Blast Overpressure Versus Noise

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People often confuse blast overpressure and noise. An over-simplistic comparison is that noise is what the human ear can hear. Blast overpressure, however, is often below the range of human hearing.

The typical range of human hearing is from 20 Hz to 20 KHz. This range can of course vary significantly between individuals. Blast overpressure may often be very low in frequency, 2 Hz or less. By definition, blast overpressure is the pressure generated by a blast that is over and above atmospheric pressure.

Although blast overpressure is often below the range of human hearing, it can cause structural response that is quite noticeable to those inside a structure. It is not possible to gauge the amount of blast overpressure by what is heard outside. How many times have you been outside near a blast, heard virtually nothing, and yet still received a complaint call that was overpressure related?

There are generally five sources of blast-generated overpressure (from the ISEE Blasters’ Handbook 18th Edition):

Air Pressure Pulse – Low frequency pressure caused by rock displacement at the face (piston-like movement or bulking of the rock mass).
Gas Release Pulse – High frequency pressure caused by gases venting through the face.
Stemming Release Pulse – High frequency pressure caused by gases venting through the stemming.
Rock Pressure Pulse – Typically insignificant air pressure generated by the ground vibration.
Noise – High frequency energy from detonating cord or surface delays.

 

Taken from White Seismology’s newsletter

Written by Randy Wheeler, President of White Industrial Seismology

 

Effect of Blasting Air Overpressure on Residential Structures

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A Summary of U.S. Bureau of Mines Research

When explosives are used to break rock in a mine or construction project, the blast produces both ground vibration and air overpressure (noise).  In most cases the atmosphere selectively absorbs the higher frequencies from a blast, leaving relatively low Read more

A Neighbor’s Guide to Understanding the Effects From Nearby Blasting Operations

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Let’s address some issues that come up frequently in conversations with neighbors concerned with the effects of blasting at nearby mining operations. San Antonio and our neighboring cities along the I-35 corridor lie on the Balcones fault line, a major limestone deposit which provides limestone materials to communities lying to the east, including major cities like Houston and Corpus Christi. Quarries in this area mine the rock, crush it into various sizes and send it to areas that cannot mine it themselves locally.

The vibrations produced by quarry blasting have been a source of concern and frustration for neighboring communities as long as people have occupied homes or businesses near these active aggregate producing operations. In order to ensure that the environmental impact from blasting on neighbors and businesses is not negative, instruments called seismographs are set in the field to record the intensity of energy that is felt wherever the instrument is located. Using data from these seismographs, or seismometers, we have learned much about the effects from blasting.

It is important to begin by explaining that what people living near a quarry may feel is a combination of ground vibration and air over pressure. In a perfect world, 100% of the energy produced by explosives loaded into the ground would go in to breaking the rock. If that were the case, no energy would be felt by anyone immediately outside the quarry. However, this is not possible. That being said, a well executed blast uses as much energy as possible in the fracturing of the rock, and leaves very little to escape into the surrounding environment. It is this escaping energy that is the topic of much neighborhood conversation and concern.

Energy that isn’t used for breaking rock travels either through the remaining rock, or through the air. A seismograph records the intensity of escaping energy using a microphone to measure changes in air overpressure (that is, over normal atmospheric pressure), and a transducer to measure ground vibration.

Escaping energy from a blast that travels through the air produces a temporary increase in air pressure much like a clap of thunder or a jet engine from aircraft traveling overhead. This increase in air pressure, called air overpressure, is measured in decibals. Air overpressure travels in a wave form and much like a wind, pushes on anything in its path. However, this pulse comes and goes much more quickly than a gust of wind. It is this wave that is “caught” temporarily by surfaces in its path, like the sides of structures, before it is quickly released. Air pressure can be an annoyance even at low levels and once it reaches very high levels, can produce the potential for damage to structures. The criteria for safe blasting levels of air overpressure have been established and are well published after extensive testing and research by the United States Bureau of Mines. Air overpressure is produced where energy escapes through fractured rock and primarily travels in the direction that the rock being blasted moves. In this image below you can see the movement of rock at detonation. Air pressure increases in proportion to the amount of energy released between the fractured and moving pieces of rock. Therefore, changes in air overpressure are more discernable along this path and can sometimes be perceived miles away. There are several factors that make controlling and predicting air overpressure more difficult than ground vibration. Some of these factors include atmospheric conditions that change constantly, such as wind speed and direction, or thermoclines. These invisible thermoclines separate air with different temperatures or air traveling at different speeds. Because of this, it is universally considered optimum conditions for blasting when there is a clear cloudless sky with no wind. However, weather conditions can change very quickly and conditions that were perfect only moments before, can degrade, resulting in undesirable changes in air pressure for neighbors.

