Geology Reference
In-Depth Information
6.2.5 Receivers and detectors
The instruments that measure voltage in DC and IP surveys are known as
receivers
. The primary design requirement is for negligible current to be
drawn from the ground. High-sensitivity moving-coil instruments and po-
tentiometric (voltage balancing) circuits were originally used but have now
been replaced by units based on field-effect transistors (FETs). Instruments
designed exclusively for low-power DC surveys may have transmitter and
receiver integrated into a single housing, and readings are then displayed
directly in ohms. Separate units are almost inevitable where high power,
from large battery packs or motor generators, is required, and in IP sur-
veys, where direct receiver-transmitter interaction must be avoided at all
costs.
To allow noise levels to be assessed and SP surveys to be carried out,
most receivers allow voltages to be measured even when no current is being
supplied. Voltage ranges, numbers of cycles (representing a compromise
between speed of coverage and good signal-to-noise ratio) and read-out
formats may have to be specified via front-panel key-pads or switches.
Usually a displayed reading is updated as each cycle is completed, and the
measurement should be accepted only after this has stabilised.
Error conditions, such as low current, low voltage and incorrect or missing
connections, may be indicated on displays by numerical codes that are
meaningless without the handbook. If all else fails, read it.
6.2.6 Noise in electrical surveys
Since electrodes can, in principle, be positioned on the ground surface to any
desired degree of accuracy (although errors are always possible and become
more likely as separations increase), and since most modern instruments
provide current at one of a number of preset levels so that fluctuations
in supply are generally small and unimportant, noise enters the apparent
resistivity calculations almost entirely via the voltage measurements. One
important factor is voltmeter sensitivity, but there may also be noise from
natural voltages, which can vary with time and so be incompletely cancelled
by reversing the current flow and averaging, and from induction in the
cables. Large separations and long cables should be avoided if possible, but
the most effective method of improving a signal/noise ratio is to increase
the signal. Modern instruments often provide observers with direct readings
of
V
/
I
, measured in ohms, and so tend to conceal voltage magnitudes.
Small ohm values indicate small voltages, but current levels also have to
be taken into account. There are physical limits to the amount of current
any given instrument can supply, and it may be necessary to choose arrays
with geometric factors that imply large voltages for a given current flow.
