Blast Vibration Basics Post #3

Blast Vibration Measurement

This is the third post in a series on blast vibration basics. Blast vibrations are an important concept to understand because all blasts produce vibrations and, in some cases, those vibrations will annoy neighbors or potentially damage nearby structures. However, in today’s environment, blast vibration damage is extremely rare due to the regulatory limits placed on blast vibrations from blasting and other construction activities.

This post focuses on blast vibration monitoring equipment, capabilities, and how to install the equipment properly. Some of this post might be on the more advanced side for your needs, and if that is the case, you may want to skip to the “Important Specifications” to help you understand how to program your seismograph correctly.

Blast Vibration Monitoring Equipment

Blast vibrations are typically monitored using blasting seismographs, which use geophones to record vibration waveforms. In recent years, manufacturers have also started to offer accelerometers for blast monitoring. This article summarizes how seismographs with geophones and accelerometer-based monitors work. A later post will cover how to set your seismograph or accelerometer up and how microphones for measuring air overpressures or airblast. This post also covers important settings and specifications that you should understand if you are operating a geophone or accelerometer.

Seismographs

Example seismograph package. *(source: https://www.ntx-int.com/prodotti/sismografo-minisupergraph-ii-msg-ii/)*

Seismographs are monitoring units that primarily consist of a data logger and a geophone. The image at the right shows all of the components that are typically included in a seismograph package (clockwise from bottom left: geophone housing with cable, geophone spikes, charger, user guide, microphone with cable, download cable, and microphone spike. center: data logger).

Data loggers are small computers which can be programmed with desired monitoring settings and record the signal that is output from the geophone housing. The data logger saves each record so that the record can be downloaded to a computer and analyzed in a vibration analysis software package.

A blasting seismograph geophone housing typically contains three geophone sensors. The geophone sensors are oriented to record vibrations on one of three axes: vertical, radial or longitudinal (horizontal in the direction of the blast), and transverse (horizontal and perpendicular to the radial direction).

The geophone sensors operate using electromagnetism. When a ground vibration passes a geophone, the geophone housing, which is coupled to the ground, vibrates with the ground. Each geophone houses a cylindrical magnetic weight mounted to the sensor case (see diagram below). The weight is surrounded by a coil of wire on its long axis, which is mounted on springs. When a vibration passes the geophone housing, the housing moves with the ground (as does the magnet), but the weighted coil stays motionless. As the magnet oscillates, the relative difference in the magnet’s movement with the coil creates a current in the coil. The current is proportional to the velocity that the magnet and case are moving. The current is measured by the data logger and converted to units of particle velocity (typically inches per second or millimeters per second).

Geophone diagram (source: Zhou, X, Y. Ruan, X. Mou, Y. Yuan, and Y. He, 2022. “A Design of Electromagnetic Velocity Sensor with High Sensitivity Based on Dual-Magnet Structure” <https://www.mdpi.com/1424-8220/22/18/6925>)

**Analogy**

Geophone sensors use the same concepts that are used when you create a magnet out of a nail, battery, and a bit of wire. If you ever worked on this science project at home, you know that the wire is coiled around the nail and connected to two battery terminals, like the image on the left. When connected to the battery, current will be introduced to the wire. The current in the wire creates a magnetic field, which then polarizes the nail and makes it magnetic.
A geophone sensor operators with the same concepts. The coil moves along a magnetic cylinder. But because the cylinder is magnetic, unlike the nail, the magnet creates a current in the coil. The data logger measures the current and converts it to particle velocity.
*source: http://www.stefanklocek.com/simple-electromagnet/*

Accelerometers

Accelerometers measure particle acceleration rather than particle velocity. Basically, accelerometers consist of a damped mass on a spring. When a vibration passes an accelerometer, the spring is compressed, exerting a force on the damped mass. The spring’s compression is proportional to acceleration (https://en.wikipedia.org/wiki/Accelerometer).

Accelerometers are either capacitive micro-electro-mechanical systems (MEMS) , Piezoresistive, or Piezoelectric. Each of these types of accelerometer has different performance specifications. If you’re interested in learning more details about accelerometer types and specifications, detailed information can be found here: https://blog.endaq.com/accelerometer-selection, https://blog.endaq.com/accelerometer-specifications-decoding-a-datasheet, and on www.pcb.com. Most of the notes on accelerometers in this post are transcribed from these sites.

