Dr. Abhishek Kumar: Welcome all to lecture
9 of subsurface Exploration: Importance and
Techniques Involved.
So far, we have discussed about different
classification of investigations, then we
started with direct method, slight test period,
trial, trenches, then we went to semi-direct
method that is mostly geotechnical investigation,
different kind of geotechnical investigation
we had discussed earlier like standard penetration
test, cone penetration test, dilatometer test,
pressure meter test and so on, 00:58 test;
what are the advantages/disadvantages of each
of those tests.
And then we have also solved typical numerical
problems like how to interpret field records
in order to identify the soil type in order
to determine different properties of the medium.
So as we know like most of the geotechnical
tests are point based methods like whatever
soil type you are determining it will be at
that particular location and at that particular
depth.
However, whenever we are interested to find
out subsurface properties of a particular
set, we are interested to find out what is
the little variation.
At times, we are interested to go for deeper
location probably where geotechnical investigations
are not feasible.
So considering the requirement for interpolation,
considering the limitation with respect to
a depth exploration at times.
Moreover, in addition to that, like whenever
we are going for any kind of geotechnical
testing, it requires a lot of time, expertise,
as well as -- because it requires a lot of
time and specific equipments, most of the
time geotechnical tests are expensive and
time consuming.
So because of these three major limitations
or issues, and in addition you can explore
particularly when large area is there where
the required number of tests are significantly
larger, you have to invest more number of
times and a huge amount of finance also, particularly
when you are going to geotechnical investigation.
So another option which comes to mind, which
is not new, but even the techniques which
are people following nowadays, even for smaller
areas for regional study, even for a local
study, site specific studies, so more and
more methods have been evolved.
In last one decade, even though because of
use of geophysical investigation particularly
for oil exploration, mineral exploration and
even for geological explorations, it has been
in use for quite some time maybe for three,
four decades now, but more and more use of
geophysical investigation has become more
permanent nowadays, because such investigations
can be done even at local scale and because
of improvement in the interpretation, because
of the significant reduction in the initial
investment regarding the setup purchase it
has -- I mean now the geophysical investigations
are -- people can afford, that's what I want
to say here.
So from today's lecture onward, we will be
discussing about what are geophysical investigations
and we will be discussing, just like geotechnical
investigations, some of the geophysical investigation
methods, what are the methods, how you interpret
the typical field records in order to quantify
the subsoil properties.
So as the name suggests geophysical investigation,
we are interested to find out -- we are interested
the soil type, we are interested to quantify
subsoil properties, physical properties of
the medium based on some ways of using geophysics.
So it comes under module 3 as first lecture
or overall it is lecture 9.
So it consists of determination of subsoil
information, which is a broad objective of
this course.
From test pits as well as geotechnical investigation,
it is time consuming as well as expensive
as I highlighted earlier.
Moreover, earlier discussed tests are generally
conducted at limited location.
So whatever you site of interest depending
upon the guidelines provided for different
kind of structure for different kind of field
investigation, even for different kind of
seismic zones and areas, geotechnical investigation
can be, I mean the entire area can be divided
number of grids, and for each grid you have
to conduct at least or more than one tests.
At times, you have to validate the findings
from geotechnical tests by one more test.
So that we have discussed, but what about
if the properties varying in between the grids,
like you did the test at two locations and
assuming like either it is same, if it not
changing between the two test locations or
whatever changes happening, it is happening
linearly.
So in those cases where interpolation is required
and considering the finance involved, you
have to go for limited number of geotechnical
tests, so more your field investigation and
interpretation part depend upon interpolation
between the field tests.
So in those cases, geophysical investigation,
because there you can actually explore a larger
area, you can even go for continuous recording,
you can even go for 2-dmentionsal methods,
also where you can have complete information
about how lithology is changing in terms of
depth, in terms of physical properties along
the survey line.
So that is another important aspect when we
go for geophysical investigation, like those
are less time consuming in comparison to geotechnical
tests and interpolation is also significant
easier or most of the time you can rather
than going for interpolation between two location
which are wide apart, you can go for more
number of tests, because these are relatively
quicker methods.
So geophysical test being indirect method,
because you are actually not getting any kind
of soil sample from the depth or you are not
taking about the sample or any kind of sensor,
you're not going to lower there.
