- Resistivity Survey
Survey Resistivity / Geoelectric Survey is a measurement of the properties of the earth’s material with the concept of measuring resistance to electric current. Rocks or minerals have different resistivity values, ranging from very low resistivity to very high resistivity. The geoelectric survey aims to measure the resistivity value by flowing current and measuring the voltage simultaneously through 2 electrodes that function as currents and 2 electrodes that function as potentials.
geoelectric survey scheme
We as a geophysical consultant provide geoelectric survey measurement services with mineral targets / can also be for infrastructure purposes and for environmental needs such as groundwater (aquifer), water pollution etc.
- IP (Induced Polarization)
The explanation of the IP effect is that the earth is storing electrical energy during the time that the current is following. This electrical energy is released only gradually when the current is turned off. In order to understand the IP effect, we need to understand the mechanisms by which the earth conducts current and how these mechanisms can store electrical energy. Current flow underground is dominated by electrolytic conduction, in which current is carried by the transport of charged ions through pore fluids.
However, this is not the case in metallic grains, in which the current flow is electronic. IP surveys are perhaps the most useful of all geophysical methods in mineral exploration, being the only ones responsive to low-grade disseminated mineralization. There are two main mechanisms of rock polarization and three main ways in which polarization effects can be measured. In theory the results obtaine by the different techniques are equivalent but there are practical differences
The local acceleration of gravity is not 9.8 m/s2, instead the gravitational acceleration responds to local changes in rock density. The gravity method Is widely used as a reconnaissance tool in petroleum exploration, where large scale sedimentary basins typically have a lower density than the basement rocks, and hence will show up as a gravity “low”. In mineral exploration, gravity is occasionally used as a follow-up method in base-metal surveys. The method is also used in engineering site surveys. Differences in rock density produce small changes in the Earth’s gravity field that can be measured using portable instruments known as gravity meters or gravimeters. The gravitational constant, G, has a value of 6.67 × 10−11 Nm2kg−2. Gravity fields are equivalent to accelerations, for which the SI unit is the m s−2 (alternatively written as the Nkg−1). This is inconveniently large for geophysical work and the gravity unit (g.u. or μm s−2) is generally used. The cgs unit, the milligal, equal to 10 g.u., is still also very popular
- SP (Self Potential)
Natural potentials of as much as 1.8 V have been observed where alunite weathers to sulphuric acid, but the negative anomalies produced by sulphide ore bodies and graphite are generally less than 500 mV. The conductor should extend from the zone of oxidation near the surface to the reducing environment below the water table, thus providing a low-resistance path for oxidation–reduction currents
- GPR (Ground Penetrating Radar)
Ground penetrating Radar (GPR) is a tool used to detect bodies buried underground at certain depth using radio wave ranging between 10 MHz to GHz. GPR consists of transmitter, an antenna connected directly to pulse source and receiver, an antenna connected to image signal processing unit.
Determining the type of antenna used, the transmitted signal and signal processing method is dependent on some matters: type of object to be detected, object depth, and electric characteristic of ground medium. Radar methods use the reflections of short bursts of electromagnetic energy (chirps) spanning a range of frequencies from about 50% below to 50% above som specified central frequency. A typical 100 MHz signal thus has a significant content of frequencies as low as 50 MHz and as high as 150 MHz. The radar frequencies of from one to several thousand MHz were originally thought to be too high for useful ground penetration, and ground penetrating radar (GPR) is a relatively new geophysical tool
- AMT/CSAMT (EM)
A broad band of naturally occurring electromagnetic radiation exists and can be used geophysically. These magnetotelluric fields are partly sourced by ionospheric currents and partly by thunderstorm activity (sferics). The most useful signals, in the frequency range from 1 Hz to about 20 kHz, are commonly referred to as audio-magnetotelluric (AMT).
These propagate down into the Earth as roughly planar wavefronts oriented parallel to the Earth’s surface. The broad AMT frequency band allows conductivity variations to be investigated over a correspondingly wide range of depths, from a few metres to several kilometres. However, the signals, like so many other things that come for free, are not always reliable. Short- and long-term amplitude fluctuations cause many problems, and unacceptably long times may be needed to obtain satisfactory readings because of low signal strengths. In particular, signals tend to be very weak in the 1–5 kHz range that is crucial for exploration of the upper 100 m of the ground. It is therefore now common to generate similar signals from controlled sources (CSAMT), and to use these to either supplement or replace the natural signals
- Seismic Refraction
Refraction surveys are widely used to study the water table and, for engineering purposes, the poorly consolidated layers near the ground surface, and also in determining near-surface corrections for deep reflection traces. Travel times are usually only a few tens of milliseconds and there is little separation between arrivals of different types of wave or of waves that have travelled by different paths. Only the first arrivals, which are always of a P wave, can be ‘picked’ with any confidence
- Geological Mapping
- Topography Survey
- Undergound Utility Service
- Well Logging
- Sondir CPT
- Boring SPT