Blasting
Units of Measurement
Ground Vibration
The movement of any particle in the ground can be described in three ways; displacement, velocity and acceleration. Velocity transducers (geophones) produce a voltage which is proportional the velocity of movement, and can be easily measured and recorded. They are robust and relatively inexpensive and so are most frequently used for monitoring. BS7385 12 gives no preference as to which is used, but does indicate that the frequency response of the transducer should be considered, especially when its natural frequency is close to the frequencies being recorded. Fortunately, it has been shown in many studies, most notably by USBM13, that it is velocity which is most closely related to the onset of damage, and so it is velocity which is almost always measured. If necessary, the velocity recording can be converted to obtain displacement or acceleration. The velocity of ground vibration (particle velocity) is usually measured in millimetres/second (mm/s) or inches/second in the US.
It should be remembered that particle velocity is NOT the same as the velocity of the wave through the ground; often referred to as the seismic velocity. This is orders of magnitude faster, ranging from 500 to 6,000 metres/second (m/s) depending on the type and quality of rock. The difference is clear when visualising a cork moving up and down in waves which are moving forward in the sea.
The ground vibration records in Figure 3 show the variation of velocity with time. Each trace has a point where the amplitude is the greatest (either positive or negative) and this is known as the Peak Particle Velocity (PPV) which has units of mm/s. Geophones are only able to respond to vibration in one orientation and so to capture the complete signal it is necessary to have three geophones arranged orthogonally (at right angles). One will be vertical with the other two horizontal, but the horizontal geophones can either be aligned with the cardinal points of the compass or they can be arranged with reference to the blast position. In the latter case, one geophone would be set along the line from blast to monitor (longitudinal or radial) and the other would be perpendicular to this (transverse).
When referring to a Resultant PPV care needs to be taken to ensure it is the true Resultant PPV, calculated by producing a vector sum of the 3 separate directional recordings (√(v2+l2+t2) where v=vertical, l=longitudinal, t=transverse) for every point on the recording. Historically, the maximum of each of the three orthogonal recordings have been used, but these may not all occur at the same instant in time, so should be referred to as the Psuedo Resultant.
Unfortunately, there is no specific guidance on which parameters should be used. Some regulators set limits in terms of resultant PPV while others set a limit in terms of a single plane.
The other important aspect of ground vibration is the frequency content, as it is known that the levels of vibration which can cause damage vary with the frequency of the vibration. Figure 1 in the Introduction shows clearly the two different sections of a ground vibration signal recorded on the foundations. While the vibration record contains a broad range of frequencies throughout its length, the first part clearly has more of the higher frequencies (shorter wavelength) than the second part. The frequency content will be determined by similar factors to the PPV, but in addition, the detonator delay interval is known to affect it.
As a ground vibration signal has a broad sweep of frequencies, it is difficult to come up with a single value which characterises the signal. Various methods have been used438, but many of these are inaccurate. The most reliable method is to convert the time signal into a frequency using a routine called FFT (Fast Fourier Transform) and then selecting the frequency (in hertz – Hz) which has the highest amplitude.
Figure 3. Three orthogonal records (V, L & T) for a blast recording
Air Overpressure
General overviews of air overpressure are given in the Themed Review L0089 (pp19-21) and by Singh430. It can be measured in any unit which measures pressure. The commonest are Pascals (Pa), which is a linear scale, and decibels (dB), which is a logarithmic scale using the ratio of the recorded pressure to a reference pressure. Unlike noise measurements, there is no weighting applied to the value in decibels and so the unit is sometimes given as dB(Lin) although this is often abbreviated to dB. Strictly, it should also give the frequency above which it is linear (e.g. 2Hz).
It is important not to confuse dB(Lin) with dB(A) which indicates the weighting which is applied when monitoring for noise. The values obtained when measuring dB(Lin) will nearly always be higher than when measuring for dB(A). For example the air overpressure from a blast may be around 95dB(Lin), but the noise may only be 60dB(A).
Continued with Potential Effects TOP Return to Introduction