So mostly, these are called as indicted method,
because you are putting the sensor at the
surface, any kind of disturbance, which the
sensor is detecting, even that kind of disturbance
is also located at the surface.
So these are call that's why nondestructive
tests, and thus lesser time consuming, because
you need not drill a bore hole, you need not
drill a trench, so you can simply put some
sensor at the surface, and by means some typical
measurements at the site of interest, you
will be able to understand what kind of soil
properties are there at the surface, what
kind of soil properties are there below the
depth at different, different layers, and
depending upon what kind of depth you are
interested to find out, you are interested
to explore, you can go for different kind
of sources, you can go for different choices
of receivers as well.
So these are overall less time consuming.
It can be done over larger areas, because
these tests even can be conducted over larger
area, because your sensors will be spread
over larger area.
You can go for tests even in 2-dimension.
So you can actually got for these kind of
tests over larger lands without interpolation,
because that will be consist of almost continuous
recording.
So there will not be any much requirement
for interpolation in between.
What it does, depending upon the physical
properties of the medium like depending upon
which kind of test you are using, the geophysical
test, each geophysical test will be able to
determine some variation in the physical properties
of the medium, either using those physical
variation direction or by using some correlation,
empirical correlation, you will be able to
determine what kind of soil deposits are there,
what are the strength characteristic of the
soil deposit, which will be thereafter you
can use it for subsurface exploration for
interpretation, for foundation design, even
for dewatering purpose also, and so and so
forth you can use it, depending upon what
is the choice of geophysical method you are
using.
So it measures the physical properties of
medium, so that you can go for subsurface
characterization even you can go for identification,
like at times it's not only used for identifying
different layers, it can also be used for
identification of buried objects, it can also
detect some unconformity, it can also detect
some like in archeological survey also geophysical
tests are very permanently used, so that you
can actually able to find out without going
for any destructive tests, because those are
very important site, you cannot blindly keep
on destructing the existing structure, so
that you can explore a deeper depth, so you
can go for nondestructive test and find out
what are the buried objects, hidden objects,
which are to be excavated out, and it can
be brought to the public -- I mean it should
be exposed to the public.
So I told you geophysical tests are very useful
whenever exploration is required, whenever
interpolation is required, whenever deeper
depths are required for exploration, very
useful nowadays, geophysical test particularly
for microzonation studies, like microzonation
-- as we know most of the time whenever we
are interested to find out what is the inertial
force because of seismic activity in a particular
region, nowadays people are going for more
and more regional studies or site specific
study or even building specific studies in
order to find out what is the seismic activity,
what is the potential seismic hazard, what
is the potential level of ground motion, which
is expected at my site of interest.
Further those ground motions will be altered
because of the presence of local soil.
So in order to take into account the variation
in local soil property at your site of interest
over which a building is located, geophysical
tests are more and more -- nowadays people
are practicing, so that you can quantify the
subsoil properties.
Once you know the subsoil properties, you
will be able to identify how these subsoil
properties are dominating in controlling the
seismic vibrations into a building.
Then second thing quality control, like your
designer suggested some minimum density of
the medium at the site of the interest, even
whenever you're going to subgrade laying or
you're going for any kind of filled up area.
So once the site is filled up, you can actually
got for some kind of quality check, you can
do some test and based on that test, it will
be able to determine what is the strength
properties for the densities of the medium,
so that you can match those densities which
you are getting from geophysical tests with
the density provided by the designer.
In order to ensure like your quality, the
guidelines which are given for filling up
the area for preparing the subgrid are those
properly matched.
So it is like quality control.
Third one is forensic investigation of failed
structures.
So whenever there is some kind of failure
at the structure, particularly like abutments
of bridges, flyovers, retaining structure,
you can actually got for forensic investigation
in order to find out, because those structures
are already failed, at times it is not advisable
to go for detailed geotechnical investigation,
because again that can trigger more distress
to the existing structure, though it has already
failed.
You can go for geophysical method, so without
creating much of disturbance to the surrounding
area of any disturbed, or failed geotechnical
structure, you can go for -- you can explore
the subsoil properties and find out whether
there was some problem during laying of subgrade,
whether there was some problem during filling
for preparation of abutment, approach roach,
or any preparation for zone of influence for
any bridge periods and so on and so forth.
So there are a number of varieties of which
in which forensic investigation for failed
geotechnical structure can be done very easily,
considering the nondestructive nature of geophysical
test.
