Applications of seismic polarization analysis
The distance between the pair of current electrodes and the pair of potential-field electrodes determines the depth of investigation the measured data. The measured IP phase indicates the ability of rocks to briefly hold an electrical charge after the transmitted voltage is turned off.
Electrical properties of the ground can be calculated from comparing the transmitted signal to the received signal. Ground resistivity ability to conduct an electrical current affects the strength of the received signal. A change in induced polarization ability of ground material to polarize at interfaces affects the shape or timing of the received waveform. When plotted, these properties reveal valuable information about faults, fractures, geologic structures, mineralization, and groundwater porosity for subsurface modeling. Resistivity and IP surveys are used extensively in minerals, geothermal, and groundwater exploration, in many environmental investigations, and for several applications in hydrocarbon exploration and secondary recovery.
Resistivity surveys are also an underused but valuable tool for many geotechnical applications. The polarization data from an IP survey adds insight for interpreting results of other common exploration survey methods. IP surveys may be done as follow-up to a CSEM or CSAMT survey in which an induced, controlled source of electric energy is also used, but in this case, to measure the magnetotelluric fields. Complex resistivity CR , also known as Spectral IP, is a multi-frequency IP method developed by Kenneth Zonge in the early s as a means of differentiating between anomalous survey responses coming from geologic alterations, from sulfides, or from electromagnetic coupling — an unwanted artifact of the IP measurement process itself.
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This method has proven extremely valuable for mineral reconnaissance surveys. Geology The geologic model is of disseminated mineralization in which the conductive sulphides are not connected but occur in an insulative matrix. Clay minerals also show a moderate IP effect. Variations in subsurface moisture content, porosity, permeability, and soil or rock type all affect resistivity measurements. Changes in imaged resistivities are used to infer changes in geology. Metallic mineralization — particularly disseminated sulphides — cause increased IP values. Certain dissolved solids in groundwater increase IP response, and clay can increase IP response if the abundance of it is within specific ranges dependent on clay type.
In structural features, such as faults, contacts generally appear as conductive plates or sheet-like features unless they have been well cemented with little porosity. For example, quartz or calcite-filled veins could be expected to have high resistivity values.
IP, CR Geophysical Methods | Zonge International
In contrast, porous shear zones could be expected to have low resistivity values relatively conductive. This is the origin of the term Induced Polarization IP. IP Response The IP response also refers to the capability of the earth material to store electrical energy in a manner analogous to a capacitor in parallel with a resistor, as shown in this simplified model : If an alternating current is applied, the IP effect appears as a phase lag between the transmitted current and the measured voltage. The resistivity of the rock can be determined from the transmitted current, measured voltage and the measuring location relative to the transmitter.
The phase lag varies with frequency and volume percent of the polarizable materials. The resistivity of the rock in most oil fields is determined mainly by its porosity and the resistivity of the fluid. An increase in the ion content of the fluid leads to a decrease in resistivity. Positive IP anomalies above oil deposits have long been observed in Russia.
Detailed studies carried out in China since have also shown IP response is stronger at and around oil reservoirs as well as along the hydrocarbon migration path. It consists of a Kw T high power geophysical current source or Kw model T that transmits the current waveform into the ground and an array of CRU-2, 2-channel data acquisition units that receive the earth's electrical response. With the IP method, the chargeability is measured simultaneously with the resistivity.
The IP method is useful for mineral exploration because sulfide minerals such as pyrite and chalcopyrite have high chargeability.
The electromagnetic survey is a method for observing resistivity like the electric survey. This method measures an electromagnetic field that vary with time and presumes subsurface resistivity distribution. The exploration depth depends on the frequency of the electromagnetic field.
Electromagnetic waves reach more deeply in accordance with decreasing frequency. Therefore, by measuring electromagnetic fields of various frequencies, you can obtain information about shallowness and depth. The exploration depth is generally from 1 km to several km.