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Part 1: Seismic techniques
by A. N. BURTON",
BSc, ARCS, FGS
SEISMIC METHOD of subsurface
investigation consists essentially of introducing seismic energy into the ground to
be investigated and measuring the time
taken for the energy to travel along a
defined path through the ground to a seismic detector located a known distance
from the energy source. The energy travel
time between source and detector is measured to an accuracy of 1 milli-second or
better, by a seismic recorder.
The principal seismic techniques in common use at the present time are:
continuous seismic reflection profiling
for marine investigations, and
for land
seismic refraction
profiling
investigations.
Continuous seismic refraction profiling is
also used in offshore investigations
and
seismic reflection profiling onshore, but
these applications tend to be more expensive although there may be occasions when
the value of the information obtained may
justify the greater expense.
In reflection profiling the time (t) required for seismic energy to travel from
source S to detector D along the path
THE
—
—
soirector,
Herts.
Hunting
Surveys
Ltd., Sorehamwood,
shown in Fig. 1 is given by:
+
Vrpr
where V,
d'........(1)
velocity of the upper
layer
x
= the
d
=
distance between source
and receiver and
the thickness of the upper
layer
Similar expressions can be obtained for
cases with more than one refiecting boundary.
refraction profiling
In
t for the path shown
in
VoVr
the expression
for
Fig. 2. is:
)
VrVo
= the
seismic velocity of the
lower layer while the other
symbols have the same meaning as in equation (1) above.
The path shown in Fig. 2. is known as
the critical refraction path since the angle
of incidence (ic) of the energy is such that
the angle of refraction is a right angle.
where
V,
Continuous
seismic reflection
profiling
articles have been written on
seismic reflection profiling. A
of these publications is listed
Numerous
continuous
vo
vi
reflection
Fig. 1. Diagram illustrating
seismic energy at subsurface interface
of
selection
in the bibliography.
Seismic energy incident on an interface
between layers of two different materials
may be either totally reflected, or partially
reflected and partially transmitted. For near
vertical incidence of approximately plane
wavefronts the coefficient of reflection R
and the amplitude transmission coefficient
T are given by:
Vo,
8
Vi
Fig. 2. Diagram illustrating refraction of seismic energy at subsurface interface
32
Ground
Engineering
V„p„+ V,p,
T
= seismic
—Vrpr
=
2Vipr
V.ps+
V p
".(3)
".(4)
where V,, V„and p, p, are the seismic
velocities and densities of the two layet3.
The term Vp is known as the acoustic
impedance of the material. R is negative
when Vr»
V,». Thus when energy is
reflected from an interface between a dense
layer overlying less dense material a 180
phase change occurs.
In general, the depth of penetration of a
seismic pulse into the seabed is inversely
proportional to the frequency of the pulse,
while the sub-seabed
layer resolution is
directly proportional to frequency. Thus, a
single energy source may not be capable
of both the depth of penetration and the
layer resolution required for a particular
investigation. Other factors affecting the
performance of continuous seismic profiling (CSP) systems are power output and
the firing rate of the energy source.
A wide variety of CSP systems are commercially available for off-shore investigations. These systems have been designed
to meet differing requirements,
ranging
from structural investigation of bedrock to
depths of several hundred
metres for a
tunnel project, to the detailed investigation
of the upper few metres of the seabed for
a submarine pipeline. A typical CSP record .
is shown in Fig. 3.
The CSP systems currently available are
sparkers, boomers, pingers and air guns.
Sparkers produce a seismic shock wave in
the sea by the explosive formation of
steam bubbles resulting from the discharge
of stored electrical energy between two
or more electrodes immersed in the sea.
Boomers operate by the explosive repulsion of a metal plate, spring-loaded against
an insulated coil, when stored electrical
energy is discharged
through
the coil.