The next one is validation of the findings.
At times, whatever geotechnical investigation
you are doing, you can also validate, so that
if a particular geotechnical investigation
has limitation with respect to the depth,
you can go for geophysical investigation for
deeper depth, so that way you will be able
to develop some correlation between some measure
of geophysical test and some measure of geotechnical
test, and that's how you will be able to determine
the outcomes from geophysical tests at deeper
depths in order to find out the equivalent
properties in terms of geotechnical testing.
So this is like validation as well as interpretation.
Then it is suitable for deeper depths where
many geotechnical tests might not be possible
to perform.
So as we know like SVG test, CPG test, dilatometer
test, each of tests has some in terms of the
depth of exploration.
So if your depth of influence is more than
that particular depth of exploration, there
must be some other ways with which you will
be able to determine what kind of subsoil
properties are there, what are the physical
properties of the medium, what are the soil
types.
So in those cases also particularly for deeper
depths, geophysical investigations are very
important.
The common methods as I mentioned here like
in earlier times people were going for more
and more for gravity survey, magnetic anomaly
method, then seismic reflection method for
mineral exploration, for studying the geomorphology
of the area, even for studying the geology
or topography of the area, people were going,
and even for oil exploration also.
So people were going more and more for these
methods, but over the period of time more
and more advancement in the technology and
more and more sophisticated and user-friendly
equipments are nowadays available.
So you can see here, I've given here the list
of different geophysical method which are
more permanently, people are using nowadays,
starting with seismic reflection survey, then
seismic refraction survey, electrical resistivity
survey, sounding and profiling survey, magnetic
anomaly method or magnetic anomaly test, gravity
method, suspension logging method, and then
we'll also give some brief idea about what
is Multichannel Analysis of Surface Waves,
that is MASW, and Spectral Analysis of Surface
Waves, that is SASW.
Those two tests are more and more prominently
people are using now for forensic investigation,
even for quality control, even for microzonation
studies.
So we will be discussing overall like these
are the list of prominent tests which comes
under geophysical testing methods.
As far as the syllabus of this course is concerned,
we will be discussing about seismic reflection
test, seismic refraction test, electrical
resistivity methods, magnetic anomaly, gravity
method, suspension logging, and as I mentioned
here, when we'll be discussing about other
methods, I'll give also some glimpse about
what is MASW and what is SASW.
Now coming about to as I mentioned here, like
geo physical tests has certain advantages,
certain limitations.
So I am going to list out here, what are the
advantages.
So what it does actually, it tries to find
out based on physical medium, based on physical
property variation or you can say, when there
is too much of contrast 
in characteristics of the medium, what it
does, there will be too much of variation
in conductivity of the medium, whether it
is related to reflection, refraction, whether
it is related to magnetic properties of the
medium, whether it is related to electrical
properties of the medium.
So it keeps on interpreting the physical property
of the medium, rather changing its field record
characteristics.
Suppose you're going for because of too much
contrast, you have to be very careful with
too much contrast.
So like you're going for any kind of field
test, you know like at the site of interest,
there are very different layers, one maybe
very soft layer, one maybe very stiff layer.
So when a particular like you go for seismic
method when any kind of waves will be there,
when it will lease the interface, when the
wave will face too much of contrast, that
will show too much of change in the properties
of the medium.
So because of this too much contrast maybe
in terms of velocity, you will also interpret
okay there is change in the medium characteristics
also.
So geophysical test targets for identification
in the physical properties, variation, because
of change in field record characteristics.
So advantages include, you can go for deeper
depth, almost limiter interpolation, because
you can go for limited interpolation, then
you can go for these tests are quick and less
time consuming.
Then you can go for less time consuming, limited
interpolation and then can validate findings
from geotechnical test.
Disadvantages, I can say here limitation,
requires -- most of the time these tests are
significantly costlier, require trained people,
because interpolation is required, at times
require larger area for investigation for
setup of instrument.
Keep in mind these are indirect methods, so
indirect, so always reliability, reliability
of field recording and interpretation 
is to be validated.
You have to get more and more confidence,
whatever interpretation you are doing, it
is correct.
Then another advantage here, you can do those
tests here over larger distances.
So that's why very much similar to your geotechnical
investigation.
You can say here, one note you can put, suggested
like at least more than one method of investigation
should be used.
