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:
[Dipole-dipole]
[Pole-dipole]
[Wenner]
[Schlumberger]
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.
RESISTIVITY
PROFILING:
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.
RESISTIVITY
PSEUDOSECTION:
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.
APPLICATIONS:
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
Ask WELLOG
about Software to perform 1D inversion.
If you are interested in applying VES to find Water, Mineral Deposits,
Environmental contaminants…
Contact info@wellog.com
for more information on VES.