Ground vibration is produced by energy escaping through the remaining solid rock, so it tends to be more discernable behind the blast. Unlike air overpressure, the intensity of ground vibration tends to be more predictable since it travels through a more solid medium.

The human body is a very sensitive seismograph, but many people are confused by what they feel, misjudging air overpressure to be vibration, and vice versa. Because energy travels through the ground more quickly than it does through the air, seismographs and neighbors alike will perceive the vibration before the air overpressure. The greater the distance from the blast, the larger the gap in time between the arrival of the two. That is why some neighbors correctly describe feeling “two blasts”. They first perceive the ground moving, then the air moving.

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Neighbors who are unfamiliar with these dynamics of blasting cannot understand why what they feel tends to be different from one blast to another. It is not uncommon for a neighbor to feel very little on one blast, and then find another blast significantly more intense. The most obvious conclusion that can be reached is that a larger amount of explosive energy was used whenever the blasts are more perceptible. In fact, the amount of explosive energy used by rock producers is tightly controlled and selected as a result of much research. Seismic data collected from each blast is analyzed, and offers valuable information to ensure that the impact on neighbors is minimized. Despite this, factors like the weather and the orientation of the blast to neighbors, make predicting the effects difficult. That’s why conscientious operations have a blast monitoring program which offers them immediate feedback on every blast and protects them from overlooking variables that can produce undesirable results.

J.R. Heck
CEO, Firmatek Seismic, LLC

Stockpile Time Machine

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This video was produced to help a Firmatek client see the changes in her inventories between measurements. With unexpected discrepancies in her accounting of a specific product, this video and the accompanying inspection by our technicians was all she needed to accept Firmatek’s measurements as the truth, make the necessary adjustments to the books and verify the true source of the discrepancy. Our client was armed with the truth.

Our Mission: Earn trust. Pursue truth. Arm the client.

Blast Perception 101

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Over the years Firmatek Seismic has monitored thousands of quarry blasts, had many conversations with property owners, and drawn many a pictures to describe what occurs when a blast takes place.  I thought it might be helpful to post this simple video to help you understand what you may be feeling when your local quarry blasts.

Compaction and Airspace: The Keys to Landfill Profitability

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The business of landfills is air, selling empty space to be filled with solid waste, and in this industry not all air is equal. Landfills have the unique ability of being able to modify the lifespan of their product without changing the amount of product they started with.  It all comes down to compaction. Volume may be fixed, but density is not, and the more solid waste you can fit into a given volume, the more valuable that space is. That is why it is absolutely necessary to have an accurate and reliable way of tracking the change in the volume of solid waste before and after compaction. Read more

Case Study: 3D Mobile Scanning used in Reserve Calculation and Mineable Modeling

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“He inquired of Firmatek, LLC to see if there was a way to quantify his reserves and help him establish a profitable plan for excavating and updating the operation as the new mine progressed.”

In April of 2010 Firmatek performed services for a client who’d purchased a piece of property with the intention of mining it out.  By drilling core samples throughout the 500+ acre property Read more

3D Laser Mobile Mapping and the X-Ray Machine

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The history of man is described in relationship to events that alter its course.  Terms like “pre-Civil War”, “B.C.” or “before the Wright brothers” are used like a thumb tack in time to give context and understanding to a story.  These turning points are often moments in time when the ingenuity of man paves the way for a revolution in thought and industry.  One such example is the medical field before and after the intro Read more

How Can I View My Point Cloud Data and 3D Models?

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With the recent increase in the use of three-dimensional laser scanners to capture real world conditions, many forward-thinking operators are searching for ways to view, use and manipulate the resulting data.  Read more