In geophysical method, you can go for at least
more than one method.
Either you can go for additional geophysical
method or you can go for geotechnical method.
So that findings whatever you are getting,
it will be validated.
I mentioned here different methods also like
seismic reflection survey, seismic refraction
survey, electrical resistivity, magnetic anomaly
methods, gravity test, suspension logging,
then multichannel analysis of surface waves,
spectral analysis of surface waves, so on
and so forth.
Now we will be discussing here onwards what
are the seismic methods, because as I mentioned
here, the first two methods are called as
seismic reflection survey or seismic refraction
survey.
So what do you mean by seismic methods, why
it is called seismic methods.
We know like because of any kind of disturbance,
which maybe because of impact load, maybe
because of explosion, maybe because of machine
operation, maybe because of waves in water,
that is water currents, there will be some
kind of disturbance or some kind of shock.
These will result in development of some kind
of shock or shockwaves, which will travel
as well as cause disturbance in the propagation
medium.
So depending upon which medium it is passing
through, it will cause some kind of disturbance.
What kind of disturbance it is going to cause,
that depends upon what kind of wave you are
targeting for.
So precisely you can tell like whenever there
will be some kind of shockwaves, whenever
there will be some kind of disturbance, because
of ever mentioned reasons, there will be different
kind of -- the disturbance at the surface
will cause some of kind waves generated at
the source, like if I am showing here, like
this is your sources, this can be anything
like exploration, explosion, it can be impact
load, it can be a machine operation, it can
be even passage of traffic also can come here,
like traffic movement.
Each of 
these can cause some kind of source.
So I am referring source to any kind disturbance.
As a result of which what will happen?
Wave will be generated.
I am showing here some kind of wave fronts.
So wave front means whatever matter is available
at this particular location, it will be undergoing
some kind of disturbance, and depending upon
how much the resistance against this disturbance,
the material offers that will help in understanding,
quantifying sub-measure of 
material property.
What I meant to say, if the wave is passing
too quickly, we can say the material is stiff
or it can be hard, because the resistance
offered to the material is significant high.
If the material is passing very high velocity,
you can call it as stiff material, hard material.
Same way, if it is very slow, slow velocity,
you can call it as soft material or softer
material.
So what is happening as a result of this source,
any kind of disturbance, actually wave fronts
are creating now.
Now you can see here, this is at the ground
level.
Okay suppose this is soil level one.
I do not know whether this soil layer is sand,
clay or it is soft medium, or it is harder
medium, because most of the geophysical methods
give you quantitative assessment, and this
is your soil layer two.
Now when you went for trenching, when you
went for test pits, when we went for geotechnical
test, we were interested to find out what
kind of soil layers are there.
What is the measurement of strength properties.
Even here also, our target will be to determine
what is the thickness and what is the -- you
can call it some ways of measurement of the
physical property of this particular layer.
Now what will happen as a result of this wave
front.
I am showing you some partial this thing here.
So once the wave front reaches this -- at
the interface, why am I calling this is an
interface because this medium suppose this
is very stiff soft medium, this is stiff medium,
because of the sudden change or too much contrast
in medium characteristics, what will happen,
the incident wave, either some wave -- first
of all, some wave will travel along the surface.
This will be called as direct some wave, you
can see it based on the wave front, will reach
at the interface and will reflect back.
Some wave might travel to the deeper medium,
so you can call it as either absorb or you
can call it as -- yeah, travel downwards.
Some wave will along the interface which keeps
on creating more and more wave fronts, because
once it traveling here, again it will be considered
additional source, which will again transfer
-- so I am putting here another receiver,
which is actually going to detect what kind
of disturbance you are getting from here.
So depending upon at which particular depth
this too much contrast or the interface is
there, or too much contrast in the medium
or you can call or interface.
So whenever the interface comes what will
happen, the wave which is -- you can call
it a shockwave which was generated because
of any kind of disturbance causes the source.
Once the wave reaches the interface, it will
experience some kind of too much contrast
in the medium characteristics as a result
of it some energy the wave was carrying, it
will be absorbed in the material, that is
traveling downward.
Some will travel, some will reflect back and
start traveling towards the surface, which
will be getting detected by the receiver.
So depending upon, you see here, because of
the wave front, it is causing some kind of
disturbance in the material.
