“Marine Seismic Data
Processing &
Interpretation”
28th June to 14th July, 2006
at
Geological Survey of India
Bhu Bigyan Bhawan
Salt Lake City, Kolkata
SUMMER PROJECT REPORT
Under the Supervision of
Dr. A. Bagchi (Director Geophysics)
Dr. R. Singh, (Assistant Geophysicist)
Bhu Bigyan Bhawan
Kolkata.
DEPARTMENT
OF GEOPHYSICS
FACULTY
OF SCIENCE
BANARAS HINDU UNIVERSITY
VARANASI – 221005
2006-2007
ACKNOWLEDGEMENT
It gives us utmost pleasure to express
our sincere gratitude to Dr. B.K. Saha (SnDyDG) for allowing us to get familiarized and gave us the opportunity
to acquire first hand knowledge in the raw field of seismic data processomg amd interpretation at Geological Survey of India,
Marine Wisng, Bhu Bigyan Bhawan, Kolkata.
We take this opportunity to express our
heartiest thanks to Dr. A.Bagchi who coordinated and managed
the training programme, extended his support and took an untiring effort to make this venture a success.
We express our indebt sense of gratitude
to Dr. R.Singh and his associates for guiding us consistently and taking an untiring effort to make us familiar with
the different processing principles in a logical manner. We would like to thank him for extrapolating our knowledge and for
all his help and support in making things understand during our processing venture which he conducted in a comfortable and
pleasant environment.
By
Dhiman Mondal
INTRODUCTION
Mankind has to defend on an alternative source for his ever-increasing industrial growth.
Fortunately for man there is yet another major region left which could serve as a source of many important minerals, oil and
natural gas. And that is the ocean around us. In fact, in the past quarter centaury, the eye of the scientists, planners,
politicians and industrialists have been turned seaward. Oil, natural gas, coal, gold, iron ore, diamonds, manganese and nickel
art some of the minerals and materials that could be extracted from the seas bottom today. Most of them occur beneath the
relatively shallow sea-bottoms to the continental landmasses. The geology of these self-areas is of continental type.
Marine geophysical technology
has to do with geophysical methodologies and technology having a bearing on geophysical exploration in the marine environment.
It is not that we have many kinds of geophysics but that our geophysical work has to be carried out in a particular environment,
which provides special opportunities and improves peculiar constraints. Whenever we have to investigate geological features
under voter, we have to resort to geophysical methods. In the case of sea-bed the entire area is under the cover of water.
The scope for geophysical is therefore enormous and the need is inescapable. The nature of information to be obtained, the
purpose for which it has to be gathered and the conditions under which the investigations have to be carried out constitute
the fundamental criteria which determine our choice of geophysical instruments, techniques and methodologies.
The tools, instruments,
techniques, concepts, methodologies etc. developed in the course of exploration on land will naturally form the basis for
geophysical exploration at sea. But major modification adaptations and in cases new technology will have to developed appropriate
to the marine environment.
Marine geophysical methods
have been widely used to determine the sub-surface structural configuration and to detect the geophysical anomalies with which
mineral, oil and natural gas deposits may be associated. Unlike on where geological survey proceeds the geophysical in the
ocean regions where there is complete cover of a formidable layer of saline water geophysics invariably precedes the geological
surveys.
Sub-bottom profiling has
become the most useful of all exploration tools in the search for mineral deposits in the sea floor. The reflected signals
from the sub-bottom profiler source and suitably amplified and recorded graphically as a continuous profile showing the thickness,
configuration and internal structures of unconsolidated sediments layers and bed rock structure including buried stream channels
with mineral, oil and natural gas potential.
Under water mineral exploration
has been pioneered the petroleum industry, which has successfully adopted land geophysical and drilling techniques for off-shore
work, now taking place in many continental shelf areas and several continental slopes areas of the world.
It is clear that the petroleum
industries, the newly emerging ocean engineering industries and government research programmes undertaken by the industrialized
countries, will continue to play a major role in advancing marine mineral, oil and natural gas exploration and evaluation.
India has a long coastline
of the order of 4,000miles, and her exclusive economic zone (EEZ) is likely to cover a sea-bed area of around 40% of her land
area. In this zone major mineral potentialities will relate to oil and natural gas but interesting and useful possibilities
by way of solid minerals including diamonds, tin, magnetite, gold, phosphate nodules lime, sand, etc. exist.
Under these circumstances it is clear that the resources should
be evaluated, expanded, adapted, augmented, geophysical exploration is a great help where the resources to investigated are
under cover. In the case of resources of the sea bed, under the cover of water marine geophysical exploration both a necessity
and an ideal approach.