Pingers operate with an acoustic pulse
generated by the oscillation of piezoelectric
or
maqneto-strictive
transducers
following the discharge of electrical energy
into the transducer. Air guns produce a
shock wave by the explosive release of
high pressure air from a pressure chamber
immersed in the sea.
Sparker and boom er energy sources
develop a double pulse of energy unless
steps are taken to suppress the second, socalled, "bubble" pulse. The latter is caused
by the contraction and explosive re-expansion of bubbles of steam formed after the
initial
energy
discharge
(see Sargent,
)
TABLE I. PERFORMANCE CHARACTERISTICS OF CSP SYSTEMS
(a) Sparkers
Descri ption
Heavy duty sparker
Sparker
Light duty sparker
Multi-electrode
sparker
Power output Firing rate
(per second)
(joules)
Frequency
band (Hz)
000-8 OXy'.25-4
50-1 000
1
Pulse length
(milliseconds)
8-20
200-1 000
2-10
200-2 000
5-10
20-200
2-20
50-10 000
3-5
200-1 000
2-4
200-1 000
2-4
Penetration
(milliseconds)
)
150
Types
available
Remarks
Penetration good in
nearly all geological
environments and useful
therefore for investigating structure in
bedrock. Unsuitable for
detailed studies of
shallow sub-seabed
geology
—EGG Sparkarry
—Aquatronics
Has been used successfully in a wide variety
of surveys including
harbour and pipeline
route surveys. Equipment is compact and
requires relatively
small power supplies
50-150
Super Sparker
—Teledyne
—Huntec Sparker
Has comparable depth
resolution to the
light duty sparker but
greater penetration
*Higher power outputs are not used, generally,
for site investigations
(b) Boomers
0,25-3
Boomer
3-5
200-3 000
)
100
Depth resolution similar
the light duty sparker,
but penetration superior.
Boomers have been used,
therefore for similar
types of investigations
to sparkers but are
preferred for water depths
in excess of 80-100m.
They have practical disadvantages of being heavy
and cumbersome and
requiring larger power
supplies.
Precision boomer
0.5-2
400-14 000
2-6
100-300
(c)
Pinger
up
to 20
1
000-12000
Air gun
0.1-4
1969). Suppression of the "bubble" pulse
reduces the power output and, hence, the
depth of penetration of the system, but
reduces the pulse length and thus enhances the layer resolution.
Pinger sources are pure tone sources
and do not give rise to a "bubble" pulse.
However, their use is limited by their
relatively high frequencies, the lowest frequency attainable with these sources being
about 1Kc/s.
The principal characteristics of some of
7-2 500
Combine good depth
resolution and good
penetration in most
geological situations.
2-50
Depth resolution
excellent but penetration
varies in inverse ratio to
the grain size of the sea
bed sediments.
Air gun
70-1 500
the better known CSP systems are listed
in Table I. This gives a general guide to the
capabilities of the various systems, but it
is advisable to consult an experienced firm
or consultant before selecting a system or
combination of systems for a particular
investigation.
the depth of penetration
In considering
of the various systems listed in Table I it
should be borne in mind that in practice
the depth to which CSP records can be
interpreted may be limited by the depth of
—Huntec
—EGG Uniboom
Pingers
0.2-1
(d)
30-50
—EGG
to that obtainable with
Capable of deep penetration in shallow to
moderate water depths,
moderate penetration in
deep water.
—EGG
—ORE
—Edo Microprofiler
—Sonia
—Bolt
—Petty
—Unipulse
Birdwell
water. This is because multiple reflections
of energy between the seabed and water
surface may mask useful reflections redepths greater
ceived from sub-seabed
than the water depth.
of the CSP method
Other limitations
are:
(i) Penetration may be reduced or not
achieved at all in seabed soils with high
acoustic impedance (e.g. very stiff or hard
boulder clay) in deep water, and highly
soils (for example, peat) in
organic
January,
1976
33
=««J«r
% .
~»
"--0g,
5
»S
I
}y
~
5 III}}00
5
~~}at 55
00
i+I
5
II
150 1)5
55}
any water depths.
