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Vibration Monitoring
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Vibration monitoring is the measurement and recording of passing movements in the ground or in a structure, using a seismograph like the one shown at left, or other device, to detect and record the movement. It is the most commonly-utilized method for identifying potential vibration damage problems pre-construction, eliminating or minimizing them during construction, and addressing them post-construction. Properly done, vibration monitoring can be extremely helpful in preventing damage to structures, understanding the nature of damage, and ascertaining its cause. Improperly done, as shown in the construction monitoring photo above left, it can be worse than worthless. Since construction companies will sometimes, albeit rarely, present copies of vibration monitoring data or reports to those who have damage claims, this chapter of the CVDG will offer an overview of vibration monitoring: how it is done, what can be learned from it, and some of the critical errors in execution and interpretation that can affect the scientific validity of vibration monitoring data and conclusions based on them. Vibration Monitoring Requirements and Responsibilities Most U.S. Federally-funded construction projects require pre-construction vibration assessment. Individual states and municipalities often require monitoring in the construction contract or in the information for bidders on the project contract. It is often not carried out, in spite of any requirement. Some contracting entities may alternatively specify no monitoring or monitoring only in connection with blasting done during the project (see Vibration Regulation for more on this). Some common construction operations (pile driving, vibratory compaction, some ground impacts from demolition operations, movements of tracked equipment, blasting) are known to exceed construction vibration standards in some circumstances or to have special characteristics which make them particularly dangerous from a vibration damage standpoint. In that light, properly-done vibration monitoring should be considered a necessary precaution and a responsibility in any project involving performance of any of those types of operations in the vicinity of structures, whether that monitoring is required by contract or not. Vibration monitoring should be initiated when the project work starts, not when the first damage report is received, since the existence of one damage claim often means that others will come shortly thereafter. Construction vibration damage is sufficiently widespread throughout the world (see Is Damage Possible? and Damage Prevalence in the Pro version of the CVDG), that, even though nearby construction work doesn't always cause damage, one cannot reasonably maintain (or support scientifically) an argument that "construction can't cause damage". Vibration Monitoring Instrumentation The device most often used for measuring ground vibration properties is the seismograph. Several manufacturers make ground vibration monitoring seismographs, primarily for use in monitoring vibrations from surface mine blasting. For that reason, such instruments are usually referred to as "blasting seismographs". They are also used in (and sold for) construction vibration monitoring. Like earthquake seismographs, blasting seismographs detect and measure ground vibration by the movement of a magnet suspended in a surrounding coil of wire. According to the Lenz Law of physics, an electrical current is created in the surrounding coil in proportion to speed of movement of the magnet with respect to the coil (i.e. the speed of ground movement, quoted in inches per second or millimeters per second). Electronics in the monitor measure this current, convert it to ground motion velocities and store the raw velocity data in the instrument memory. Each of the three directions at right angles to each other (longitudinal or radial, transverse, and vertical) has its own separate measuring coil in the seismograph transducer head, since vibrations often differ in important ways along different measurement directions. For accurate measurements to be obtained, the seismograph measuring head ("transducer") must move as the ground moves, i.e. it must have full "ground coupling" through effective anchoring to the ground.[15]
This photo should not be taken as showing a proper installation of the seismograph. There are numerous problems in this "professional" installation which contradict the seismograph Operator's Manual instructions and other guidelines set by industry and government groups. Sadly, such poorly carried out vibration monitoring is far too common in "studies" done in connection with construction work, especially when the "studies" are motivated by damage reports. Proper Installation and Use of Seismographs Seismograph data can become significantly degraded in value or even meaningless if improperly acquired or if the seismograph is, seemingly at least, intentionally set up to avoid detecting vibration events which violate standards. Seismograph installation should be done in accordance with operator's manual instructions and industry standards.[10] While there is a substantial list of easily-observed elements involved in a proper and meaningful seismograph installation,[9],[10] some "errors" in installation are particularly detrimental to acquisition of meaningful data. In one "professional" vibration monitoring example, the seismograph "trigger level" (that velocity of vibration above which the seismograph is caused to record a vibration event) was set so high (in some cases above 0.5 in/sec - ten times the recommended value)[10] that few events were recorded, even though the bridge expansion pile driving operation that was being monitored was routinely violating some or all U.S. construction vibration standards at the seismograph sites. There was at least one home damaged in connection with this work.