The image at the right is a diagram of a PCB Piezotronics ICP® Accelerometer. This type of accelerometer is piezoelectric, which is the most widely used for acceleration testing and measurement. A piezoelectric accelerometer uses a sensing crystal attached to a seismic mass. When accelerated, the seismic mass induces stress on the crystal, which results in a proportional electric current output. The current, which is proportional to the acceleration, is measured by a data logger.

MEMS variable capacitive DC accelerometer construction *(source: https://www.pcb.com/resources/technical-information/mems-accelerometers)*

MEMS accelerometers are sensors manufactured using microelectronic fabrication methods. The small mechanical sensor is typically mounted on silicon so it can be placed on a computer chip. MEMS are typically capacitive accelerometers, meaning a they operate based on a seismic mass under acceleration causing capacitance changes (www.pcb.com).

These are the types of sensors used in cell phones and other electronic devices to measure acceleration. They’re inexpensive to manufacture, small, power efficient, and operate well at low frequencies (down to zero hertz). They can also mount in any direction. Some types can also measure direct current, which means they can be calibrated to account for gravity to a certain extent. The downside of MEMS units is that they are still newer technology, some can suffer from poor signal to noise ratio (noisy), and some can have poor resolution.

Which Type of Sensor is Best for Blast Vibration Recording?

Today, most blast vibration monitors in use are blasting seismographs with geophones, but accelerometer technology is advancing quickly and manufacturers are starting to offer more accelerometer-based options. Depending on your application, you may prefer one option over the other. The following are some basic recommendations based on my experience. Your application may vary, so make sure you understand what you’re monitoring, what data you need, and the results you require before you choose.

Geophones

In the United States, geophones are the easiest option to use for compliance monitoring (making sure vibrations don’t damage structures) . In other countries, monitoring requirements and regulations may differ, so look up your own regulations to make sure you understand the monitoring requirements.

For vibration compliance monitoringin the U.S., regulations and monitoring guidelines suggest that the geophone should be buried and placed near a residential structure. Most U.S.-built geophone housings are separate from the data logger, which is useful for this application because there’s no need to bury the logger. All of the regulations and guidelines use limits with units of particle velocity and were built around using geophones. For this reason, most compliance monitoring applications use geophones…but this does not mean accelerometers cannot be used.

Geophones can also be used for troubleshooting and are typically on hand at a lot of mining operations. They’re relatively easy to operate as well.

If you need to convert to displacement (integral of velocity), the particle velocity record from a geophone is one step closer to displacement than acceleration. Less conversion calculations usually mean the final calculation has greater accuracy. But you need high sample rates and bit rates or the vibration will not be converted correctly.

Accelerometers

Accelerometers are becoming more prevalent in the blast vibration monitoring world. They’re becoming less expensive and easier to obtain. Users can be trained quickly on them and they can typically be mounted in any orientation, making them easier to mount in a mining or construction operation for troubleshooting.

In mining operations, it’s difficult to bury a sensor because you’re dealing with solid rock or larger muck where you probably want to place the sensor. In this case, you can mount the sensor to solid rock, but level, solid rock is difficult to find. With accelerometers, you can mount them at any angle, so there’s no need to find a level mounting location. Underground, they can be mounted sideways on the rib or upside down on the roof, which keeps them out of the way of mobile equipment.

Accelerometers are great for troubleshooting. There are many available sensor options, so near field monitoring can be accomplished easier without the unit decoupling (becoming detached or rattling during the vibration event). If you already have a seismograph, you may be able to purchase an accelerometer sensor for your seismograph, so reach out to your manufacturer and ask them about this possibility.

Accelerometers may not need to be calibrated as often as geophones and typically operate at a wider frequency range than geophones. Accelerometer options may have higher sample rates and bit rates than most geophone-based seismographs, so you may have better luck converting to displacement from acceleration.

Either way, both types of units can be used in most applications. Regardless of which type of sensor you pick, the following settings and specifications are important when setting up your sensor.

Important Settings and Specifications

This section covers important performance specifications and values that you may be able to program into a seismograph. This may not cover some specialty monitoring scenarios, but it should cover most standard monitoring requirements that you will come across.

Waveform vs Bargraph Mode

Most seismographs offer waveform mode, bargraph mode, or combination mode (both at the same time). Waveform mode is used to record the entire vibration waveform for a blast event. This is important if you wish to understand the nature of the vibration, the frequencies and amplitudes, troubleshoot your blast, and/or do detailed analysis on your vibration waveform.

Bargraph mode records the peak amplitude value during a certain time period, say 10 minutes. The maximum value at every 10 minute interval for, say, 24 hours is recorded and plotted in a bar chart. The purpose of this mode is to show the maximum vibration values over a long period of time to ensure those values did not exceed a maximum allowable value.