(ii) Rough sea conditions with high ambient noise levels may make operation of
CSP systems impractical due to masking of
the seismic signal by the noise. This problem may be overcome to some extent by
mounting the energy source/detector system on the hull of the vessel, beneath the
zone of turbulence, as with the ORE pinger,
or by deep towing of the system.
8z Refraction profilin
Published literature on the seismic refraction method is even more abundant than
that on the CSP method. However, an
excellent review article published recently
(Green, 1S74) provides a very useful guide
to the literature and the present stage of
development of the method.
Rocks and soils normally transmit two
main types of seismic shock wave, namely
shear (S) waves and longitudinal
(P)
waves. The relationships
between
the
velocities of shear and longitudinal waves
(denoted V, and V, respectively) and the
elastic moduli of the material (assumed to
-.===-'IIII
aI
-'=:-=-~iINK),.
z
Qg ~J
is} 5 f}}sI
Fig.
3.
Typical CSP profile
~
i%%i(
AM }li}5.~
f, }}Il:,II!
be isotropic) are:
}%-'I(xi
fj"..':::
V„=
where n
~ ~
)
"A"-'k}}
'tsfp3 gm
'!r,00''.00,.„".0 I„.: '
w'll
} 3ri@9$
@,
, 5,, @AC.~%w,
'f'I:XL':,'L5%,.'*)B5}h0
~J /:"
..;-'I},
I
'.<
('JJI '.-, '
aECC~%a, =,:-'4 =:--;8::}}-0}." .~f-',=';":z..-J-
I
Fig. 4. Typical 12-channel seismic refraction record
—; =,.„(5g
V,
=
the shear modulus
of the
material
K = the
bulk
modulus
of the
material
—the bulk density of the material
p
Clearly V, is greater than V, and, therefore. P-waves are generally the first to
arrive at the seismic detector(s), and, with
most recording equipments currently used
in site investigations,
they mask subsequent arrivals. A typical seismic refraction
record, consisting of P-waves, obtained
using a 12-channel equipment, is shown in
Fig. 4.
Shear wave detectors are available, and
are used where measurements
of shear
modulus and Poissons Ratio are required.
Since the shear modulus of fluids is zero,
TABLE II. PERFORMANCE CHARACTERISTICS OF REFRACTION PROFILING SYSTEMS
Instrument
RS-4
Manufacturer
Dresser Sie
RS-44
Dresser Sie
Recording
channels
12
24
Recording
Gains
speed
Standard 11.6in/sec
Max
23.2in/sec
Filters
AGC
Adjustable from
100 to 6 000 in
ten 6 db steps
1-10 db vernier
Standard 11.6in/sec
Max
23.2in/sec
ditto
4 positions
Off
—No filter
Blaster
Timing lines
Integral
10 millisecs
Separate
10 millisecs
Separate
2 millisecs
Separate
2 millisecs
Separate
Digital time
measurements to
desired
points on
seismic
wave form
16 Hz
35 Hz
75 Hz
TRIO
ABEM
12
Controlled:—
50, 100, 200cm/sec
5 to
Uncontrolled
8 steps
1
500
adjustable
in
High and low
3-4m/sec
ABEM
TRIO
Bison
ENHANCE- Instruments
MENT 1570
SIGNAL
ER 75
Electrotech
24
SINGLE
12
sensitivity
ditto
Sweep time
Engineering
Geophone Gain
Control
Adjustable from
Oto10
Recording time
2 000
adjustable
Ground
in
ditto
milliseconds;
25, 50, 100, 250
500 switched
0.2-0.4 secs
34
2 AGC
positions
ditto
Integral
S-waves are not transmitted by water.