These and other kinds of gross "errors" in seismograph installation and use, while not present in every or even, necessarily, a majority of examples, occur often enough that it is wise to document every seismograph installation by photo or video and to check every bit of seismograph data obtained for evidence of such "mistakes". Since properly installing the seismograph and running it in accordance with operator's manual instructions and industry recommendations takes no more time or money than ignoring such guidelines, there is no justifiable scientific reason why such incompetencies should occur. For more on this topic, see the CVDG Pro's Vibration Data Analysis and Vibration Data Issues sections. Memory Limitations of Some Blasting Seismographs When installed properly, blasting seismographs are capable of providing accurate recordings of ground movement in blasting settings. However, many standard blasting seismographs are not ideally suited for monitoring construction vibration, due to limited internal vibration event memory. The seismographs shown in the photos in the CVDG, in their standard configuration used, have a specified event capacity of "Up to 13 one-second events (1024 sample rate, four channels recording)"[1] in the Histogram Combo mode, employed for all recording on the projects in which the seismographs were utilized. The capacity for events was substantially lowered from this number in one job, because the seismograph was set to record for five seconds, not one. Although the normal seismograph memory is usually more than sufficient for most blasting settings, construction work can, and did, generate far more vibrations
than memory could hold.
In that road reconstruction project, vibration events filled seismograph event memory hours before the end of the work day on 23 of 29 days for which seismograph data were produced (of 39 days monitored).[8] For the partial data produced in that job, waveform events were available (although not all produced) for 297 events; insufficient event memory caused the loss of well over 1500 waveforms, as shown by ASCII export of the data for those histograms which were not "lost" by the vibration monitoring firm. Insufficient event memory means that, at best, very limited data will be recorded for many of the strongest construction vibrations, severely limiting the ability to draw meaningful scientific conclusions from such data and crippling the estimation of damage potential, even when the data are not "lost" by the vibration monitoring contractor. While some of the latest blasting seismographs have greater event memory that may be sufficient in some construction situations, most earlier generation seismographs in use today have the memory capacity limitation. Considering that, in the road reconstruction mentioned several times in the CVDG, some days had over 400 seismograph-detected "events", even many of the modern seismographs will lack enough memory to record all construction vibrations. These seismograph memory limitations can be so critical in construction vibration settings, and so prejudicial in legal proceedings, that their impact is discussed in more detail on a separate page in the CVDG Professional Edition, Seismograph Memory Limitations. Other Ground Vibration Measurement Instruments Other kinds of instrumentation can also be used to detect and measure ground vibration and its effects on structures. Vibration or its effects can be monitored using seismographs, accelerometers, displacement gauges, strain gauges, and inclinometers. Seismographs and accelerometers measure properties of the vibration, while the others characterize effects of vibration on structures. The use of these devices and what they measure is discussed in more detail in the CVDG Pro chapter, Vibration Measures. They provide different, but complementary, information to that provided by seismographs. While these other equipment types are valuable and used in scientific studies, they are rarely seen in construction or blasting site vibration monitoring nor are their directly-measured properties considered the best indicators of damage potential. The accelerometer sensor technology of cell phones and tablet computers, along with development of several software programs to access the sensors, has made it possible for homeowners and others to carry out "do-it-yourself" basic vibration monitoring at very little additional cost and with minimal investment of time. The topics mentioned in this paragraph are discussed in more detail below. Handling of Vibration Data
The waveform event report (as shown
just above at right) shows the
detailed characteristics of an individual detected vibration whose PPV is above the "trigger level" set for the instrument. Vibrations having PPV's below the operator-set trigger level are not recorded as detailed
events. Vibration traces for all three
directions appear in the report. T The histogram event report (see in the next heading below for an example) provides a graphical depiction of the general vibration levels during the entire run of the seismograph, as well as indications of the largest vibrations detected during the run. The printed histogram does not depict all the histogram data taken by the seismograph. It only shows data at those intervals which can be fitted on the printed sheet. One must export the histogram data to ASCII or other compatible format, then examine them in a spreadsheet or database program to see the details of all the vibrations. Reading Vibration Monitoring Results Construction companies and the vibration monitoring subcontractors they hire
have a vested monetary interest in concluding that vibrations are non-damaging to structures.
