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 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 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:
- Grass
- Isolated slab on stone or
concrete
- Loose earth
- Any soft material
- Inside a structure except
on a basement floor
- 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.
