VIBRATION AND SEISMIC WAVES

1. SEISMIC WAVES

Seismic waves are waves that travel through the earth. These waves represent the transmission of energy through the solid earth. Other types of wave transmission of energy are sound waves. light waves, and radio waves. Earthquakes generate seismic waves. The science that studies earthquake is seismology, the name being derived from the Greek word seismos meaning to shake. Some of man's activities such as blasting and pile driving produce seismic waves which people can feel. Seismic waves are divided into two large classes, body waves and surface waves.

• Body Waves

Body waves travel through the mass of the rock, penetrating down into the interior of the rock mass. There are two kinds of body waves: compressional waves and shear waves.The compressional wave is a push-pull type wave that produces alternating compression and dilatation in the direction of wave travel, such as occurs in a stretched spring. The shear wave is a transverse wave that vibrates at right angles to the direction of wave travel. The motion of a shear wave can be seen in a rope which is strongly flexed at one end. The rope moves up and down, but the wave travels outward towards the other end. Liquids do not transmit shear waves.

A schematic representation of the motion for compressional wave and shear wave is shown in Figure 1 and Figure 2.

Figure 1: Compressional Wave

Figure 2: Shear Wave

• Surface Waves

Surface waves travel over the surface of rock mass but do not travel through it. The depth to which the rock is affected by the wave motion is approximately one wave length. Surface waves are generated by body waves which are restrained by physical and geometrical conditions from traveling into the interior of the rock mass. Surface waves produce the largest ground motions and are the large energy carriers.


2. CAUSE OF SEISMIC WAVES

Seismic waves are elastic waves. Elasticity is a property of matter which causes a material to regain its original shape or size if it is deformed. A very familiar example of elastic behavior is that of a stretched rubber band which springs back to its original length when released. Rock materials are highly elastic and thus produce strong elastic or seismic waves when deformed. Deformation occurs in two ways, a change in volume which is a compression or in shape which is a shear.

Material resist deformation and this resistance is called an elastic modulus. If the deformation is a compression, the resistance is measured by modulus of incompessibility or the bulk modulus. If the deformation is a shear, the resistance is measured by the modulus of rigidity or the shear modulus. Thus, there are the two types of seismic waves, compressional and shear.

Operations such as blasting will always produce vibration or seismic waves. The reason for this is a quite simple. The purpose of blasting and other such operations is to fracture rock. This requires an amount of energy sufficient to exceed the elastic limit. When this occurs, the rock fractures. As fracturing continues, the energy is used up and eventually falls to a level less than the strength of the rock and fracturing stops. The remaining energy will pass through the rock, deforming it but not fracturing it because it is within the elastic limit. This will result in the generation of seismic waves. A simple schematic representation of compression and shear is shown in Figure 3 and Figure 4.

 Figure 3: Deformation by Compression

 Figure 4: Deformation of Shear

3. WAVES PARAMETERS

The fundamental properties that describe wave motion are called wave parameters. These are measured and quantified when discussing wave motion or vibration. Consider the simple harmonic wave motion illustrated in Figure 5 and represented by the equation:

 Figure 5: Wave Motion and Parameters

Period and frequency are reciprocals so that:

Wave length L is the distance from crest to crest or trough to trough measured in feet and is equal to the wave period multiplied by the propagation velocity V.

4. UNDERSTANDING VIBRATION INSTRUMENTATION

 4.1 Seismic Sensor

The function of vibration instrumentation is to measure and record the motion of the vibration earth. In basic scientific terms, this is a seismograph comprised of a sensor and recorder.

The sensor is in fact three independent sensor unit placed at right angles to each other. One unit is set in the vertical plane, while the remaining two units lie in the horizontal plane at right angles to each other. Each sensor will respond to motion along its axis. Three are necessary to completely determine the ground motion. The three units are enclosed in a case as shown in Figure 6.

 Figure 6: Seismograph Sensor

The configuration of the sensor case varies with the manufacturer, and may be round, square, rectangular, or triangular.