So depending upon what kind of disturbance,
the nature of the disturbance causing the
material or the kind of movement this material
that is soil particle in this particular case,
so nature of soil particle movement, that
will help you in understanding what is the
characteristic of this medium.
So generally based on there can be different
kind of waves, which can possibly be generated
from the source and depending upon the nature
of each kind of waves, it will be causing
different kind of soil movement.
So I am telling here, nature of the soil particle
movement will change, will vary from wave
type to wave type.
So when discuss about seismology, when people
started interpreting about different layers
of the earth, it is understood like precisely
the kinds of waves which are generating because
of any kind of shock can be classified as,
I am calling here shock like the shock which
can actually cause some kind of disturbance
to the material.
So we can precisely classify it as P wave
or primary wave or compressional wave, which
are causing compression and rarefaction to
the material.
You can call it as P wave.
Then secondary wave or S wave or shear.
So first one was causing compression and rarefaction,
this is causing some kind of shear, you can
call it a shear wave.
So you see here, when a primary wave is passing
through the medium, it is causing some kind
of compression, expansion compression, rarefaction,
like compress and then it will bring it to
its original position.
So with respect to second position, it will
be expansion, but finally it will commit to
its original position.
Then second one is shear wave.
So whenever the wave is passing like this
is the direction of wave propagation.
So what it does, once it is shear wave, it
will be causing shear in mutually perpendicular
direction.
So particle motion will be either perpendicular
to the wave or other one will be perpendicular
to this board.
So depending upon what is the -- and which
plain the shear stresses are getting dull
or in which plain the particle is undergoing
some kind of shearing, it is called SV or
called as SH wave, but both of them are causing
-- I mean it is happening because of passage
of shear wave.
So in order to -- I mean when shear wave is
passing, the material is offering resistance
in terms of shear strength of the material.
And the third one is Rayleigh wave, which
is precisely caused when this body waves interact
with the surficial layer of the Earth.
So these are called Rayleigh waves or it cause
elliptical motion in the soil particle.
So whenever particularly the Rayleigh wave
is passing, the particle motion will be like
this.
So there will be some kind of elliptical motion.
The last one is called as Love wave.
Here the particle motion will be translational,
like side wave motion will be there.
These again are classified as surface wave,
because more prominent and near the surface
of the ground.
So these are precisely four kinds of waves.
Whenever we go for seismic methods, there
are precisely four prominent kinds of waves.
Again, you can have other waves, which are
mostly generated because of reflection happening
from heterogeneities present in the medium,
like there can be any local pockets which
is available in the medium.
So whenever that local pocket is there, again
that will be considered as additional source
of disturbance of additional source from which
the waves are reflected or some energies is
getting contained.
So this will be again recorded at the receiver
at later stage.
Those are again called as -- I mean those
waves are called to give you an example, those
are called coda waves, which are generating
because of the heterogeneity present in the
medium.
Whenever we go for the interpolation, we generally
consider any soil layer, this is homogenous.
So whatever material property you are getting,
it is like applicable throughout the material
unless there's too much contrast or there's
too much change in the medium characteristics.
Otherwise, up till that depth, till which
there is not too much contrast in the material,
the entire material will be characterized
as one kind of medium.
So this is like -- so you will be able to
-- when ever we are interested to go for seismic
methods, we are interested to find out based
on this, we are actually inducing some kind
of vibration and depending upon what kind
of disturbance it is creating, we will be
interested to record those disturbance and
depending upon the characteristics or time
of arrival of those disturbances at your receiver
end, you will be able to interpret the physical
property of the medium.
Now one question which comes to mind is how
you quantify which kind of wave you are actually
determining like you have put the receiver,
but receiver whether it is recording P wave,
whether it is recording S wave, whether it
is recording other waves.
So that depends upon what kind of disturbance
you are actually creating at the source.
So whenever you are going for vertical impact
load or explosion, mostly your shock will
be dominated by P wave dominant.
So if you put a receiver and this is a shock
or this is your source, mostly it will be
containing your -- maximum content or maximum
energy will be transferred through P wave.
Similarly, if you are taking some horizontal
striking, of a rod, mostly it will be contained
by S wave dominant.
So if this kind of disturbance is there where
you are actually -- some rod is there, which
you are striking horizontally against any
later movement, resisted/restricted against
later movement.