Energy sources used in seismic refraction profiling must be able to produce
sufficient energy at a frequency
which
matches the band-pass frequency of the
In
detecting and recording equipment.
practice the two energy sources commonly
used are gelignite explosions and the impact of a dropping weight, or sledge hammer blow on a steel plate embedded in
the ground. The latter source is used where
depth of investigation is not greater than
about 10m, the former for deeper investigations. A dropping weight device is illustrated in Fig. 5.
At any single firing of the energy source
of the released energy
the proportion
transmitted
downwards
into the ground
under investigation determines the depth
of investigation achieved. Burying an explosive charge in the ground and tamping
is advantageous, but if the surface layer is
composed of loose unconsolidated material
or peat it may be necessary to insert the
gelignite charges below the surface layer
to obtain the required penetration.
The characteristics
of some of the
refraction seismic systems commercially
available at the present time are listed in
Table II. A problem affecting the performance of all systems is that cultural and
atmospheric noise occupies the same frequency band as the seismic signal and may
mask the latter. This problem has been
overcome by the integrating seismograph
developed
by Bison Instruments
Inc.,
which adds the signals
received from
repeated firings of the energy source at
each location. This reinforces the seismic
signal because it is in-phase, whereas the
random noise tends to cancel out.
The seismic refraction method will give
reliable results
the following
providing
conditions are satisfied:
(i) The seismic velocity of successively
deeper subsurface layers increases with
depth below ground level.
(ii) The velocity to thickness ratio of each
subsurface layer is less than a certain critical value in relation to the overlying and
underlying layers.
(iii) The seismic velocities of the subsurface layers are effectively constant over
the length of the geophone spread.
If condition (i) is not fulfilled, and a low
velocity layer is present, critical refraction
does not occur. The seismic energy is
refracted towards the normal to the interface and passes downwards into a higher
velocity layer before being returned to the
surface by critical refraction. Thus, the low
velocity layer, called a "blind-zone", is not
detected, and consequently the calculated
depths of the deeper refractors will be
greater than the true depths.
If condition
(ii) is not satisfied a thin
layer will not be detected because the
energy refracted by it will be masked by
energy refracted by the underlying higher
velocity layer. This is known as the "hidden-layer" effect, and results in the calculated depths of the deeper refractors
being less than the true depths.
Condition (iii) is probably fulfilled less
frequently than the other two particularly
in the case of superficial
deposits and is
probably the most frequent cause of depth
determination
errors in the interpretation
of seismic refraction records. Velocity variations in the bedrock are less serious in
that they affect depth calculations to a
smaller degree than the velocities of overlying layers and are usually detectable.
These limitations indicate the importance
of boreholes to control the interpretation
of the geophysical data, not only for correlation of seismic velocities with geological
strata, but to check whether the conditions
mentioned above are satisfied.
Tidal zone investigations
Seismic investigations in the tidal zone,
between high and low water, are carried
out by a combination of CSP and seismic
refraction methods. The CSP method is
used, at times of high water, to obtain
information
as far inshore as the vessel
and conditions will allow; the refraction
profiles are carried as far offshore as possible during the low water periods. The zone
of overlap between the two methods
should be of the order of 500m, if possible,
to ensure a good correlation between the
two methods.
of seismic records
Interpretation
Interpretation of seismic records is very
much a question of the skill and experience
of the interpreter. The latter should be a
qualified geologist as well as a geophysicist
since the end-product of the interpretation
is essentially a geological profile. A knowledge of the engineering classification of
soil and rock materials is also useful in
I
Fig. 5. Drop-weight
IIJ,
applying the results of correlation boreholes to the interpretation of seismic survey work.
Seismic velocities and hence the thicknesses of sub-seabed layers cannot be
determined
directly from CSP records
obtained using an energy source and one
detector with fixed separation (see equation (1) above). Seismic velocities are
determined from correlation boreholes or
by the seismic refraction method.
