Resistivity
Our High Resolution Resistivity™ (HRR) method is ideally suited to acquiring 2D and 3D images of the lateral and vertical resistivity of the subsurface.
Resistivity is a volumetric property measured in ohm-m that describes the resistance of current flow within a medium. Its inverse, conductivity in Siemens/meter, describes the ease by which current will flow through a medium.
The earth’s resistivity is a function of soil type, porosity, moisture, and dissolved salts. The resistivity method detects and maps changes or distortions in an imposed electrical field due to heterogeneities in the subsurface.
Data are acquired systematically along a survey line by transmitting electric current (I) into the earth and recording the resulting potential (V) at increasing distances away from the current electrode. In this manner, a cross-section of apparent resistivity values is constructed of the subsurface.
Array Configuration: M & B are remote reference electrodes; A and N are roving electrodes and moved systematically along the survey line.
Measurements are typically acquired using the pole-pole array configuration. The term “pole-pole array” refers to a particular arrangement of four electrodes that are used to transmit current and receive the potential voltages. An electric field is established by applying electrical power (I) between two electrodes (transmitting pair or Tx). Electric potential (V) is measured by sampling received voltages using a data acquisition card connected to two additional electrodes (receiving pair or Rx). One of the electrodes of the receiving pair (Rx) and one electrode of the transmitting pair (Tx) is used as a remote grounding reference. The remotely-located reference electrodes must be located far enough from the active survey area that they do not influence the local electric field. A “transfer resistance” value (V/I) is obtained by dividing the electrical potential (V) by the applied electrical current (I). These two remote electrodes remain in a fixed location while the two “roving” electrodes are moved within the survey area. The remote electrodes have minimal influence on the readings, so, we use the term “pole-pole” to indicate that we are only using two active electrodes.
Example Cable Layout
Uses of Resistivity:
Heap Leaching
Electrical resistivity surveys are most reliable as a first-order target recognition tool. In this mode, sufficient background data are needed to distinguish the entirety of the target and confirm the extent of its edges. A target will not be identified if the variations in properties of the background material are similar in contrast and scale to those associated with the target. Assuming the target can be identified, the next order of interpretation is the relative degree of target size and intensity. Low resistivity regions may have discernable features that identify relative concentrations within the target. Finally, if data are of exceptionally high quality, i.e., the data are free from significant noise and have been acquired properly, they may be correlated to specific observed phenomena to develop relationships that convert directly geophysical data to hydrogeological or metallurgical data. This scenario requires that empirical models be developed from observed, co-located geophysical and metallurgical data. The empirical models are then used to translate and extrapolate the geophysical data to obtain metallurgical values over the site. Resistivity can be used to locate preferential flow paths of dry areas in the heap.
This figure shows a 2D resistivity slice of a leach pad at a depth of 20m. The orange and brown areas indicate higher resistivity, and can be interpreted as drier areas.
Pond Liner Leak Detection
This method is applied at the pond using a specialized marine cable with stainless steel electrodes at approximately 10 foot spacing to measure an induced electrical field (i.e., electrical voltage). The electric field is established by transmitting electrical current on two electrodes, with one electrode located between the bottom and top liners and the other electrode within the pond itself. The principle of resistivity as a leak detector is that as long as the top liner is tight (no leaks), then no electrical current can flow between electrodes between the liners and in the pond. The resistivity system uses a 12 volt deep cycle marine battery and outputs a low powered signal which does not present any danger to the liner or operators in the pond. If no leaks exist within the top liner, the ensuing electrical field would show a similar response (equi-potential lines) throughout the pond. If some seams of the liner are delaminated, or if small tears or holes exist in the liner, then these areas would allow electrical current to flow through the openings, thus causing anomalous normalized voltage readings centered over these areas. The resulting data is contoured to form a spatial map of the distortions within the pond which are interpreted to result from leaks within the liner.
Example geophysical results of an HDPE liner test. Dark blue indicates areas where current is leaking through the inner liner.
Resistivity for 3D Imaging
A 3D survey is superior to a 2D survey because considerably more data are collected to define the electrical properties of the subsurface. However, 3D surveys usually take longer to acquire and require more resistivity equipment. Resistivity data are collected based on a 3D data acquisition method that makes use of different electrode arrangements. The surface electrodes are distributed across a uniform grid to optimize the inversion models used in the data analysis and interpretation. The significantly larger amounts of data associated with a 3D survey, relative to a 2D survey, makes an optimized geometry crucial to reduce modeling run times and analysis. Further resolution is possible by adding depth electrodes to a surface electrode geometry, whereby electrical current and voltage measurements can be made near or within a target. Depth electrodes have the added benefit of being further from near-surface infrastructure and associated electrical interference and noise.
Three-dimensional View of Calculated Resistivity for Alternative Inversion Approaches. The green area shows the possible plume of ionic particles, with the yellow areas being less resistive and therefore a greater concentration of ionic particles.