Some of those obtaining or using vibration data may also be ill-prepared to understand the real meaning of vibration monitoring
results (e.g. histogram at left). Some of the problems you should look for are detailed in the CVDG Pro Vibration Data Issues chapter. Extensive additional information on vibration monitoring, report reading and analysis, interpretation and standards can also be found in the CVDG Pro. Analysis and Interpretation of Vibration Monitoring Data Proper
and careful analysis of vibration data is laborious and time-consuming, but essential.
At right are shown just four of the over 50 pages of tables which I prepared, relating
to the detailed analysis of the partial set of produced vibration data from one road reconstruction
project. Each table looks at the same data in different ways and provides a view
and analysis of different elements of the data. For meaningful conclusions to be drawn from vibration data, they have to be properly obtained (i.e. following seismograph manufacturer and industry group instructions and recommendations), analyzed fully and interpreted correctly. All the data must be used, not just carefully selected parts, and they must form a consistent, or, at least, non-self-contradictory picture. Finally, such conclusions must be reported honestly with all supporting data behind each conclusion present in the report, so that others can compare the data and report text and judge the accuracy of the conclusions. Otherwise, the conclusions can have little or no scientific validity. Vibration monitoring data analysis, interpretation, reporting and use is such
an important and large area that many chapters in the CVDG Professional Edition are
devoted to various aspects of these and related topics.
For help in reading and understanding vibration monitoring data, read the
CVDG Pro section, Vibration Data Reports.
For more on possible problems to look for in analyzing the data, see the Pro version chapter, Vibration
Data Issues. The CVDG Pro section, Vibration Data Analysis, has a much longer
and more detailed listing of specific vibration monitoring issues.
Vibration Standards and Their Proper Use Even if vibration monitoring data can be read and understood, they won't be of much value in the absence of some independent, accepted means of linking them to the potential for causing damage. Because the mining industry has faced vibration damage claims related to blasting activities for many years, most of the data on structural effects of vibration, and standards for interpreting vibration data in that light, are based on studies done by or for the mining industry. Unfortunately, these blasting-related studies and standards for very short duration vibrations, while often used by construction companies and state agencies to justify their positions, provide limited help in evaluating the potential for damage from long-lived construction vibration caused by heavy equipment. A critical distinction between blasting runs ("shots") and construction vibration is explicitly acknowledged in USBM RI 8507, the basis for the frequently used OSM blasting standard:
Thus, use of blasting standards and vibration limits for non-blasting, long-lived construction vibrations is ill-advised and scientifically questionable. Of course, even well-motivated contractors may not think about or understand these differences between blasting and construction vibration. Proper use of vibration standards is a long and important topic which is investigated further in the CVDG chapter, Vibration Standards and in several chapters in the CVDG Pro. Distance Makes the Vibration Become Lesser, .... Vibrations die off with distance, although perhaps not as fast as most people would think. The lessening intensity with distance is the rationale behind what are called "scaled distance" and similar vibration propagation calculations accepted and used by the mining industry to estimate vibration effects at distances other than those measured or using different amounts of explosives than those measured. The Federal Transit Administration's Noise and Vibration Manual (formally titled Transit Noise and Vibration Impact Assessment) also provides a version of a "vibration attenuation equation" and necessary reference values for that equation to calculate expected vibration intensities at various distances from different types of standard construction operations.[5] This equation can be used, within its limitations and when no seismograph vibration data exist, to provide an estimate of the vibration PPV produced in a construction operation. The CVDG chapter, Vibration and Distance, provides an extended introduction to how vibrations move through the ground and how their velocities can be approximated using the FTA equation, along with some cautions about the proper use of such calculations. The Vibrationdamage.com Ground Vibration PPV and Safe Distance Calculator, available free to registered users of the free Construction Vibration Damage Guide for Homeowners, uses the FTA equation to estimate ground vibration velocities, as well as afford estimates of safe distances from construction operations. It can also be used to calculate blasting vibration PPV's, maximum charge weights using the scaled distance approach and blasting damage probabilities. Users of the FTA equation should note that the benchmark vibration velocities for