The sub-surface layer resolution which
an experienced interpreter can derive from
sparker and boomer CSP records is often
considerably better than the figures for
pulse length given in Table I would indicate. This is due to the fact that significant reflections frequently can be followed
through the "bubble" pulse trace on the
CSP record (see Sargent, 1969). Thus,
layer resolutions of 3 milliseconds for the
EGG Sparkarray and 1 millisecond for the
Huntec sparker are obtained in practice.
Precision boomers, which are designed
to remove the "bubble" pulse by operating
near the sea surface, may give a layer
resolution better than 0.4 milliseconds in
favourable conditions.
The identification
of phase reversed
signals received from interfaces between
a dense layer and underlying
less dense
material
is possible by an experienced
interpreter
the energy pulse is
when
asymmetric and half wave rectified (see
Gauss, 1970). This is of considerable
importance in site investigations
for the
foundations of offshore structures, and for
dredging feasibility investigations.
In refraction work the seismic velocities
of the subsurface layers and layer thicknesses can be determined directly from the
seismic data (see equation 2 above). V,
and V, in equation (2) can be determined
by plotting a graph of t against x (see
Fig. 6). The diagram in Fig. 6 shows the
results of four firings or shots of the energy
source. Shots made close to the end geophones of the spread (end-shots) produce
the two profiles composed of two linear
segments
representing
different seismic
velocities. The V, segment represents the
direct wave travelling in the surface layer,
while the V, segment
represents
the
energy refracted from the lower layer. The
thickness d of the surface layer can be
calculated either in terms of the time intercept t, and the velocities V, and V,, or in
4I
devicein operating position behind Land Rover
January,
1976
35
X
c
Fig. 6. Diagram illustrating
subsurface layer
DISTANCE
time-distance
terms of the critical distance xo and these
are
velocities. The respective formulae
given below:
V,V,
Vo'j~
(Vr'
2
«,(v,
—v.)<
""(6)
""(7)
The purpose of firing from each end of
the spread is to take account of any dip
of the lower layer.
lines for the shots
The time/distance
beyond the ends of the spread (out-shots)
show a single velocity V„ indicating that
all the first arrivals from these shots travelled in the lower layer. This enables the
velocity of the latter to be determined
more accurately and confirms that it is a
true refracting layer. If the change in slope
between V, and V, for the end-shots had
been caused by a 'lateral change in velocity,
then a similar time-distance graph would
have resulted from both the end and
out-shots.
Equation (6) above enables a depth
determination
to be made at the end of
each geophone spread in terms of the
half-time intercept t,. It is sometimes possi-
2
ble to calculate the effective value of the
half-time
intercept under each geophone
so enabling depth calculations to be made
along the entire length
of the spread
(Hawkins 1961).
The theory outlined above can be extended to three or more layers for both horizontal and dipping strata.
Measurements
of the compressional
wave velocity (V„) can be used to derive
values for the dynamic modulus of elasticity of subsurface
layers. Brown and
Robertshaw (1953) obtained the following
empirical expression for Youngs Modulus:
E
=
V,
'
10-'b/in'-.
(6)
This relationship appears to be valid for
values of V, greater than 3km/sec and for
materials in the density range 2.2 to 2.6.
36
Ground
Engineering
or seabed cores, the geological strata
present on the site and their depths.
(ii) At the detailed investigation stageto check the continuity of strata between
boreholes; determine the dynamic moduli
materials;
of sub-seabed or sub-surface
of hard strata;
estimate the rippability
locate the position of geological faults and
other bedrock structures.
A seismic survey carried out as part of
a preliminary
site investigation provides
basic information on the geology of the
site, such as depth to bedrock. This information
can be used for planning the
detailed site investigation, by drilling and
other direct methods, to give the best
results for the least expenditure.
investigation,
the
During the detailed
interpretation
of the seismic work carried
out during the preliminary stage should be
continuously up-dated to take account of
the new borehole information available.
