Natural potential measurement involves making use of geophysical methods of recording electromagnetic field components (gravimetric, electrical, magnetic, etc.) of the earth, without the contribution of an artificial sources signal generating. The method is often applicable in geological and hydrogeological exploration on large areas, away from interference.
NATURAL POTENTIAL MEASUREMENT METHODS
- Magnetic Field Intensity and Vector
- Microgravimetry
- Self Potential (SP) Surveying
- Horizontal-to-Vertical Spectral Ratio (HVSR)
* APPLICATIONS OF GEOPHYSICAL INVESTIGATIONS:
- Archeological Surveying
- Borehole Investigation
- Determining Geological and Hydrological Conditions
- Lake and River Prospecting
- Mapping Unexploded Bombs (UXO)
- Minerals Exploration
- Research on Inorganic Contaminants
- Utility and Buried Objects Detection
- Wind Farms and Photovoltaic Sites
* Other methods used in geophysical investigations:
Applications of natural potential measurements:
- Mapping geological boundaries
- Identification of fractures in the bedrock
- Mineral Exploration
- Geological mapping
- Detection of septic systems
- Abandoned wells
- Detection of buried tanks
- Characterization of buried waste
- Detection of buried weapons
- Delimitation disposition pits / trenches
- Buried fuel tanks
- Detection of cables and pipelines
- Unexploded ordnance (UXO)
- Old settlements
- Observation posts
- Archaeological battlefields
- Submerged tanks and landfills
Magnetic Field Intensity and Vector
Magnetic prospecting maps variations in the magnetic field of the earth that are attributable to changes of structure, magnetic susceptibility, or remanence in certain near-surface rocks. Sedimentary rocks generally have a very small susceptibility compared with igneous or metamorphic rocks, which tend to have a much higher magnetite content.
In mining exploration, magnetic methods are employed for direct location of ores containing magnetic minerals such as magnetite. Intrusive bodies such as dike can often be distinguished on the basis of magnetic observations alone.
Magnetic investigations found a wide application in engineering geophysics where are used primarily for the detection of ferrous materials buried near surface – pipes, cables, unexploded bombs (UXO), containers etc.
Applications of magnetism:
- Mapping geological boundaries
- Identification of fractures in the bedrock
- Mineral Exploration
- Geological mapping
- Detection of septic systems
- Detection of buried tanks
- Detection of buried weapons (UXO)
- Delimitation disposition pits / trenches
- Buried fuel tanks
- Detection of cables and pipelines
- Old settlements
- Old fields
- Cemeteries
- Historic structures
- Historical complex
- Observation posts
- Archaeological battlefields
- Search anchors dragline excavators, etc.
- Submerged tanks and landfills
Microgravimetry
The gravimetry method is useful wherever the formations of interest have densities that are appreciably different from those of surrounding formations. It is an effective means of mapping sedimentary basins where the basement rocks have a consistently higher density than the sediments. It is also suitable for locating and mapping salt bodies because of the generally low density of salt compared with that of surrounding formations. Occasionally it can be used for groundwater studies and for direct detection of heavy minerals such as chromites.
Gravimetry measurements are routinely made in conjunction with marine seismic work and are used as a minor supplement. Gravimetric investigations find a wide application in engineering geophysics is often used for mapping and detection of underground voids, tombs, the archeological sites, tunnels, burial, etc. inhomogeneities.
Microgravimetry applications:
- Bedrock depth
- Investigations diapire
- Identification of fractures in the bedrock
- Mapping of sand and gravel deposits
- Detection of septic systems
- Mapping and detection cave
- Deposits filling
- Determination of the sliding plane
- Investigations on displacements
- Detection of voids and cavities
Self Potential (SP) Surveying
Self Potential (SP) Surveying, one of the oldest geophysical methods, involves passively measuring natural earth voltages. The method is commonly used in hydro geological and geotechnical applications.
The SP method involves measuring natural earth voltages using two electrodes at the surface. The natural earth voltages are typically caused by moving water and ions through pore spaces of soil and rock (electro kinetic potential). The potential drop (voltage) is measured between two non-polarizable electrodes at specific locations along the survey line. Typically, one electrode is placed in a fixed location, while the mobile electrode is placed at each reading location.
The depths of investigation for typical SP surveys range between 5 and 50 feet. The surveys are commonly conducted along with resistivity surveys in order to correlate features. Areas of water seepage are indicated by a sharp increase or decrease in the measured voltage. The data are presented as a graph of voltage versus survey location.
Horizontal-to-Vertical Spectral Ratio (HVSR)
Seismic prospecting method HVSR (Horizontal-to-Vertical Spectral Ratio) consists in recording the vibrations produced by ambient noise in three directions for a few minutes to determine the fundamental resonance frequency. To apply this method is used a single geophone especially designed for this purpose.
Estimating sediment thickness deposits and bedrock geometry is an essential component in numerous geological and hydrogeological studies. Some studies do not require or do not help pay for the acquisition of high resolution so would benefit from using HVSR method, the low cost with a solid estimate of the thickness of sediments and bedrock depth at a point.