appropriate uses of construction equipment given in that reference[5] are indicated
to be "representative", not characteristic, of all such pieces of equipment, all
methods of use of the equipment, or of current models of construction equipment. Improper use or use of different equipment than
that indicated in the standard could produce larger or lesser vibrations. Indeed, measurements of vibratory compactor vibrations in a road reconstruction project showed peak velocities more than a factor of three higher than those
indicated in the referenced FTA table (as shown in the diagram at right). ...But It's Not That Simple Simple distance-based calculations do not tell the whole story of damage potential. As vibrations move ("propagate") through the ground, their frequency distribution changes from a typical broad spread of high and low frequencies to what are referred to as "low frequency wave trains" (see USBM RI 8507)[2]:
The transformation from high to lower frequencies is important, as the reduced frequency of the vibration wave trains is close to the natural resonance frequencies of the house. At the resonance frequencies, vibrations in the house grow with additional or long-lasting vibrations, rather than dying out. Thus, distant vibrations with resonant components can be more damaging than vibrations lacking those components which originate nearby, especially when they last for longer than a few seconds, as is typical in construction vibration. Vibrations which may not be damaging close to the site of their inception, may become damaging at larger distances (typically, a few hundred yards for blasting vibrations), even though they are less intense at the greater distance. For that reason, having your house at some distance from the work may not guarantee vibration damage safety in all instances (see CVDG Pro for a more complete discussion of this topic) nor can distance always be used as a legitimate scientific argument against damage causation. In our own case, the largest single crack that we observed developed when the construction work was a block away, although the cracks and other damage became more numerous and widespread as work approached the house. As a rough guide, if you can hear the construction operations in the distance, there might be reason to be concerned in at least some circumstances,[7] depending on what operations are being carried out, how they are being done and how effectively the vibrations are attenuated (i.e. decreased in peak particle velocity) in your area and changed in frequency by interactions with the ground as they travel. The role of distance in vibration damage is discussed much further in the CVDG chapter, Vibration and Distance, and in the CVDG Pro chapters, Applying Vibration Standards and Calculating Vibration Amplitudes. Locating and Using Blasting Seismographs Vibrations can spread out in all directions from the source, but usually not equally so, for reasons discussed on the CVDG page, Vibration 101. Because the frequency distribution, intensity and interaction of vibrations with structures depend so strongly on distance and location, good vibration monitoring will use two or more properly installed and calibrated seismographs in tandem, one placed near the source of the vibration and one placed, perhaps, a hundred yards away. The second seismograph should be placed to avoid interference from vibrations from any additional operations going on at the same time as those producing the source vibrations of interest. Of course, the intensity (velocity) of vibrations at the distant seismograph will be lower, in the absence of vibrations from other operations, but the point of the second seismograph is to detect and quantify the low frequency wavetrains[2], as discussed above. Their frequency distribution and duration are at least as important as their measured velocities (peak particle velocities, PPV's). Such monitoring should take into account the construction schedule, with specific emphasis on operations known to have damage potential, and the locations of sensitive structures near the work.[19] Vibration Monitoring Data Reporting It is common in the field of vibration monitoring simply to quote the maximum PPV observed for a given time period, location or operation, with, perhaps, a single "zero crossing frequency" (a simple, but often inaccurate, approximation of the frequency of the largest vibration component - see Vibration Frequencies for more information) for the quoted vibration. While this simplistic approach may seem adequate to some, it has little scientific meaning or validity in the context of assessing potential for damage. As discussed on several of the CVDG chapters, including Vibration Standards and Resonance and Fatigue, damage potential is dependent on both the maximum velocity of the vibration and its full frequency distribution.
