Drilling & Blasting
Seismograph
Calibration
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What the user should know
he dictionary definition of calibration
is: to determine by measurement or
comparison with a standard, the
correct value for each scale reading on a
device1. Blast consultants and blasting
contractors assess risks every day, but
how often do they consider the
importance of, and the risks associated
with, the calibration of their
seismographs and the accuracy that they
provide? Can the calibration provider
back up the seismograph user in times of
trouble and provide documented
evidence that the equipment was
functioning within specification? Can their
calibration procedures withstand
independent scrutiny and be shown to be
traceable to a national standard for
accuracy of measurement? These are
important documents that, in a court of
law, provide tremendous support in
showing that blasting and monitoring
activities are being conducted
professionally and competently.
Why calibration is
important
The recommended interval for the
calibration of most types of measuring
equipment, including seismographs, is one
year. Across most of North America and
many parts of Europe regulatory
authorities actually require seismographs
to be calibrated on an annual basis.This
requirement has been, or is being,
adopted by several other countries
around the world and some specific
projects even require that seismographs
be calibrated every three or six months.
Calibration is important to ensure that
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the instrument is performing as it was
designed to, and measuring accurately the
true ground vibration and airblast.
Although seismographs are designed to
be used in rugged environments, they are
still a sophisticated electronic monitoring
devices. Preventative maintenance,
therefore, becomes an important part of
the annual calibration process. Many
manufacturers and their authorized
calibration facilities will perform upgrades
and preventative maintenance, often free
of charge, provided the units are regularly
serviced.The upgrades often include
product enhancements of both hardware
and software and result in expansion of
the functionality of the equipment.
How a seismograph is
calibrated
During the calibration process the
geophones are mounted on a shake table
with a reference sensor and excited at a
specific frequency and amplitude. At this
point the sensors are checked for the ‘as
found’ condition of each channel.This
must be performed before any
adjustments are made.These results will
allow the user to assess all of the
vibration records that have been
recorded since the last time the
seismograph was calibrated and
determine the impact, if any, on these
records.The seismograph is then adjusted
to match the level being recorded by the
reference sensor and the geophones are
checked to ensure they are within
specification across the required
frequency range of the equipment.
Microphones are calibrated in a similar
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manner.They are exposed to a sound
source at a specific frequency and sound
pressure level and the ‘as found’
conditions are recorded.The
microphones are then adjusted to match
a reference microphone and, like the
geophones, checked to ensure they meet
the specifications for the required
frequency range.
To ensure compliance, it is very
important that all sensors are tested at
several frequencies within their required
range. For example, if a calibration
service provider uses a device called a
piston phone to calibrate the
microphones, the microphones are only
checked at one frequency and one
pressure level.This frequency is typically
about 250Hz and is at the high end of
most seismographs’ frequency response.
This is not a valid procedure because
single-point calibration does not provide
any indication of the response at other
frequencies.The seismograph may be
reading high, low or nothing at all at
other frequencies within its specified
frequency range.
Some seismographs are meant to have
the geophone, microphone and dataacquisition unit calibrated as a system.
Generally, this allows the entire system to
be calibrated more accurately. However, a
limitation of instruments that are
calibrated in this way is that the
geophone and the microphone are
matched to the acquisition unit and may
not be interchangeable with other
instruments, even if they are of the same
model from the same manufacturer,
without a decrease in overall accuracy.
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Drilling & Blasting
Other seismographs may require the
geophone, microphone and dataacquisition unit to be calibrated as
independent assemblies.This type of
seismograph should maintain its accuracy
when sensors from compatible models
are interchanged.
To help maintain the integrity of the
recorded data, most seismographs have a
sensor-checking function. Some users
confuse the purpose of this function with
that of the yearly calibration.The sensor
check can provide valuable information
about the sensors and their set-up. If a
sensor has not been installed or
connected properly, the sensor-check
function will provide some indication of a
failure. In general, the sensor check will
induce an electrical pulse into the sensor
that will cause the mechanical
components within it to move.The
seismograph in turn measures this
movement, as if it were a true vibration,
and the response is recorded.This
response is then analyzed to make sure
the sensors are operating within an
acceptable range.This provides a very
good indication that the sensors and unit
are working properly, however the sensor
check is not a calibration check and
cannot replace the annual calibration
process.The sensor check does not
compare the measured result against an
external traceable reference sensor, nor
does it test all the electronic circuits that
are integral to the geophone response.
What the calibration
certificate means
When a seismograph is calibrated by an
authorized calibration facility, the facility
will issue a Calibration Certificate.This
provides a list of reference equipment
used in the calibration process. In order
for the calibration to have any validity, the
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reference equipment must be more
accurate than the equipment being
calibrated, and must itself be calibrated by
equipment of even greater accuracy
which is traceable to a nationally
recognized standard, such as the National
Institute of Standards and Technology
(NIST) in the United States. Needless to
say, the agency performing the calibration
must be qualified to do so.The
Calibration Certificate should also
contain details of the model, serial
number, date the instrument was
calibrated and who performed the
calibration. Should an authority question
a user’s seismograph records, this
certificate provides documentary proof of
professional calibration.
Importance of
manufacturer-certified
calibration
There are individuals and companies
worldwide who are not certified by the
manufacturer to perform seismograph
calibration, yet continue to do so. It is in
a user’s best interests to have their
seismograph calibrated by the original
manufacturer or an authorized agent.
The key factor here is the integrity of
the measured record should it ever be
called as evidence in a court of law. Using
a manufacturer-certified facility ensures
that proper procedures are followed
when the seismograph is calibrated.This
removes any uncertainty associated with
the reliability and measurement accuracy
of the seismograph.The use of
unauthorized calibration facilities may
cause problems in court if the reference
equipment is not traceable, or if the
certification documents are incomplete.
In some instances, unauthorized facilities
have even been known to calibrate
equipment without ever updating the
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calibration date within the instrument. If
an improper calibration date is printed
on vibration records, it may call into
question the validity of the report itself.
Moreover, if a seismograph ever
requires repair, the manufacturer has the
best experience and knowledge to find
and fix the problem.They will also ensure
that any replacement parts meet the
specifications for the equipment.
Unauthorized facilities do not have access
to test specifications, procedures or parts
lists and some of these facilities may use
substitute parts that do not meet the
functional requirements, which can lead
to inaccurate vibration levels being
reported.
Calibration service providers should
always:
—be authorized and trained by the
manufacturer
—have reference equipment calibrated
to a traceable standard
—provide copies of the Calibration
Certificates for reference equipment
—record the ‘as found’ condition as
part of the calibration procedure
—test the microphone and geophone
sensors at multiple points within
their stated frequency range.
Conclusion
When a seismograph is sent for its
annual calibration the user should
remember to manage the associated risk.
The factors listed above will help to
lower this risk and provide assurances to
the regulatory authority that vibration
levels are being proactively monitored. T
Reference
1. Dictionary of Scientific and Technical
Terms, 5th Edition, McGraw-Hill.
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