Part 4: What do the Data Mean?
By Charles G. Spencer, Ph.D.
Now that you know what all the numbers represent, what exactly are they telling you and how do you know when they exceed regulations?
Plots of data are obtained by using the “Image” link in the far right column.
When you initially click on the “Image” link, it will download a PDF file with a graph of the waveforms for the radial, transverse, vertical and air vibrations. There are also data about the PPVs recorded for each at the top of the graph. This graph shows the amplitude of the vibrations over time following the triggering of the seismometer’s recording function.
If you click the little circle above the data table labeled “FFT” and then click the Image link, you will get a PDF with two plots. At the top is the waveform vs. time plot, but with a different y-axis scale. Below that you get another set of graphs. Those show the amplitude (energy) spectrum at all frequencies recorded. Higher amplitudes at lower frequencies are something to watch for.
If you click the little circle above the data table labeled “Curves” and then the Image link, you get a PDF with another set of graphs. Again, the waveforms are reproduced at the top. Below are plots of the Velocity vs. Frequency data. It shows the PPV for the frequencies recorded. The straight lines on those graphs are the thresholds used by the U.S. Bureau of Mines (USBM). Blasting vibrations must be kept below those lines to avoid violating regulatory standards.
Now, about those regulations. They are minimal. The State of Missouri requires that PPVs remain below thresholds established in 1989 by the USBM (now the Office of Surface Mining Reclamation and Enforcement). If you’d like to read that entire report, here is a link where you can download it: www.osmre.gov/resources/blasting/docs/USBM/RI8507BlastingVibration1989.pdf. That report is the “USBM RI 8507” you should see highlighted next to the “Additional Analysis” heading.
The state statutes also establish where seismometers should be located, based on a calculation called scaled distance. The requirement is that monitoring is necessary at the closest offsite structure for which the calculated scaled distance is 55 or smaller. Problem is, you have to know what charge is used to make that calculation, and that information is not made public.
Furthermore, the state regulations exempt from the requirements operators that are voluntarily using seismographs. As I said, minimal.
The USBM limits were developed based on actual blasts and damage assessments, but they are also statistical averages, and given all of the possible variables that influence the transmission of seismic waves, they may or may not be appropriate to any specific situation.
For frequencies 40 hertz and higher, the maximum allowable PPV is 2 inches per second. For frequencies below 40 hertz it drops to 0.5 inches per second at 10 hertz.
As for sound, the maximum allowable is 133 decibels.
The recent approval by the Missouri Mining Commission for the use of explosives at the Star Excavations site on Quarry Park Road in Lee’s Summit has led to concern on the part of many nearby residents. The Tribune asked Dr. Charles Spencer, a local geological consultant, to provide some basic information on the nature of blasting and its effects. His article will appear over the next four weeks.