher water content than the denser, less porous basement material either side of the fault. This type of geological setting is well suited for electrical and electromagnetic methods. The geological material surrounding the fault contact may generally be more fractured and contain material of a different material to that either side of the fault. This type of geological setting is also well suited for seismic and potential field (gravity and magnetic) methods.

• Seismic Reflection is a geophysical technology which is directly related to the acoustic impedance between geological materials; the greater the acoustic impedance the greater seismic reflection. This technology is most useful where the density contrast between geological materials is large, such as in and around fault zones and where a fault is present large features can be observed in the interpreted seismic layers.
• Electrical Resistivity Imaging (ERI) maps the differences in the electrical properties of the subsurface if an electrical resistivity contrast exists between materials. The ERI technique is highly dependent on the porosity and water content of the earth material e.g. the greater the water content the larger the electrical conductivity. Fault zones may contain fault-gauge regions which consist of clays or other similarly conductive geological material. Therefore, an accurate gradiential boundary may be observed where a strong electrical contrast exists between a highly porous, low resistivity, fault-gauge regions and a less porous, less saturated, higher resistivity basement geology. ERI can also be used to map changes in basement structure which may be associated with faulting. Where a strong electrical contrast exists between the basement geology and the overburden material, an accurate gradiential boundary will be observed.
• Electromagnetic (EM) methods are quick and effective at highlighting regions of faulting. The geological material surrounding the fault contact may generally be more porous and contain a higher water content than the denser, less porous basement material either side of the fault. Where a strong electrical contrast exists between the higher electrically conductive fault gauge material and lower electrically conductive basement material, the general depth and extent of the fault may be mapped.
• Gravity can be an effective technique for detecting faults. This method measures changes in the density of the subsurface. If a fault is present a difference in the measured gravity data may be observed due to a shift in basement material causing the basement to be higher/lower to the surface either side of the fault. Also, geological material surrounding a fault may be of a different density in comparison to the geological material either side of the fault. If this density contrast Is large enough a feature will be observed in the gravity dataset. Gravity tends to be used in conjunction with electrical and electromagnetic methods, as these methods target the contrasts in electrical properties in and around the fault zones.
• Magnetic methods, similar to gravity methods, can be an effective technique for detecting faults. This method measures changes in the magnetic properties of the geological subsurface. If the geology is magnetic in nature, if a fault is present a difference in the measured magnetic field may be observed due to a shift in basement material. Also, geological material surrounding a fault may be of a different magnetic property in comparison to the geological material either side of the fault. If this magnetic contrast Is large enough a feature will be observed in the magnetic dataset. Magnetic methods tend to be used in conjunction with gravity methods, as this method targets contrasts in geological density in and around the fault zones.

The extent and depth of paleochannels can be interpreted using most geophysical techniques. The strong contrast between the less dense paleochannel overburden material and the denser basement material, makes seismic and gravity methods suitable for paleochannel mapping. Also, the electrical difference between the paleochannel overburden and basement material provides a strong electrical contrast, which also makes GPR, electrical and electromagnetic methods suitable methods for mapping the depth and extent of paleochannels.

• Seismic Refraction can be used to measure the P-wave (Vp) velocity of soil overburden and basement material. The measured Vp can be cross-correlated with borehole data, used to estimate rippability using rippability charts and calculate engineering moduli when combined with Shear-wave velocity information (Vs) (e.g. Poisson’s Ratio).
• Seismic Reflection is a geophysical technology which is directly related to the acoustic impedance between geological materials; the greater the acoustic impedance the greater seismic reflection. This technology is most useful where the density contrast between geological materials is large, such as between the overburden paleochannel geological material and basement. When a large acoustic contrast between the overburden and basement is present the extent of the paleochannel feature can be observed in the interpreted seismic layers.
• Ground Penetrating Radar (GPR) is a geophysical technology which is directly related to the acoustic impedance between geological materials; the greater the acoustic impedance the greater seismic reflection. This technology is most useful where the density contrast between geological materials is large, such as between the overburden paleochannel geological material and basement. When a large acoustic contrast between the overburden and basement is present the extent of the paleochannel feature can be observed in the interpreted seismic layers.
• Electrical Resistivity Imaging (ERI) maps the differences in the electrical properties of the subsurface if an electrical resistivity contrast exists between materials. The ERI technique is highly dependent on the porosity and water content of the earth material e.g. the greater the water content the larger the electrical conductivity. A paleochannel may contain overburden material which is highly porous and contains a higher water content in comparison to the less porous basement material. An accurate gradiential boundary for the base of the paleochannel may be observed where a strong electrical contrast exists between a highly porous, low resistivity, paleochannel regions and a less porous, less saturated, higher resistivity basement geology.
• Electromagnetic (EM) methods are quick and effective at highlighting paleochannel regions. The paleochannel geological material may generally be more porous and contain a higher water content than the denser, less porous basement material. Where a strong electrical contrast exists between the higher electrically conductive paleochannel overburden and lower electrically conductive basement material, the general depth and extent of the paleochannel may be mapped.
• Gravity can be an effective technique for detecting paleochannels. This method measures changes in the density of the subsurface. If a paleochannel is present a difference in the measured gravity data may be observed due to a deepening of the basement material in the paleochannel region.

Freshwater, or salt-water intrusions and fresh water-saline interfaces can be delineated using electrical and electromagnetic methods. These techniques are highly dependent on porosity and water content of the subsurface geology, but are also highly influenced by the salt content of groundwater or geological layers e.g. salt-water or clays.

• Electrical Resistivity Imaging (ERI) also targets regions of conductivity variation associated with fresh-water plumes or salt-water intrusions. ERI is a preferred methodology for showing the depth extent of a fresh-water plume/salt-water intrusion along a profile line. By arranging ERI cross-sectional profiles in a parallel orientation, it is possible to periodically map over calendar-time any observable migration of fresh-water plumes/salt-water zones between profile lines. It is also possible to monitor any changes in dip/strike direction of the migrating groundwater anomalies.
• Electromagnetic (EM) methods are quick and effective at highlighting and monitoring fresh-water plumes or salt-water intrusions. Depending on depth from surface, extent of the plume/intrusion, closely-spaced parallel EM profiles can laterally constrain electrically high conductive zones often associated with salt-water intrusion. In a saline environment, fresh-water plumes are also generally less conductive than the surrounding higher electrically conductive saline geological conditions. Through repeat surveys over calendar time it is possible to observe and monitor the extension, contraction or migration of plume/intrusion zones.