A Review of Seismic Reflection Method

Seismic reflection method is a geophysical site investigation method that is extensively used to ascertain the geometry and physical characteristics of subsurface structures. Read on to know more about the developmental history, equipments and method, theory, data analysis, productivity and costing, advantages and disadvantages of the method.

A Review of Seismic Reflection Method

1A Review of Seismic Reflection Method
Table of Contents
A brief summary of the method..................................................................................................................3
Developmental history.................................................................................................................................4
Equipments and method:............................................................................................................................5
Theory:........................................................................................................................................................6
Data analysis................................................................................................................................................7
Productivity and costing:.............................................................................................................................8
Advantages and disadvantages.................................................................................................................10
References.................................................................................................................................................11
Appendix...................................................................................................................................................11

2A Review of Seismic Reflection Method
A brief summary of the method
Seismic reflection method is a geophysical site investigation method that is extensively used to
ascertain the geometry and physical characteristics of subsurface structures. In the USA, the
method is employed in hydrocarbon exploration, investigation of crystal structure, groundwater
studies and determination of shallow faults. In this method, a source of seismic energy is set off
near the surface. When the seismic waves emanating from the source encounter an interface
between two subsurface layers with distinct physical properties, the seismic energy is partly
transmitted and partly reflected. Measurement of the time taken by the seismic wave to travel
from the source to the interface and back to the surface gives us the depth of the layers. The type
of rock or sediment can be deduced from the velocity of the wave (Ucl.ac.uk, 2019).
The state of practice equipment consists of a source and a receiver. The source can be a
mechanical vibrator or an air gun. A Geophone on land or a hydrophone in water is generally
used to receive the reflected waves. The data that is gathered is stored in a magnetic tape and is
later processed and interpreted. The latest state of the art methods include the four dimensional
survey which aids in detecting changes in the hydrocarbon reservoir during the production
process.
The seismic reflection method can yield detailed representations of the features below the
surface. Often, it is possible to detect hydrocarbons directly through this method. However, the
method requires several source and receiver positions. Hence, it can be quite expensive.

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4A Review of Seismic Reflection Method
Developmental history
The academic development of the theory of reflection and refraction at an interface began when
the British geophysicist Cargill Knott derived the famous Knott’s equation in his 1899 research
paper. Later in 1919, Karl Zoeppritz’s equations were published. They provided more lucid
expressions for the amplitudes of the transmitted and reflected waves. During the First World
War, the time taken by seismic waves - that were produced by the recoil of a heavy gun- to
return to its source became a topic of active research. Although these attempts did not come to
fruition, they inspired future research work on the topic.
In 1921, a group of geophysicists (Karcher, Haseman, Ohern and Perrine) conducted
experiments in Oklahoma, in which they used a device known as the seismograph. The results of
the experiment proved that reflection seismography can yield an accurate representation of the
layers below the earth’s surface. Soon, the method was adopted by industries looking for oil and
gas reserves. By 1932, around thirty reflection seismography crews were working for a company
called the Geophysical Service Inc. (GSI) in the USA. The data was recorded on paper by means
of photo-oscillographic methods and wasn’t processed later. Recordings on magnetic tapes began
in the 1950s and the common mid-point (CMP) method of gathering data was introduced
towards the beginning of the next decade (Anon, 2019).
The digital revolution that began in the late 1950s and extended into the 1960s coincided with
the development of digital recording and data processing. The late 1980s witnessed further
advancement in technology in the form of three dimensional surveys and the use of computer
workstations for interpreting data. Data interpretation has advanced to such an extent that it is
now possible to gain information about the hydrocarbon reserves, stratigraphic features and
lithology from seismic reflection methods (Content.lms.sabis.sakarya.edu.tr, 2019).

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Equipments and method:
State of practice equipment:
There are two kinds of state-of-practice sources that are used in seismic reflection soundings.
They are the explosive and the Vibroseis sources. Gelatin dynamite, pentolite and ammonium
nitrate are the commonly used explosive sources. Vibroseis sources are hydraulic vibrating
devices that are mounted on heavy trucks and can send a range of frequencies into the surface.
These sources can also be operated in concert to produce stronger signals (Hajnal et al., 2018).
The next kind of equipment that is indispensible to seismic reflection soundings is the receiver.
The geophone is the most commonly used receiver which coverts seismic vibrations into an
electric voltage. A transducer provided with the geophone converts the motion of truck into a
potential difference. In measurements under water, a hydrophone is used (Archive.epa.gov,
2019).
A seismograph is another state-of-practice digital device that is used for recording and
controlling the data gathered in a seismic reflection survey(Liner, 2016).
State of art equipment:
The completely digital three component VectorSeis geophone is the best example of a new
generation state of art equipment that is gradually gaining popularity. Fiber optic seismic sensors
are gradually replacing geophones in the collection of reflected sesismic waves.
Nowadays, computer clusters and supercomputers are used to process the voluminous data
yielded by seismic reflection surveys.
Method:
The seismic reflection survey consists of the following steps:
 A source of energy such as a Vibroseis , air gun or dynamite releases seismic
(acoustic) waves that penetrate the surface.