Otherwise fi later movement is there, then
it will not cause much -- I mean most of the
energy will be confined to the surface itself,
and it will not cause any kind of disturbance
or any kind of shockwaves or shear waves at
deeper level.
So this is like when we are interested to
find out, when we are interested to determine
P wave velocity, we can go for vertical impact
of explosion, when we are interested to find
out S wave velocity, you can go for -- accordingly
like depending upon what kind of source is
there, you can decide which kind of wave your
receiver is detecting here.
Now as I've been mentioning here, because
this is method of exploration.
So depending upon your velocity of the medium,
we call it as VP that is primary wave velocity.
This is called VS, so depending upon what
kind of material is there, I am going to give
you here some chart, and then the value of
VP in kilometer per second.
Let's see what is the overall velocity of
primary wave of propagation through different
material.
So to give you an example, if air is a medium,
the primary travel travels at a rate of 0.3
kilometer per second, which is almost close
to the velocity of the sound.
When we go for water, the primary wave velocity
increases from 1.4 to 1.5.
Similarly, ice is like for different medium,
what is the primary wave velocity, which if
some seismic recording station is there, you
will be able to detect the primary wave, because
of any seismic activity happening in the other
parts.
Then clay, you can have primary wave velocity
from 0.5 to 2.1.
Then sand, you can have primary wave velocity
from 0.2 to 2.0.
Then granite, you can primary wave velocity
as high as 4.5 to 6.0.
If you remember, granite we had also discussed
like this is very intact rock.
So it is offering more and more resistance
to any kind of primary wave passage or any
kind of movement which the primary wave offers
in terms of compression or rarefaction.
That's why the primary wave velocity is like
this.
So based on seismic method, if you are able
to quantify your primary wave velocity is
in this range or this range or this range,
that is giving you an indication that a medium
through which this primary wave is propagating
and getting detected by the receiver, this
medium is either granite, sand, clay and so
on and so forth.
So that's how you go for interpretation of
subsurface medium.
You try to measure some wave, the physical
properties of the medium, and then depending
upon what is the range of physical properties
of different medium directly particularly
when you're going for seismic methods or indirectly
when you're going for other geophysical methods,
you will be able to identify what kind of
medium is there, what are the physical properties
of the medium, and at times what is the depth
of the medium.
So this is again as I mentioned here like
each of these properties are related to physical
properties of the medium.
So let's see how these like VP that is primary
wave velocity , it can be related to the elastic
constant like [K + 3/4G/?]1/2 where K is bulk
modulars of the medium, this G is shear modulars
of the medium ? is mass density of the medium.
So based on this you can understand whenever
your primary wave velocities give you indication
of some other physical properties, elastic
constant of the medium, and VP is P wave velocity.
Same way you can quantify, because shear wave
is causing disturbance in terms of shear,
like shear failure, so it is directly related
to your shear velocity.
One, two so VS is your S wave velocity.
So in the previous table I had given you the
range of primary velocity for different medium.
Same way you can determine the shear velocity
of the medium.
Third is VP/VS itself can be correlated to
[2 (1-?)/(1-2?)]1/2, so that's how VP and
VS ratio of a medium is related to the -- is
almost constant where this is called is Poisson's
ratio.
So this way you'll be able to determine -- once
you know one value, you will be able to determine
based on the recording time of the kind of
wave, the wave which is being dominated by
the source depending on what is your choice
of the source, you'll be able to determine
VS value, VP value, once you know both the
values, you can determine Poisson's ratio
or vice versa.
So just by measuring this time of travel or
time of arrival of different kind of wave
between the source and registry, but you will
be able to quantify what kind of medium is
available.
Now let's see about seismic.
So in order to go further into detail, we
will be discussing about what is seismic reflection
survey, seismic reflection we will be discussing
in next class.
Now as I mentioned earlier also, we are interested
to find out what is the thickness of the medium.
We are interested to find out what is the
P wave velocity of the medium, and because
for any disturbance or any reflection to take
place at the interface, I am considering this
interface's indication of too much contrast
in the medium characteristics.
I am writing here interface.
Otherwise, interface is not there or if the
medium contrast is not significant, what will
happen this bound the ratio downward unless
there will be bound for too much contrast
in the medium.
Now I am keeping a source here.
This can be explosion, it can be vertical
impact; I am keeping a receiver here, very
much similar to the previous slide.
So because of this there will be wave fronts,
okay.
