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Blast Vibration and Seismograph Section
Accuracy of Blasting Seismographs
By Bob Turnbull
The International Society of Explosives Engineers
(ISEE), at its 1995 annual conference, established a
Seismograph Standards Working Group to develop performance standards for blast vibration seismographs.
This group was transformed into the Blast Vibration and
Seismograph Section in 1997 with 5 working committees
one of which was the Seismograph Standards
Committee. Standards are essential to ensure that vibration records from different seismographs would report
the same vibration levels within a stated tolerance. In
2000, the ISEE through the work of the Blast Vibration
and Seismograph Section published the “Performance
Specifications for Blasting Seismographs.” This document is available on the ISEE web site at
www.isee.org/sections/blast.htm.
Why is this important?
While there are several factors that can affect the
vibration records taken from different seismographs, the
performance specification is an important first step
towards ensuring consistency in the recorded values
from different seismographs. This is true for seismographs of the same model type or seismographs from
different manufacturers.
This specification addresses several key issues such
as frequency response, accuracy, sample rate and the
density of the geophone. Even though the specification
does not address the issue of phase and cross-talk
response, adhering to it will help to produce similar
results from different seismographs. It is generally
accepted that the majority of the vibrations due to blasting are in the frequency range of 4 to 125 Hertz (Hz).
The performance specification addresses this by maintaining a tighter tolerance within this frequency range.
Figure 1
Figure 2
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According to the performance specification the seismograph must meet the following criteria:
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The Journal of Explosives Engineering
March/April 2006
34783 ISEE
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Figure 3.
Figure 4.
To evaluate conformance to this performance specification, each individual sensor of the seismograph
should be exposed to a vibration or sound pressure
source that covers a multitude of frequencies throughout
the response range. The resulting amplitude response
from each channel can then be listed in a table or, if
enough points are tested, plotted on a graph. If the
graph includes the ISEE accuracy guidelines, anyone can
quickly evaluate conformance to the specification. A
sample of these graphs for a ground and an air overpressure channel are shown in Figure 3 and Figure 4
respectively.
Why is it important to compare each sensor to the
specification?
Depending on the tolerance of the individual components used in the seismograph the actual value of
these components will be different. With potentially
hundreds of components being used in each seismograph this tolerance could have a significant affect.
Additionally, fully testing each sensor ensures that a
component of proper value is in the correct location
within the circuit. The compliance response graph is
one way to ensure that your particular seismograph will
meet the performance specification set out by the ISEE.
The only method of ensuring that your specific seismograph meets this ISEE guideline is to compare the
results of each individual sensor to the performance
specification. The ISEE has taken the first and most
important step in establishing guidelines to provide
some consistency to the vibration recording equipment
used to monitor vibration produced from blasting. The
phase and cross-talk response criteria still need to be
considered to fully address the performance of a blasting seismograph.
Bob Turnbull is a technical services manager at Instantel.
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March/April 2006
The Journal of Explosives Engineering
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