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 Each subsurface contrast between the various layers reflects about 0.1 -10% of the
incident energy, depending upon its physical properties. The rest of the energy is
refracted.
 The seismic energy that returns to the surface is detected by the geophones. The
transducers provided with the geophones record the ground motion during this period.
 The geophones are arranged in a line and a switch is used by the operator to select the
active geophones for a particular recording.
 The analogue voltage response of the geophone is transferred to a seismograph,
where it is filtered, amplified and digitized.
 The shots are transferred from the seismograph to the computerized processing
system which eliminates noise, de-convolutes the data, places events at the correct
positions and performs the necessary filtering of velocity, frequency and coherence.
 Seismic data inversion is carried out on the processed data to convert it into an
abstract model of the system that is being investigated (Hajnal et al., 2018).
Theory:
Just like any other geophysical method, the seismic reflection method exploits a particular
physical characteristic of subsurface features. This characteristic is the acoustic impedance
across the interface of the two layers. Acoustic impedance (Z) is defined as:
Z=v
 is the density of the medium and v is the velocity of the seismic wave. The amplitude of the
reflected wave increases with increase in the impedance contrast between the two layers. The
proportion of reflected energy is given by the reflection coefficient R. R is related to the
impedance contrast as shown below:

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The subscript 1 indicates the first layer while the subscript 2 refers to the second layer.
Negative values of R indicate a phase change upon reflection.
The travel time of the reflected wave from the source to the receiver can be related to the
depth of the interface through the equation:
4s2=4h2 + x2
⇒ v2t2=4h2 + x2
⇒ t = (4h2 + x2)1/2/v
The travel time is t, h is the depth of the interface and x is the horizontal displacement of the
ship or the truck carrying the equipment. By measuring the travel times t at different offsets
x, the depth h and the velocity v can be calculated.
For multilayered profiles, root mean square velocity or average velocity should be used (Li et
al, 2016).
Data analysis
In reflection seismology, only the reflections are relevant to the analysis and any other kind
of data collected by the receiver is called ‘noise’. To make the collected data more amenable
to interpretation, it has to be processed extensively.
 De-multiplexing is the first step in which data from all receiver locations at a given
moment is rearranged to a format in which data for a particular receiver location at
all times is clustered together.
 The next step is the trace editing step in which noise due to surrounding machinery
or malfunctioning geophones is eliminated (Ureña et al., 2018).
 In the gain recovery step, the fall in amplitude with successive measurements is
corrected. Automatic gain functions can be employed as long as one is conscious of
the fact that the actual information has been lost (Liag-hannover.de, 2019).

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 Vertical time of travel can change because of a change in elevation of the source and
near-surface variations in velocity. This problem of alignment of reflection data is
solved by adding an appropriate time shift to the recorded curve.
 An important aspect of the data inversion process is the migration operation in which
the data points are placed at their correct subsurface locations. The dip angle is given
by the relation:
tan  = t/x
where t is the difference in travel times and x is the offset. After migration,
the dip angle is:
sin ’ = t/x
 In acoustic impedance inversions, the relation between the reflection coefficient and
the impedance is used to invert the data and obtain the acoustic impedances. The
seismic impedance sections that are generated can be directly linked to the lithologic
features.
 Generally, a computer workstation is used for interpretation of data. Jump correlation
and loop tying are the two fundamental procedures for interpretation (Schuster,
2017).
Productivity and costing:
The performance and cost analysis report presented below is that of a 3D seismic reflection
survey conducted by Naval Facilities Engineering Service Centre located in Port Hueneme,
California. The organization conducts surveys for commercial purposes and provides its services

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to both government and private organizations. The survey was conducted in Alameda,
California in the year 2000.
The cost of a 3D seismic reflection survey depends on factors such as the extent and depth of the
survey area, the minimum resolution required to locate the desired target, the geologic and
cultural conditions of the site and the accessibility of the site. The surveys that look for targets at
shallow depths require densely packed detector arrays, hence they are costlier than the deeper
investigations (Kana et al., 2015) .
Cost analysis
The daily expenses included:
 Labor: $ 1900/day
 Material cost: $ 400/day
 Fuel for the trucks $ 200/day
 Rental/Lease costs: $ 400/day
Subtotal $ 2900/day
The breakdown of the cost in terms of each activity is given below:
Research and planning for the survey $ 12,834
Running the survey $ 71,655
Data processing and interpretation $ 25,415
Performing attribute analysis $ 15,682
Drilling and sampling $ 20,849
Laboratory analysis cost $ 20,400
Generation of reports $ 37,532