So like this, wave front, it reached here.
Again, some portion of energy will be reflected
back.
I kept here, I mean keep in mind like the
angle of incidence here will be equal to the
angle of reflected wave.
Because it is reflection so, the angle of
inclination will remain the same.
So this is known as reflected wave.
So in addition to this, there will be some
kind of wave which is traveling along the
surface, which is called as direct wave.
So at the receiver if you see the receiver,
you can see two kind of disturbance.
First time will be the arrival time of direct
wave and then it might minimize, and then
again you will have second wave, which will
be like arrival time of reflected wave.
Now I am giving you signature of receiver
signal.
This is very simplest form.
As more and more actual field condition keep
on adding here, this will become more and
more complicated.
So I can say simplest form of receiver signature.
Now I am trying here -- so I will be interested
to go for two recordings of time at receiver.
One is time of arrival of direct wave.
So when your receiver detects direct wave,
direct wave time of arrival.
That will be called as 'td'.
Second one is time of arrival of reflected
wave that is 'tr'.
Now you see here, for a known distance between
the source and receiver that is L, what will
be the VP value.
That is VP1 for layer number one.
VP1 can directly be called as L/td, like for
known distance between a source and receiver
if you know what is the time of first disturbance
to be detected the receiver.
You will be able to determine what is the
value of VP1.
Now you see the second one, you are interested
to find out thickness of the medium.
So that time of arrival of reflected wave
is a function of, so you say it is OAB that
is called as OA + AB that is the length divided
by VP1.
VP1 is the propagation part velocity, so this
P wave velocity along the propagation path
of reflected wave between the source and receiver.
Now this can be called as -- so OA and AB
are equal.
You can call it as 2AB/VP1.
Now because this incidence angle is equal
so this length will be L/2 and this another
one will be another L/2.
So you can call it as 2 times -- see this
one you're interested to find -- suppose OA,
you are interested to find out that is AB
equals to -- so this will (L/2)2 considering
this as OAC as right angle triangle or ACB
as right angle triangle.
So (L/2)2 + H2 that will be the length of
AB divided by VP1.
Take this 2 inside the square root, so that
will be L2 + 4H2 square root divided by VP1,
so tr.
So for known values of tr, L, VP1 known -- so
this is equation number one, this is equation
number two -- your value of H can be determined
from equation, estimated from equation two.
So by means of one source, one receiver, based
on detection of time of arrival of direct
wave, time of arrival of reflected wave, that
is tr and td, you are actually able to determine
what is the P wave propagation velocity.
Based on your P wave propagation velocity,
you can quantify whether you medium is sand,
silt.
Of course, you can better classify in terms
of stiff medium, soft medium, hard medium,
and then you can also determine what is the
thickness of the medium.
So this is the overall thickness of the medium.
This is called the seismic refraction method.
So let's solve numerical example here in order
to understand better like what is the way
you can actually quantify this medium characteristic.
So the numerical is like a loose deposit of
overconsolidated clay is underlain by bedrock.
Okay, just before solving this numerical,
I would like to highlight, like the numerical
problem which I showed earlier it was related
to this, the derivation which I showed you
here it was considering relatively leveled
ground.
Same way if you go for inclined surface, so
where this is the source and this is your
inclined interface considering at an angle
a.
So this problem, here also you're interested
to find out what is the thickness, what is
the soil type here.
What is the thickness?
So thickness maybe I can tell you here Da.
What is the inclination here?
This can be solved by keeping the source and
receiver.
So two receivers actually you can keep, one
on one side, other receiver you can keep either
here or you can keep here itself.
Again, you will be having two measurements
here.
so this is -- Da is the depth of investigation
at 51:05 both the receiver and the source.
So suppose tR/S is the time of arrival of
reflected wave when receiver and source at
same location.
If you know this, you will be able to determine
what will be the value of Da that is equals
to tR/S/2 VP1.
VP1 you can determine -- if you know the value
of this, that will be equals to L. So VP1
again based on detection of direct wave arrival
that will be called as t direct at receiver.
So that will be called tdR.
So you using first one -- I am calling this
one as first one, because based on this we
will be determining first the VP1 value, put
that VP value there and you will be able to
determine what is the Da.
Now sina you will determine, so that will
give you what is the angle inclination.
