INST/MAN/V9000/06/23.02.21 Page 5
TRANSDUCER MOUNTING
One of the most critical aspects of vibration monitoring is the placement
and coupling of the measurement transducer. The accuracy of any seismic
reading is only as good as the quality of the connection between the
transducer and the ground surface.
This connection, known as coupling, is vitally important in obtaining valid
results. While it may seem that placing a sensor on firm ground, and even
spiking it into the ground, should provide good coupling, often it does not.
In order to be sure that the seismograph is faithfully recording the actual
motion of the ground, the link between the ground and the sensor must be
no less than perfect.
The potential for decoupling becomes greater with high particle velocities,
associated with high-frequency vibration, i.e. high accelerations. Thus, the
problem of decoupling is most often seen when recordings are made at
short distances. This is when the highest frequencies are present, as well
as the highest particle velocities.
At acceleration levels greater than 0.2g the transducer should be buried
completely when the measurement surface consists of soil. When done in
a careful manner, this is probably the ultimate way of assuring that perfect
coupling with the ground is obtained. However, if not done properly, this
method can also lead to erroneous readings.
A sandbag can also be effective if used properly, being loosely packed and
large enough so that, when placed on the sensor, all sides of the bag are
resting firmly on the ground surface. When measuring on a firm surface,
attachment can also be by means of resin or bolting. When recording
within a building, bolting or high modulus resin should be used on vertical
surfaces.
Measurements on floors having compliant coverings should be avoided.
British Standard guide 7385: 1990 discusses the mounting of transducers
located within buildings in greater detail. The same document discusses
monitoring being undertaken at different levels within a building to identify
dissipation or magnification as well as simultaneous monitoring on the
building foundation and ground outside to establish transfer functions.
