| GEO-GRAF |
send e-mail
|
| What Is GPR? -
Geophysical Equipment Descriptions - Radio
Frequency |
|
| |
| |
| RF techniques
are successfully utilized by Geo-Graf to
locate and “trace” most types of
metallic buried utilities such as water pipes,
gas mains, electric cabling, reinforced concrete
sewers, grounding cables, fuel lines and other
piping associated with USTs and ASTs. |
|
| |
| The
basic RF instrument will consist of separate transmitting
and receiving units. The RF device utilized by Geo-Graf
has a transmitting unit that generates a wave at a
frequency of about 83 KHZ. This wave readily
penetrates the surface and subsurface. When the
transmitter is in proximity of a subsurface conductive
object, the wave signal will be conducted along the
length of the object. When the receiver is an appropriate
distance away from the transmitter, it is positioned to
detect and process any similar signals emanating from the
ground. Thus, when the transmitting signal is
applied to an electrically continuous buried pipe or
cable, the signal will be conducted along the length of
the utility, enabling the receiver to detect and locate
the exact position of the center of the pipe. With
the transmitting unit remaining stationary, an operator
can walk with the receiver, sometimes noting the position
of the utility up to several hundred feet away from the
transmitter. This is referred to as “tracing”
or “marking out” the utility. |
 |
| |
| The
ability to “mark out” a utility is based on the
soil parameters, the conductivity of material comprising
the utility, and the ability to place a signal onto the
utility. For conductivity, the pipe/cable must be
metallic or have metallic reinforcing and must be
electrically continuous. This means that any broken or
separated pipe segments, loose joints or large valve
systems will hinder the ability and distance the signal
can travel along the pipe. However, sometimes it is
difficult to get the signal onto the pipe. The
easiest way is through direct pipe contact via a surface
valve, meter or the pipe itself. Often these are
not accessible in the field, but the signal can also be
induced onto the pipe. The transmitter can be
placed on the ground above a suspected pipe. The
signal will be transmitted through the ground to the
utility, enabling the receiver to detect it. In
most situations “blind tracing” is done.
This is where an operator of the transmitter and an
operator of the receiver walk over an area in
anticipation of transmitting onto a utility, and
subsequently receiving a signal from the ground.
This search method is advantageous with respect to
locating abandoned or “forgotten” utilities and
“surprise” utilities. |
| |
| Once
the pipe is located, often an approximate depth can be
obtained. In the case of a direct pipe contact, a
very accurate depth measurement is obtained.
However, in inducing a signal onto the pipe, the signal
strength is somewhat degraded because of the different
soil layers, and because the signal is less focused than
when it is directly conducted onto the pipe. As a result,
a depth range can be obtained with a larger margin of
error. |
| |
|