The sensor is usually an electromagnetic transducer which converts ground motion into electrical voltage. Inside the sensor is a coil of wire suspended in a permanent magnet field. The magnet is attached to the sensor case and cannot move, but the coil suspended in the magnetic field by springs or hinges is free to move. Any movement of the coil relative to the magnetic field will generate an electrical voltage proportional to the speed of the coil movement. If the coil moves slowly, a small voltage is generated. If the coil move fast, a large voltage is generated. When the ground vibrate, but the suspended coil inside will tend to remain motionless due to its inertia, thus producing relative motion between the coil and the magnetic field, resulting in the generation of an electrical voltage.

A schematic diagram of the sensor transducer is shown in Figure 7.

 Figure 7: Sensor Mechanism

 

The recorder takes the voltage output of the sensor, converts it back into motion, and produces a visual record of the ground motion. Since the sensor consists of three mutually perpendicular independent units, there will be three traces on the record, one for each sensor unit. This record is then ready for analysis and interpretation.

The recorder changes the output voltage of the sensor into motion by use of galvanometer. When a voltage is generated at the sensor, a current will flow through the circuit causing the galvanometer coil to move. Thus the electrical energy has been changed back into motion and may be amplified in the process. The recorder also puts timing lines and calibration signals on the record. Finally, the recording of the motion may be done photographically or by heat stylus.

The ground motion may also be recorded on magnetic tape. To obtain a record from magnetic tape, it is necessary to have a playback system and a chart recorder. This system is somewhat more involved, but adds increased flexibility. The tape can be played back at different amplifications or for varied analysis techniques. In addition, many events (i.e., blast) can be recorded on a single magnetic tape. The cassettes are inexpensive and easily available.

4.2 Seismograph System

There are many seismograph system, or simply seismographs, available today, each of which performs the basic function of measuring ground motion. The many variations are a response to need, constraints, and advancing technology. A brief description of the main types of seismograph will be helpful.

• Analog seismograph is a three component system that produces a record of the ground motion. It is called analog because the record is an exact reproduction of the ground motion only changed in size, amplified, or de-amplified.

• Tape seismograph is the same as the analog seismograph, except that it records on a magnetic tape cassette instead of producing a graphic record. A record of thee ground motion is obtained by use of a playback system and a chart recorder.

• Vector sum seismograph is the standard seismograph system consists of three mutually perpendicular components. The vector sum seismograph performs the mathematical calculation electronically; that is, it squares the value of each of the components for each instant of time, adds them, and takes the square root of the sum. It then produces a record of the vector sum. 

• Bar graph seismograph is a three component system that differs in its recording system. Instead of recording the wave from the ground motion at each instant of time, only the maximum ground motion of three components is recorded as a single deflection or bar whose magnitude can be read from the record graph. This is a very slow speed recording system which can be put in place and left to record for periods up to thirty or sixty days.

• Triggered seismograph is an analog or tape seismograph which automatically starts to record when the ground vibration level reaches a predetermined set value, which triggers the system.

Most seismographs are equipped with meters that register and hold the maximum value of the vibration components and the sound level. Other seismographs are equipped to produce a printout which gives a variety of information such as maximum values for each vibration component, frequency of vibration for the maximum value, vector sum, and sound level. Blast information such as date, blaster number, time, location, job designation, and other pertinent information can also be added to the printout.

4.3 Vibration Parameters

Wave parameters were discussed earlier. Vibration parameters are the fundamental properties of motion used to describe the character of the ground motion. These are displacement, velocity, acceleration and frequency. As a seismic wave passes through rock, the rock particles vibrate, or are moved from the rest position. This is displacement, When the particle is displaced and moves, it then has velocity and can exert force that is proportional to the particle's acceleration. These fundamental vibration parameters are defined here:

• Displacement is the distance that a rock particle moves from its rest position. It is measured in fractions of an inch, usually thousandths.

• Velocity is the speed at which the rock particle moves when it leaves its rest position. It starts at zero, rises to a maximum, and returns to zero. Particle velocity is measured in inches per second.

• Acceleration is the rate at which particle velocity changes. Force exerted by the vibrating particle is proportional to the particle acceleration. Acceleration is measured in fractions of “g”, the acceleration of gravity.