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Total cost $ 204,187
Expenditure on drilling and sampling is highly dependent on the geologic conditions of the site.
The drilling costs at Alameda are usually low because the site is not located on bedrock terrain.
An inexpensive direct push method of drilling was employed.
Advantages and disadvantages
The seismic reflection method is the most widely used method for geotechnical site
investigation in the USA. It has numerous advantages over the other survey methods.
 The method can detect both horizontal and vertical variations in the properties of
subsurface features. The vertical variation is detected through vertical sounding
profile (VSP) (Content.lms.sabis.sakarya.edu.tr, 2019).
 At times, direct detection of hydrocarbon reservoirs is possible.
 The greatest advantage of this method is that it allows the mapping of multiple
interfaces in a single shot.
 The data can be interpreted to reveal more complex geologic features than other
methods.
 The method can yield complex subsurface images.
 Seismic reflection can work with small source-receiver distance, unlike the
refraction method.
 The effectiveness of this method does not depend on the velocity variation with
depth (Schmelzbach et al., 2016).
However, just like any other method, reflection seismology has its limitations as well.
 The method uses expensive and time consuming data processing techniques.
 The data collection process is expensive because the method requires several source
and receiver locations.

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 Data interpretation demands significant expertise in the method.
 It is impossible to directly detect contaminants at their usual levels in hazardous spills.
Thus, the seismic reflection method offers remarkably accurate images of subsurface features
at a high cost. Before selecting this method, it is essential to ascertain whether high
resolution or cost effectiveness is the chief necessity of the survey.

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References
1. Anon, (2019). [online] Available at:
https://eesc.columbia.edu/courses/ees/lithosphere/labs/sonar/sonar.html [Accessed 6
Aug. 2019].
2. Archive.epa.gov. (2019). Seismic Reflection Methods | Environmental Geophysics |
US EPA. [online] Available at:
https://archive.epa.gov/esd/archive-geophysics/web/html/seismic_reflection_methods
.html [Accessed 6 Aug. 2019].
3. Content.lms.sabis.sakarya.edu.tr. (2019). [online] Available at:
http://content.lms.sabis.sakarya.edu.tr/Uploads/33502/46613/1450y04015_erol_akde
mir-esra_b%C4%B1y%C4%B1kl%C4%B1o%C4%9Flu.pdf [Accessed 6 Aug.
2019].
4. Liag-hannover.de. (2019). [online] Available at:
https://www.liag-hannover.de/fileadmin/user_upload/dokumente/Grundwassersystem
e/BURVAL/buch/033-050.pdf [Accessed 6 Aug. 2019].
5. Ucl.ac.uk. (2019). An Introduction to Reflection Seismology Data Processing.
[online] Available at:
https://www.ucl.ac.uk/EarthSci/people/lidunka/GEOL2014/Geophysics4%20-
%20Seismic%20waves/Seismology/An%20Introduction%20to%20Reflection
%20Seismology%20Data%20Processing.htm [Accessed 6 Aug. 2019].
6. Kana, J.D., Djongyang, N., Raïdandi, D., Nouck, P.N. and Dadjé, A., 2015. A review
of geophysical methods for geothermal exploration. Renewable and Sustainable
Energy Reviews, 44, pp.87-95.
7. Schuster, G.T., 2017. Seismic inversion. Society of Exploration Geophysicists.
8. Schmelzbach, C., Greenhalgh, S., Reiser, F., Girard, J.F., Bretaudeau, F., Capar, L.
and Bitri, A., 2016. Advanced seismic processing/imaging techniques and their
potential for geothermal exploration. Interpretation, 4(4), pp.SR1-SR18.

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9. Hajnal, Z., Pandit, B., Takacs, E. and Annesley, I.R., 2018. Integration and Cost
Saving Utilization of the Seismic Reflection Technique in the Athabasca Basin,
Canada (No. IAEA-CN--261).
10. Liner, C.L., 2016. Elements of 3D seismology. Society of Exploration Geophysicists.
11. Li, W., Deffenbaugh, M., Gillard, D.G., Chen, G. and Xu, X., ExxonMobil Upstream
Research Co, 2017. Method for estimating subsurface properties from geophysical
survey data using physics-based inversion. U.S. Patent 9,696,442.
12. Ureña, M., Benito, J.J., Ureña, F., Salete, E. and Gavete, L., 2018. Application of
generalised finite differences method to reflection and transmission problems in
seismic SH waves propagation. Mathematical Methods in the Applied
Sciences, 41(6), pp.2328-2339.

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Appendix

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