WELLOG VERTICAL ELECTRICAL SOUNDING (VES)
Vertical Electrical Sounding (VES) uses an array of electrodes, an electric power source, and voltage and current measuring devices to measure the resistivity of the earth. The concept of sounding is based on increasing electrode spacing to measure further into the subsurface.
The Schlumberger method is a surface Geophysical Electrical method that uses a linear array of electrodes to measure resistivity of the earth. Other arrays in use are the Wenner array, dipole-dipole array and pole-dipole or mise a la masse.
THE ELECTRODE ARRAY:
Stainless steel rods (electrodes) are driven a short distance into the ground. In the most common array, the dipole-dipole array, four electrodes are used. The spacing (a) between electrodes may be, for example, 50 feet. The first two electrodes are connected to a power source (sender) that generates a voltage and causes current flow in the ground. The second set of electrodes (receiver) are spaced at incremental intervals (n) equal to distance “a” and connected to a voltmeter in order to measure voltage. Receiver electrodes may be of the porous pot electrode design in order to reduce reaction with soil. With transmitter current, receiver voltage and electrode geometry, apparent resistivity for the dipole-dipole geometry is determined as follows:
Apparent resistivity = ra = (k) * V/I = (p * a * n * (n+1) * (n +2)) * V/I
WELLOG has free calculators for use in calculations for the following methods:
A polarity switching system is used to reverse the current flow on a frequent interval, for example, once every second.
By incrementally increasing the spacing between electrode pairs, progressively deeper resistivity measurements are made. Using n=1, n=2, n=3 spacings between electrode pairs gives an approximate incrementally deeper reading below the surface. Increasing the spacing between electrode pairs in increments of 50 feet provides resistivity from 50 feet incrementally deeper sections. Large, high power systems are used for deep VES. Small, portable, low power VES systems are used in shallow applications. The midpoint between electrode pairs is considered the measurement point.
A related method of survey is Resistivity profiling. Resistivity profiling involves maintaining a fixed separation of electrode pairs and moving along a selected survey line. Survey lines are usually chosen to cross perpendicular to the strike of known mineralization.
A more complete method can be used that provides a profile along a survey line that includes determination of resistivity at various depth. Obtaining data collected from different electrode pair spacings, a resistivity pseudosection is produced of the subsurface. The method is performed in the field by placing stakes at a predetermined interval. The transmitter (sender) is positioned first at two stakes at one end of the survey line, while measurements are made at sequentially more distant receiver pairs. Then the transmitter is advanced to the next pair and the process is repeated.
Calculations can be performed on the data obtained from each point surveyed and a resistivity profile is created. When multiple layers are encountered, inversion software is used to create a two dimensional view. Combining 2D views gives a 3D view of areas having contrasting resistivity.
While performing a Resistivity survey, it is customary collect IP data. Most surveys include Induced Polarization (IP) as Percent Frequency Effect and Metal Conductivity Factor (MCF) using data collected in both time domain and frequency domain measurements. A measurement of Spontaneous potential (SP) is also made.
Complex Resistivity (CR) can be obtained from spectral analysis using frequencies from .1 to 100 cycles per second.
This method has been used to find formation faults, formation bedding, bedrock, water saturated aquifers, mineral deposits, and hydrocarbon formations including Coal.
VES works most favorably in applications having good resistivity contrast between a target and surrounding formations
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