You can determine based on the formula which
is as VP12 t that is time of arrival of reflected
wave, that is tr -- yeah, time of arrival
of reflected wave at receiver, plus time of
arrival of reflected wave -- direct wave at
the source when both are kept at the source
and receiver, then (trR - trS) - L2 over 2
tR/S . VP1.
L. So tr, this is called as the time of arrival
of reflected wave, then you kept the receiver
at the source itself.
This is the time of arrival at the receiver.
Okay, I can call it as receiver 1, R1.
So time of arrival of reflected wave at receiver1,
time of arrival of again reflected wave when
source and receiver are kept at some location.
L is the distance between the source and the
receiver1.
tR/S is the time of arrival between the source
and the first receiver -- source and receiver
both are kept at the same location.
So that's how you can determine the value
of a.
Once the value of a, value of Da, once value
of VP1 all are known, that will help you understanding
who variation of interface layer is taking
place and what is the physical property of
the medium.
Okay.
So this is a numerical problem.
A loose deposit of over consolidated clay
is underlain by a bedrock.
Previous subsurface investigation in the area
suggest that the bedrock is almost horizontal.
So almost horizontal means you have to take
up the first example where the bedrock was
almost horizontal and you determine the thickness
as well as primary velocity of the medium.
During a seismic reflection survey, the receiver
marks the arrival of waves as 41 milliseconds
and 267 milliseconds.
So based on that difference between the two
values you can understand the first arrival
will be direct wave, second arrival will be
for time of arrival of reflected wave as a
result of impact loading, which is kept at
37 meter from the receiver.
Determine P wave velocity, thickness of soil
layer and also using the Poisson's ration
0.3, determine the value of S wave velocity.
So this is the problem.
So how we will solve this problem?
First of all, write what are the things given
here.
So we have time of arrival of reflected wave
that is 267 milliseconds, time of arrival
of direct wave which is given as 41 milliseconds.
Remember, be careful with the units and then
distance between the source and receiver which
is given as 37 meters and Poisson's ratio
which is given as 0.3.
We are interested to find out VP, H and VS.
you can call it as one also because one layer
is there.
So going with the derivation which we did
for seismic reflection survey first part when
the bedrock is horizontal.
So first one will VP1 that will be equal to
L/td that is 37/41, 41 is given in milliseconds,
so you can covert it to seconds.
That's how you will get to know what the value
of VP that is 902.44 meter per second.
Now second one you are interested to find
out, the thickness.
So you can directly determine the thickness
that is H. You can determine as tr equals
to square root 4 . H2 + L2/VP1 that is root
4*H2, you have to determine, plus 372 over
VP1 is 902.44, so you solve it and this tr
value is given as (267*10-3).
So if you solve this equation, you will get
to know the value of H = 109.04 meter.
The third part is determination of VS value.
So we know like VP/VS is given as [2 (1 - ?)/(1-
2?)]1/2, nu is given as 0.3, so you will be
determine the value of [2 ( 1 - 0.3)/(1 - 2*0.3)]1/2
that will come as 1.87 that is VP/VS.
So VS you can call it everywhere 1 also, VS1
will be called as VP1/1.87 that is 902.44/1.87
that comes out to be 482.59 meter per second.
Be careful with the units, otherwise you will
end up in -- though you will report your value
in meter per second, but your time of arrival
is given is milliseconds, because this is
considering the layer thickness of just 119
meters, and considering the VP velocity of
300 millimeter per second of that order, so
it will take more time for -- I mean it will
take relatively lesser time rather it should
be quantified in terms of seconds.
So with this timing, I hope this example in
today's class had given you sufficient idea
like what is the source, what is the receiver
and depending upon the choice of the source
what kind of wave be dominating and what will
be the -- how the energy contained or generated
at the source will be contained to the receivers
depending upon the choice of the source, and
once you know the choice of the source depending
upon the time of arrival detected at the receiver,
you will be determining -- corresponding either
P wave velocity, S wave velocity and so on
and so forth.
So that's how you can determine and then based
on the numerical solve you can understand
how you determine, how you interpret the properties
of the medium.
So you can determine the VP value, VS value,
Poisson's ratio once it is known to you or
you're able to detect VP and VS both, you
can determine the Poisson's ratio itself.
And then once you know the medium density
also, which probably you can determine from
geotechnical test, you can also determine
the value of shear modulars, bulk modulars
and other things.
So thank you all.
[Music]
