Geophysical Surveying

Resistivity

 
 
 
Resistivity

Resistivity is one of the earliest forms of geophysical prospecting. Its base principle makes use of the different electrical conductivities of sub-surface features in order to map-out underground variations and anomalies. Unlike its often used counter-part, magnetometry, resistivity survey is sensitive to geological changes adding another dimension to survey, making imaging of subsurface levels more accurate, though more complex to analyse.
Probes are placed into or above the ground, which is itself used to complete a "circuit". The electrical current passing through the ground travels from one probe to the other and the conductivity of the substances beneath the probes cause a specific resistance to be calculated. For example, high water content resulting from the presence of a clay lens, a loosely back-filled trench or metal will result in a comparatively low resistivity. Accordingly, the presence of a poor conductor such as stone building foundations, well drained sandy soils, or voids will produce high resistance.

Multiple Probe Array (Syscal family)

  1. Multiple probes (called electrodes) are placed at equal distances across a transect or over an area. Up to 96 electrodes can be used in a single set up.
  2. Up to 10 readings can be taken simultaneously and an internal switch allows every possible combination of two electrodes to be used to calculate the resistivity at corresponding depths. The result is the rapid collection of data forming a tomographical data set, allowing for the creation of a fully 3D image of the site.
  3. Alongside the ability to choose the specific depths surveyed by appropriately separating the electrodes, the user is also able to adjust the strength of the current entering the ground. This means that the survey can be tailored to the requirements of the customer and to the specifics of the site environment

Ohm Mapper

  1. Distinguished by pulling an array of non-intrusive receivers and transmitters along a transect. Since the probes do not have to be inserted into the ground, time is saved both in setting out equipment and also in the data collection process.
  2. Allows the user to survey in otherwise difficult terrain such as concrete or ice fields.
  3. By alternating the distance between the transmitter and receiver, a depth profile can be created and a 2D or 3D profile of the site generated.

Twin Probe Array

  1. The most basic array comprising a single dipole-dipole arrangementof twin probes; “mobile” and “static”.
  2. Depth of survey is defined by the distance between mobile probes – 1m separation equates to approx 1m ground penetration of current whilst resolution depends upon spacing between readings – typically 0.5m to 2m.
  3. Differentially spaced probes can be used to obtain deeper data and used to create tomographical surveys.
  4. Whilst equipment cost is low and data processing straightforward, such survey is slow, particularly if using multiple probe separations. Depth capabilities are logistically limited, and extremes of moisture levels can result in poor data quality.