Thus, simple quotations of PPV's, even if accompanied by the zero crossing frequency, can be misleading and are of limited value in estimating damage potential. One must take into account the complete frequency spectrum of the vibration, as derived from FFT analysis. Most vibration data are reported as single zero crossing frequency PPV's, lacking any frequency spectrum analysis whatsoever. In all too many cases, FFT analysis of the frequency composition is not even possible for the strongest vibrations, due to early filling of inadequate seismograph event memory with waveform data. Documenting Vibration Data Vibration monitoring is relatively rare on construction projects,23 despite the well-known potential for damage. It is most commonly done only after a damage report. When it is done, vibration monitoring reporting quality frequently is so deficient that the provided information fails to support adequately any scientifically verifiable and meaningful conclusions, despite guidance provided in various publicly available standards, documents and regulations to aid in preparing proper and defensible reports.[13] In my experience, it is rare, indeed, to encounter a "professional" vibration monitoring report which meets even basic scientific reporting standards. Some of the most commonly missing pieces of critical information or defects are:
It is the vibration monitoring technician's scientific responsibility to provide enough information in his report, including at very least the critical types listed above, to allow his work to be both reproduced and evaluated by other scientists. Any vibration monitoring report lacking such basic information should be considered unreliable, at best. These issues are further explained and extended, with actual construction vibration examples, in the CVDG Pro chapters, Vibration Data Issues, Seismograph Memory Limitations, Reporting Vibration Data and Vibration Signatures. Should I Allow Vibration Monitoring? You might get a request to allow monitoring on your property during a construction job, or the technician may simply install a seismograph on your property without permission, as was done routinely in the case with which I am most familiar. You should think through whether or not you wish to allow vibration monitoring on your property. Your best aid in making such a decision is to find out as much as you can about any previous monitoring done on the project, prior to the request to you. It is probably wise to delay granting permission until you have seen and analyzed any previously generated reports and data, with an eye to the possible mistakes that can be made in vibration monitoring (see several chapters of the CVDG Pro for much more help on this topic). Since the vibration technician probably works for the contractor, you can anticipate that any errors made in seismograph setup, use procedure and data interpretation will likely favor the contractor and/or project sponsor. Whether problems exist or not in the monitoring, delaying a decision on allowing it is about the only leverage you will have in getting the monitoring data, short of filing a lawsuit. If you
don't allow monitoring, you may not be able to stop monitoring in the public right-of-way
portion of your yard (i.e. that part of your yard bordering the street, reserved
by ordinance for street or sidewalk expansions - example of such an installation at right). If monitoring proceeds anywhere
in your yard or immediately adjacent to it, you are highly advised to videotape
or photograph each and every installation of the seismograph, noting the time
of installation and removal, to the extent you are aware of them.
Do-It-Yourself Vibration Monitoring Many
modern smart phones and tablet computers have an entire suite of sensors, including
acceleration sensors ("accelerometers") and a GPS location sensor. These can be accessed by software
("apps") to turn the phone or tablet into a do-it-yourself vibration monitor (see
example at right, using a small, generic tablet running a free vibration monitor
app). The many available free or paid apps for vibration monitoring on devices have various capabilities. You just have to make sure you get one that works with your device operating system (iOS in iPhones and iPads, Android for most other smart phones and tablets). For iOS, the Apple Store has a number of vibration measuring apps; for Android, the Google Play Store is the place to go. With small tablet computers available for as little as $30 US and cell phones with similar capabilities just about everywhere, anyone experiencing or concerned about construction vibration should seriously consider investing that relatively small amount of money to use the tablet or a phone as a mini vibration monitor. When no vibration monitoring is done by the contractor, as is typical, homeowner-acquired vibration data assume a critical importance. Currently, the sensitivity of the device accelerometers is limited to about 0.01-0.02 g[21] (i.e. that fraction of the Earth's vertical gravitational acceleration), about the same as typical vibrations caused by construction equipment. However, this sensitivity is adequate both to detect and measure the most damaging atypical vibrations (e.g. those associated with pile driving and vibratory compaction done too close to structures). It is also possible to use a mobile device or laptop computer as a data acquisition tool coupled to a factor-of-ten more sensitive external accelerometer. While more expensive (under $1000 total), such external accelerometers can measure the entire range of heavy equipment-caused vibrations of interest in possible damage situations - at a small fraction of the cost of a seismograph. Because these devices measure acceleration, not velocity (PPV), it is usually desirable, or necessary, to convert the measured accelerations to peak particle velocity by numerical integration of the data.[20] The CVDG PDF editions include a chapter about free vibration tools available here at Vibrationdamage.com. That chapter provides direct links to the tools, including our free template for Excel and other spreadsheets, which will convert the accelerations measured by the device sensors to peak particle velocities, as well as help document and analyze the data. To learn more about smart phone and tablet computer sensors and how to use a phone or a tablet properly to do your own monitoring,[22] read the CVDG Pro's DIY Vibration Monitoring and its several related additional chapters on this topic. Vibration Monitoring and its Impact This long chapter of the CVDG gives a sense of the power of vibration monitoring to both prevent and correct damaging vibrations from construction. When properly based on accepted construction vibration limits, implemented in accordance with publicly available guidelines, and applied thoughtfully on construction jobs using heavy equipment, it can be invaluable in stopping preventable damage to structures. I hope this discussion also provides some understanding of the many ways in which vibration monitoring can be misused and misreported to produce an unrealistically low impression of damage potential - one which is neither factual nor scientifically supportable. Vibration monitoring is rarely done, even when required by the construction contract, until damage to a home or structure occurs, if then. As a direct result of this failure, virtually nothing is known about the actual vibrations which may have been responsible for the damage. Yet, it is widely acknowledged by those working in the area of vibration effects that construction project vibration monitoring is a necessity.[17],[18],[19] Of course, such monitoring must be carefully done, in accordance with accepted industry guidelines, to have any predictive value or scientific validity. Monitoring, with real-time alerts for the construction crew, should be carried out from the beginning of any project involving heavy equipment use within 500 feet of structures. Such a policy affords greatly improved damage protection for homes and other structures at minimal cost (typically well under 1% of the project cost), while, incidentally, providing the contractor with litigation support in the event that damage from construction operations is claimed.