• Frequency is the number of vibrations or oscillations occurring in one second, designated Hertz (Hz).

Vibration seismographs normally measure particle velocity since the standards of damage are based on particle velocity. There are, however, displacement seismographs and acceleration seismographs. Also, velocity seismographs can be equipped to electronically integrate or differentiate the velocity signals to produce a displacement or acceleration record.

5. VIBRATION RECORDS AND INTERPRETATION

5.1 Seismograph Record Content

Normally a seismograph record will show the following:

Four lines or traces running parallel to the length of the record. These traces are the vibration traces, while the fourth trace is the acoustic or sound trace. (There may not be an acoustic trace.)

Each of the four traces will have a calibration signal to show that the instrument is functioning properly. Timing lines will appear as vertical line crossing the entire record or at the top only, the bottom, or both top and bottom. An example of a typical seismogram, or vibration record, is shown in Figure 8.

 Figure 8: Vibration Record

One vibration trace or component is vertical, the other two horizontal. The components are usually specified as follows in Figure 9.

• Vertical: motion up and down, designated V.

• Longitudinal or Radial : motion along a line joining the source and the recording point, designated L or R.

• Transverse : motion at right angles to a line joining the source and the recording point, designated T.

Figure 9: Vibration Components

The sensor normally has an arrow inscribed on the top. By pointing the arrow toward the vibration source, the vibration traces will always occur in the same sequence, with the arrow indicating the L component also the direction of motion will be consistent from shot to shot. The instrument manufacturer will indicate the proper sequences.

Each trace represents how the ground is vibrating in that component. If the seismograph is measuring velocity, then each trace shows how the particle velocity is changing from instant to instant in that component.

Similarly, if the seismograph is a displacement system or an acceleration system, the trace will show the instant to instant change in these parameters. The acoustic trace shows how the sound level changes with time.

5.2 Record Reading and Interpretation

The maximum vibration level is found by measuring the largest amplitude, either up or down from the zero line on any given trace. This value, in inches (usually fractions of an inch, e.g., 0.54), is then divided by the instrument gain setting. The result is the maximum ground motion. Figure 10 illustrates the operation.

Figure 10: Measure of Vibration Amplitude and Period

Assuming the seismograph is a velocity seismograph, the result in 0.108 in/s. If it is a displacement seismograph, the the result is 0.108 inches. If it is an acceleration seismograph, then the result is 0.108 g's.

To measure the frequency of vibration, it is necessary to examine the wave motion on the record. A sinusoidal wave would have equal amplitudes for the crests and through, and the distance between successive crests or troughs measures the period of the wave or the time for one complete oscillation. This is an ideal condition that does not normally occur on vibration records. That is, successive equal crests or trough do not usually occur. Hence, ti will be necessary to modify the procedure. The procedure will be illustrated for both the ideal case and for the normal occurring case.

Figure 10 shows several approximately equal successive crests. The distance between these successive peaks is measured by counting the number of timing lines from the first to the second peak. There are three,and since each timing space has a value of 0.01 seconds, then:

Now assume that the vibration record looks as following Figure 11.

 Figure 11: Half Period Measurement

Since there are not successive peaks or troughs of full equal amplitude, measurement will be made from the trough to the crest, this is a half period. There are 2.7 timing spaces from the trough to the crest in the first T/2 space going left to right. It is a trough to crest value since each timing line represents 0.02 seconds.

Notice the difference in T/2 spacing values. The second T/2 spacing represents the zero crossing value. The third spacing is a crest to trough value. Each of these spacing values will yield a different frequency. When records are read manually, this is a judgment call by the analyst and depends on this experience and expertise. The time values were read as multiple of the timing spaces. In practice it may be necessary to measure fractions of a timing space.

Frequency determined by reading from the record is highly susceptible to error. Frequency values read electronically by the seismograph system are much more precise and reliable. Early investigations and research show a wide scatter in data, and much of it no doubt is due to to the difficulties associated with reading frequency off a record.