OVERVIEW OF PHYSIOGRAPHY OF OCEAN FLOOR
The
floors of the oceans are not plain as believed earlier. They are rugged and complex with world’s longest mountain ranges,
deepest trenches and largest plains. The development of the Sonic Depth Recorder has made it possible to measure depth for
mapping the ocean floor indirectly with the help of sound waves. The echo or the sound returning after striking the ocean
bottom forms the basis of this device. The data reveals many complex and varied features which rival anything about relief
features on the land.
In general, the ocean floor can be divided into four major divisions:
1)
The continental shelf
2)
The continental slope
3)
The continental rise and
4)
The abyssal plain.
Besides, there are many associated features including ridges,
hills, seamounts, guyots, trenches, canyons, deeps and fracture zones.
Numerous island arcs, atolls, coral reefs, submerged volcanoes, and sea-scraps add to the variety of submarine features.
1) Continental Shelf:
The continental shelf is a gentle seaward sloping surface extending
from the coasts towards the open sea. The seaward edge of the continental shelf is usually 150-200 meters deep. The shelf
varies in its width.
Along the eastern coast of India, also a wide strip of Continental Shelf is noted. However,
the average width of Continental Shelf is about 70 kilometers and mean slope is less than one degree.
The Continentals Shelves are mostly covered by sediments derived
form rocks on land. Some of them are under laid by sedimentary strata while others by the igneous and metamorphic strata.
There are various types of shelves including glaciated shelf, coral reel shelf, shelf of large river, shelf with dendritic
valleys, and the shelf along young mountain ranges. Old beaches and moraines can both be identified on the shelves.
The shelves of the world are of great use to man.
About 20% of the world production of petroleum and natural gas comes from shelves.
2) Continental Slope:
At the edges of the continental
shelf, the seaward slope becomes considerably steep, the angle of slope varying from 2 to 5 degree. This steep slope, which
descends to a depth of about 3,660 meters from the mean sea level is known as Continental Slope. The Continental Slope joins
the shelf to the deep ocean floor.
3) Continental Rise:
Where the continental
slope ends, the gently sloping Continental Rise begins. The Continental Rise has an average slope of between 0.50 to
10 and its general relief is low. With increasing depth the continental rise becomes virtually flat and it merges
with the abyssal plain.
4) Abyssal Plain:
Beyond the continental
rise lie the deep sea plains known as the Abyssal Plains or Abyssal Floors. They are areas of deep ocean floor found at a
depth of 3000 to 6000 meters. They occupy about 40% of the ocean floor and are present in all major oceans and several seas
of the world. They are uniquely flat with a gradient of less than 1:100. They are bounded by the hills on the seaward side.
The Abyssal Plains are covered by sediments both of ferruginous and shallow water origins. In general, Abyssal Plains are
more common where land-derived sediments are in great supply. The irregular topography is buried forming relatively flat areas
due to the large supply of sediments.
The other common features are:
a) Submarine Ridges
b) Abyssal Hills
c) Submarine Trenches and Deeps
d) Submarine Canyons
e) Banks, Shoal and Reel, etc.
BATHYMETRY
The topography of the
sea floor is the most essential for all exploration programmes. The maps representing the topography of the sea floor are
termed as Bathymetry Charts. There are two methods available for observing the sea bed topography:
1) Echo Sounding
2) Side Scan Sonar
1) The Echo Sounder:
Echo Sounder is used to
make discrete measurement of the depth below floating vessels along profile, from which water depth charts and topographic
profiles are constructed.
A pulse of sound
emitted by the sounder produces an echo from the sea bed. The sound pulse has traveled to the sea bed and back and the time
interval between transmissions of the pulse and detection of the echo is measured.
Depth
= ½ Vt
Where V : Velocity of sound in sea water
t : Two-way
travel time
The depths obtained by echo sounding have
to be corrected for the depth of transducer below the water, tides, salinity and temperature. The corrections for depth of
transducer are made in all the cases but in some recorders procvisi9on exists for adjusting the zero of the records. The corrections
for tides are made in shallow areas usually in depths less than 20m of water or if there are special requirements, these might
be made in deeper water also. The temperature and salinity effects the velocity of the sound and consequently the depth measured
but these corrections assumes importance in some areas only.
2) Side Scan Sonar:
Side Scan Sonars are used
to detect changes in sea bed topography variations in sea-bed materials and the position of wrecks or pipe lines.
This principle is similar to radar but uses,
instead of radio-waves, a pulsed beam of sound which is scanned through the under water darkness to detect changes in sea
bed topography and variations in sea-bed materials. To increase the coverage obtained per each survey lines, dual channel
systems are developed in which transduces are mounted in a towed fish so that separate beams are scanned to each side of the
ship. Thus picture of the sea bed upto a few hundred meters on either side of the ships course can be obtained.