Trigger Level

The trigger level is the input signal value that triggers a seismograph to start recording. If the level is set too low, you may record many false triggers. If the level is set too high, you may miss the vibration event. Today, thanks to the amount of memory available on seismographs, you’re better off setting the trigger level low to make sure you record the event you’re interested in.

Amplitude Range

Blasting seismographs typically operate to +/-127 mm/sec (5 in/sec) or +/-254 mm/sec (10 in/sec). If you have an older seismograph that allows this selection, the lower operating range may allow you to record with more vertical precision because a 12-bit file, for example, with a range of +/-127 mm/sec has the same number of vertical divisions as a 12-bit file with a range of +/-254 mm/sec; therefore, the vertical readings will be twice as accurate for the lower range option.

Sample Rate

Sample rate is the number of digital samples that are taken of an analog signal in one second. The data logger must digitize the input continuous vibration signal, so any recorded signal is digital and made up of sample points. Enough samples are required so that the vibration peaks are not cut off or the reading will be inaccurate (artificially low).

The sample rate for a seismograph is recommended by the International Society of Explosives Engineers (ISEE) to be a minimum of 1000 samples per second. If greater accuracy or detail is required or vibrations have relatively high frequencies, the sample rate can usually be set to a value greater than 1000 samples per second. Personally, I prefer setting the sample rate as high as possible because I usually use vibration monitors for troubleshooting. I can always downsample to reduce the file size later on, but I can never increase the sample rate to show any important missing data.

Bit Rate

The bit rate is similar to sample rate, except it describes the precision of the digital samples vertically with amplitude rather than horizontally with time. Low bit rates mean the vertical precision of the digital vibration record is low, so a vibration may exhibit plateaus in the data. A higher bit rate will be more precise, so those plateaus will be more accurate. Typically, seismographs have operated in the 12-bit and 16-bit ranges, but modern sensors may operate at 20-bit resolution or higher.

With today’s hard drive sizes, data loggers can record more records with higher sample rates and bit rates. There is less need to operate at low values or reduce file sizes because there is less risk of filling a data logger’s hard drive.

Maximum Record Time

Maximum record time is an important factor in determining if a seismograph can monitor the entire blast vibration waveform. Some older seismographs had record length limits to keep vibration file sizes small. Now, most seismographs record up to 20 seconds of data in a single record. This is important, especially underground, because blast times can be up to 20 seconds long. It is important to ensure the entire vibration can be recorded by the seismograph.

*Sample amplitude-versus-frequency calibration response graph (source: 2022 ISEE Performance Specifications for Blasting Seismographs).*

Frequency Response (Range)

The 2022 ISEE Performance Specifications for Blasting Seismographs recommends that a blasting seismograph is accurately measure vibrations that have a frequency content from 2 to 250 Hz.

Manufacturers typically calibrate seismographs once per year. They will mount the seismograph on a shake table, input a vibration at a predetermined amplitude, say 25 mm/sec or 1 in/sec. They will input that amplitude at a range of frequencies. The seismograph must report the same reading as the input amplitude at each frequency value.

For a range of 4 to 125 Hz, the seismograph must report the vibration amplitude to within 5% or +/-0.5 mm/sec (+/-0.02 in/sec) of the input signal, whichever is larger (see image to the left where the two thicker black lines provide the allowable limits for a constant input value). Outside of this range, the seismograph is allowed some variability because geophones have poorer response at low frequencies and high frequencies.

Of note, the range from 2 to 4 Hz and from 125 to 250 Hz requires the response to be within +5% to -3dB (70.7% of the input voltage) of an ideal flat response (see two thicker black lines in the diagram on the left). Geophones have limited response at low frequencies, so they are electronically programmed to operate down to 2 Hz. The response requirement is therefore less restrictive between 2 and 4 Hz and allows some roll off. Geophones also have a reduced response above 125 Hz, so the same leniency in frequency response applies for frequencies over 125 Hz. Usually blast vibrations don’t occur at these out frequency ranges, so the roll off is not a concern for most monitoring applications.

The performance specs are available at isee.org under digital downloads to anyone (members and non-members) (https://isee.org/digital-downloads/740-2022-isee-performance-specifications-for-blasting-seismographs/file).

Acknowledgements

Thanks to Scott Newman with Nomis Seismographs and Stefan Miller with Shottrack for reviewing some of the finer points of this article for correctness!