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E-mail to drzeigler@vibrationdamage.com or send online questions or comments about Vibrationdamage.com.
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At
right is a photo of a 
In another construction example in a different state, involving multiple damaged homes, seismograph installation was regularly done on top of loose rocks
or even leaf piles by the vibration "professional",
as shown in the photos near the top of this page and at left, preventing
the three mounting spikes of the seismograph head from making full (or, in the installation at left, any) contact with the ground. This mode of installation directly contradicted Operator's Manual instructions for the
These
often filled the seismograph event memory in under a minute in a road reconstruction job. As repeatedly documented in that project, the amount of
memory normally present in many blasting seismographs is often grossly
inadequate for recording waveforms for an entire day of construction work. 
Once
recorded, the data stored in a blasting seismograph or other device's
memory are downloaded to a personal computer for printing and analysis with appropriate software.
In the case of the Blastmate III seismograph shown above, the software for Windows®
(screenshot at left)
is called Blastware® (or, in recent versions, Thor®), and can be obtained free from
Virtually all of these views proved
important in understanding what data were present and what they really said, versus what was claimed by
the construction contractor, its insurance company, its lawyer, its vibration monitoring sub-contractor and the municipality 
For detailed
tips in analyzing the large amounts of vibration data generated in monitoring, see
our CVDG Pro chapter, Vibration Data Handling. The CVDG Pro chapter, Vibration Data Analysis Steps, provides a comprehensive step-by-step procedure for doing thorough vibration data analysis, along with tips about
what to look for in the analysis.
Fundamentally,
the reason for this is that blasting events differ dramatically from construction
vibration. Blasting at an active mine or quarry occurs perhaps once every day
to every few days; it produces vibrations which last a few seconds at most (more
typically, less than one second for the direct blast vibration). On the other hand,
construction vibrations can go on for minutes, hours, days or even months. The diagram
at right shows one of many examples where construction vibration repeatedly occurred
and persisted at levels above the Federal Transit Administration
construction standards (shown above) for minutes
at a time. This difference brings into play
Vibration wave interference effects can also cause vibration calculations to be
Many
types of construction vibration, particularly those resulting from ground impact (pile-driving,
dumping of large rocks, etc.) or ground impact-like events (tracked heavy equipment "drive-bys"),
have broad frequency distribution spectra (velocity vs. frequency plots) (e.g. see tracked excavator drive-by vibration frequency spectrum at right). These show significant intensity at the resonant frequencies of homes,
even if the peak frequency and PPV of the vibration may not be of any special concern.
Such broad spectrum vibrations cannot be adequately or appropriately described by a single frequency
PPV, nor can their risk potential be predicted meaningfully by using such a simple-minded approach.
Your record may
well be the only reliable documentation of the way the monitoring was done.
Vibration monitoring done on behalf of contractors
can be so variable, in execution (note "professional" installation at left for an
example of how not to install a blasting seismograph), interpretation, and reporting,
that it might be highly advisable to do your own vibration monitoring,
as a check on the contractor monitoring. That may be beyond the financial capability
of most homeowners. You can easily spend $10,000 or more to buy a seismograph vibration
monitor or hire someone to perform the monitoring using a seismograph. However, this picture
has, at least in principle, changed dramatically with the availability of technology in smart phones and tablet computers.
If your cell phone or computer screen automatically changes its orientation from portrait to landscape mode as you rotate it, chances are it has acceleration sensors, at a minimum. 