Acoustic level are read in much same way, except that there is usually a base level below which the instrument does not respond. (What is the point in recording sound levels in the normal environmental range?) This base level value is then added to the computed value to give true sound level reading. The following example illustrates the procedure.

 

Sound level changes less than 3 dB are normally not noticeable to human ear.

5.3 Field Procedure and Operation Guides

Site selection is the first item of procedure. This is usually determined by a complaint or sensitive are which needs to be checked. If there is no such problem, then place the seismograph at the nearest structure that is not owned by or connected with the operation. The seismograph distance should always be less than or at most equal to the distance to the structure.

When dealing with residents or persons in the vibration affected are be factual and direct. Emphasize that the purpose of the seismograph measurement is to protect them and their property from vibration damage.

Place the sensor on solid ground. Do not place it on:

1. Grass
2. Isolated slab on stone or concrete
3. Loose earth
4. Any soft material
5. Inside a structure except on a basement floor
6. Concrete or driveway connected to a blast area

Failure to observe these precautions will result in distorted readings that are not representative of the true ground vibration. Level the sensor, some sensor have bull eye level on top for this purpose. Other can be leveled by eye. Make sure the sensor is solidly planted. In cases of large ground motion it may be necessary to cover the sensor with a sand bag, spike it down or dig a hole and cover it with earth otherwise the sensor may be decoupled from the earth and the vibration record will not represent the true ground motion. Remember that the ground displacement is usually only a few thousandths of an inch so do not expect to see the decoupling of the sensor.

Most sound measurement is made with a hand held microphone. Hold the mike at arm's length away to avoid reflection of the sound from your body. Regardless if the microphone is used set up on a stand or hand held, do not set it up in front of a wall. This will prevent sound reflection from the wall.

5.4 Practical Interpretations

The seismograph record can be used for much more than obtaining the peak particle velocity. It can be helpful in engineering the blast and provide information to the operator as to how to achieve the best vibration control as well as optimizing the use of the explosives energy to break rock. Assume one has a seismograph record that exhibits on large peak in the center of the wave trace. That large peak has a particle velocity of 2 in/s. Also assume that no other peak on the record is larger that 1.0 in/s. The one large peak is at 2 ms controlling how we design and execute all our blast in the future. In blasting, it is not the average vibration value that counts, it is the maximum. Therefore, common sense would dictate that if one could reduce that 2 in/s peak to 1 in/s, it would not only be better for the residence in the area but also would be more economical for the operator because he would not have to go extreme measures to try to reduce this one peak value.

What does this large peak mean from a practical standpoint? It was indicated that if it occurred in the center of the record, it is indicating that something occurred that was of unusual nature approximately half way through the blast. If one assumes that these peaks and the vibration record indicate energy release over time, then the record tells one that for some reason significantly more seismic energy was obtained approximately halfway through the blast. If all blastholes were loaded the same, this indicates there is inefficiency in the blasting process approximately half way through the blast. Now go back to the blasting pattern and determine approximately where the problem resulted. One might be able to then correct the problem so that one does not have to go to unusual measures to reduce vibration. A common problem which occurs is that if blastholes are wet, one commonly does not place as much energy in the wet portion of the hole as in a totally dry hole. This is because cartridge product is used instead of, for example, bulk ANFO. The smaller diameter cartridge product may not have as much energy as ANFO and therefore, the vibration level would increase. How one handles wet hole situations can greatly effect the vibration generated from the blast.

Another common problem is drilling inaccuracy. If a blasthole within the pattern has an excessive burden at the time it shoots, vibration levels go up. The seismograph record, therefore, can be used as a diagnostic tool to determine where within the blast the problem occurred which resulted in the higher vibration level.

Ideally, if one looks at vibration records and assumes that the peaks indicate energy release over time. Common sense would dictate the one would like to see all peaks near equal throughout the entire record. If this would occur, the explosive energy is being used efficiently and is reducing vibration to a minimum.

In the past, to expect a vibration record to have near identical peaks would have been considered an academic solution which was not practical in the field, however, today with advanced technology this type of vibration record can be achieved on blasts that are well engineered.