Seismographs can be traced back many centuries to the Chinese and Egyptians using devices that would allow marbles to be rolled or cause objects to fall to determine levels of vibration.Many advances have taken place, particularly in the past twenty years, in blasting seismographs. It was not long ago that blast monitoring equipment was based on analog technology that was traditionally used for monitoring earthquakes. Blast records were recorded on magnetic tape, film or shown by marks on pressure sensitive chart paper.
A blasting seismograph is used to monitor, record, analyze, display and print ground vibration (motion) and airblast resulting from a blast event. It is used to measure the seismic wave of energy transferred from one point to another.
Seismographs today are compact, light weight and battery operated for extended field use. Seismographs perform self-tests for checking operation and calibration. Many systems incorporate very detailed computer vibration analysis software.
A standard seismograph utilizes four channels for signals from four sensors; three for ground motion and one for sound (air pressure).
The data from blast events is represented as peak measurements of ground motion and airblast readings. These readings may be displayed on a liquid crystal display (LCD), or printed on paper either in the field with a printing seismograph, or connected to an external printer or computer printer for printing away from the blast site. Modern seismographs also show these readings graphically in a printed waveform. Many seismographs also have the ability to store multiple events in memory for future use or to transfer to a computer for storage, analysis and printing.
The data typically shows peak particle velocity (the speed that a particle moves per unit of time), peak displacement (the distance particles are moved by the seismic wave), peak acceleration (rate of change in velocity per unit of time), and the frequency of the movement (the number of cycles per second (hertz) that the particles vibrate). Other vibration analysis information may be included such as peak vector sum, vibration frequency analysis and comparisons to standards such as OSM, USBM, DIN and other world standards. Each regulatory body may require different standards and limits for blasting depending on the type of structures and the potential for damage.
While legal and regulatory considerations have provided the largest impetus to the development of blast monitoring seismographs, today the technology is increasingly utilized to verify blast design, drilling patterns, sequencing, etc. An increasing area in the use of blasting seismographs is near field monitoring. Sensors are placed very close to the blast which allows the recording of information that can be used to analyze the blast design and results to improve overall performance.
Advances in blasting seismographs closely parallels the advances made in microprocessors and personal computers. Seismographs today use the latest microprocessor technology resulting in the development of the digital seismograph.
Analog signals received form the vibration sensors are converted to digital data, processed and either displayed, printed or stored in non-volatile solid state memory. This means that the blast events are not lost even if the batteries are completely discharged. This data is constantly being compared to the trigger level set by the operator of the seismograph. When the trigger level is exceeded the seismograph collects and records the data for the selected record time (set by the operator in seconds) plus a half second of pre-trigger record time.
Sophisticated software programs are available that not only perform basic analytical functions and report printing for compliance monitoring, but also integrate and differentiate, and perform filtering to eliminate unwanted frequencies.
The following components are attached to a seismograph for collecting data:
VIBRATION SENSOR (Geophone)
A vibration sensor (geophone) typically has three sensors that measure movement in three directions - vertical, radial and transverse. Peak particle velocity is measured in inches/second or millimeters/second.
A vibration sensor has an internal weight that when moved creates a voltage. This analog signal is processed by the seismograph.
A microphone is used to measure airblast. Airblast is a temporary pressure pulse above the atmospheric pressure level. Maximum sound pressure level (pspl) is measured in pounds per square inch (psi) or Pascal (pa) and converted to decibels (dB).