In view of this it is advisable to retain
the services of the geophysicist employed
on the preliminary stage survey during the
detailed investigation stage.
plots for seismic
energy refracted
from
a
V, is also related to rock quality. Deere
et al (1967) have shown that:
s
(
x 100
V, Laboratory
R.Q.D. (9)
This relationship is used in practice to
estimate the rippability of sub-surface rock
formations.
Shear wave velocity (V,) measurements
can be used to derive the dynamic shear
modulus of sub-surface materials from the
relationship:
n
............(
10)
= Vstp
(see equation (5) above)
Equations (8), (9) and (10) should
be used with caution and due regard
their
inherent
only
to
limitations,
but provided
is exercised, useful infor-
proper control
mation can be obtained.
Seismic techniques
investigation
in
site
Seismic techniques are used to a much
than
greater extent in site investigations
any other geophysical methods. Offshore
a sub-bottom CSP survey, combined with
a side scan sonar and depth sounding
survey all carried out at the same time, is
almost standard procedure at the present
time. Onshore, seismic refraction is not
employed so extensively. This difference
in utilisation
is due mainly to economic
factors. Offshore, the costs of obtaining
sub-seabed
information
and
by drilling
coring are so high that 8 seismic survey
pays for itself many times over by providrequired for planning
ing the information
an economical
drilling
and coring programme.
Onshore, although the seismic
method is capable of performing the same
role, the cost advantages are far less obvious, since onshore drilling is less expensive
than offshore drilling,
and land seismic
survey costs more than marine seismic
survey per unit length of line surveyed.
The role of seismic methods in site
investigations generally can be summarised
brieRy as follows:—
(i) At the preliminary investigation stage
to determine as accurately as possible,
in conjunction
with correlation boreholes
—
Future developments
In the marine field the major developments in progress at present are towards
operations in deeper water. The attenuation of the seismic signals by a deep water
column limits the use of present surface
or near-surface towed systems to water
depths of about 150m. To overcome this
problem deep towed sparker and boomer
systems have been developed and trials
were carried out in the North
Sea
last year. However, there is a practical
limit to the depth of water in which systems towed behind a surface vessel can be
employed to survey the seabed. When this
limit is reached seabed survey work will
have to be carried out from submersibles.
In shallower
water, improvements
in
existing instrumentation
can be expected
to continue both as regards data collection
and data processing. These improvements
should lead to improved results from seismic surveys for harbour and other shallow
water investigations.
Onshore also, instrumentation
is being
steadily
improved,
but
probably
the
greatest scope for advance in this area is
in the way seismic techniques
are employed in site investigations. A recent article (West and Dumbleton, 1975) concluded that the best way of using geophysics
in site investigation
was to integrate it
fully with other methods. If this advice is
followed by engineers it is considered that
the resulting improvement in the value of
the data obtained from the geophysical
work would be increased considerably.
References
J.
P. D. and Roberrshaw,
(1953): "The
in-situ measurement
of Youngs Modulus for rock
by a dynamic method" Geotechnique 3.7.283.
Deere et al (1967): "Design of surface and near
surface construction in rock" Proc. 8th Symposium
Rock Mechanics. Minnesota 237-302.
Gauss, G. A. (1970): "Acoustic techniques for
georogicar
studies with particular reference to
dredgmg problems" Norspec 70.
Green, R. (1974/i "The seismic refraction method
a review" Geoexploration 12, 259-284.
Hawklns. L. V. (1961): "The reciprocal method of
routine shallow seismic refraction investigations"
Brown.
—
Geophysics 26, 6. 806-819.
Kelland, N. C. (1970): "Improved interpretation
using a dual channel continuous seismic profiling
system"
European
Association
of Exploration
Geophysicists.
Safgenr, G. E. G. (1969): "Further notes on the
application of sonic techniques to submarine geological
investigations"
Ninth
Commonwealth
Mining and Metallurgical Congress.
West and Dumbleton (1975}: "An assessment of
in
geophysics
site investigation
for roads ln
Britain" Transport and Road Research Laboratory
Report 680.