MARINE GEOPHYSCAL TECHNIQUES
Marine Geophysical Techniques commonly employed includes Seismic, Gravity and Magnetic
methods of exploration, though others methods, viz. Radioactive, and Electrical are also some times used. Another
technique which has made considerable progress in recent years is the marine geothermal investigation. The instrumentation
involves in marine surveys are different; from those on land operated ones.
SEISMIC METHODS
The seismic method is by far the most important geophysical technique and its predominance
is due to high accuracy, high resolution and great penetration.
The objective of seismic exploration is to deduce information about the rocks, the structure of subsurface formation especially
attitudes of the beds, from the observed arrival times and to lesser extent from vibration in amplitude, frequency, phase
and wave shape.
Seismic waves are messengers that convey information about the earth’s interior. The capacity of a material to be temporarily
deformed by passing seismic waves can be described by its properties of elasticity. These physical properties can be used
to distinguish different materials. Thus a medium having anomalous structural feature affects both velocity and direction
of a propagating seismic waves. Observation of such effects is the essence of seismic prospecting.
The basic technique of seismic exploration consists of generating seismic waves and measuring the time required for the wave
to travel from the source to a series of geophones usually disposed along a straight line directed towards the source. From
knowledge of travel times and the velocity of the waves the path of the seismic waves are reconstructed.
Sedimentary basins are composed of layers of different kinds of rocks mostly deposited smoothly on one another. There is a
‘velocity interface’ a change from one velocity of propagation to another, at each change from one kind of rock
to another. Density of rock enters into, but velocity contrasts are usually much greater, so people tend to speak of velocity
interface without monitoring the density difference. Moreover, the rigidity(shear) modulus(defined as the stress-strain proportionality
constant in case of simple shearing-tangential stress) being the physical property possess by the solids only and cannot be
applied to ideal liquids, this property is used to distinguish different interface. The higher the rigidity the greater is
the energy transmission and vice-versa.
In seismic prospecting
we require both types of approach, travel time and waveform studies. The first approach is for the purpose of defining the
subsurface anomalous structure and the second mainly for enhancing the signal to noise ratio, and for investigation subsurface
stratigrahic anomalies.
PROPAGATION OF SEISMIC WAVES
Being a wave motion, seismic waves spread out from the source following the propagation principles. Wave types speed and propagation
directly vary in accordance with the physical properties and dimensions of the medium. The simplest medium is the one which
is homogeneous, isotropic and perfectly elastic. Is such and idealized modes the waves travels along straight ray paths, and
with constant velocity.
In nature, media are bounded and commonly stratified into layers of different physical properties. In such circumstances a
seismic wave suffers from a number of changes every time it hits an interface wave speed, propagation direction spectral structure
and energy content all changes as the wave passes from layer to layer. In addition, new wave phases may be generated at interface.
SEISMIC REFLECTION METHOD
The seismic reflection method by far the most widely used geophysical technique. With this method the structure of subsurface
formation is mapped by measuring the times required for a seismic wave generated in the earth by a near-surface explosion,
mechanical impact or vibration to return to the surface after suffering reflection from interfaces between formations having
different physical properties. The reflections are recorded by detecting instruments responsive to ground motion. They are
laid along the ground at distances from the point of generation, where the far offset distance from the source are generally
nearly equal to the depth of investigation. Variations in the reflecting times from place to place on the surface usually
indicate structural features in the strata below.
PRINCIPLE OF SEISMIC REFLECTION METHOD
The principle of seismic reflection method is based on Snell’s law.
The principle holds
for normal incidence or for a small dip (upto 20o). For greater dipping interfaces the principle does not offers
the desired result.
“Acoustic Impedance” is the principle parameter which defines the existence of an interface and thus the structure
of subsurface formation can be mapped.
For reflection, an interface exists if both velocity and density of the media in the adjacent layers are different. “The
parameter which expresses the combined effect of velocity (v) and bulk density (ρ) is called the acoustic impedance (Z),
where,
Z = v * ρ
Thus the greater the contract in the value of the acoustic impedance the stronger the reflection becomes.
The reflection coefficient depends upon the acoustic impendence as given by the following relation,
R = (Z2 – Z1)/ (Z2 + Z1)
= (ρ2v2 - ρ1v1)/
(ρ2v2 + ρ1v1)
Where Z2 and Z1 are
the acoustic impedance of second and the first layer
BASIC COMPONENTS OF MARINE SEISMIC ACQUISITION
Seismic
reflection profiling is accomplished by towing a sound source that emits acoustic energy in timed intervals behind a research
vessel. The transmitted acoustic energy is reflected from boundaries between various mediums of different acoustic impedances
(i.e. the water-sediment interface or between geologic units). Acoustic impedance is defined by the bulk density of the medium
and the velocity of the sound within that medium. The reflected acoustic signal is received by a ship-towed hydrophone (or
array of hydrophones), which converts the reflected signal to a bipolar analog signal. The analog signal from the hydrophone
can be filtered and displayed on a graphic recorder. The analog signal is digitized and logged in digital format. The digital
data can then be processed further and plotted on paper or imported to computer mapping programs for interpretation.
Seismic Streamer:
· The streamer should be digital type and meet the standards of the industry.
· The in-water streamer electronic modules
should be capable of digitizing the analogue
data.
· All streamers sensors should be in good
electrical and mechanical condition throughout the period of survey and also have sufficient spares for normal operation.
· The hydrophone in the streamer as well
as individual channel response should conform to the manufacturers’ specification.
· The streamer should be naturally buoyant
and should be towed at a depth of 6 to 7m.
Depth reading will be recorded
as per objective of investigation.
· The streamer shall be fitted with acceleration
canceling type of hydrophones and should have adequate number f calibrated depth transducers.
SEISMIC PROFILING SYSTEMS
Acoustic Source:
The different types of energy sources employed to-day in offshore
seismic work differ from those used for land work. The process of generation energy in marine work is not applicable for land
seismic survey. Similarly, marine work calls for different types of receiving transducers to substitute the geophone which
is used on land.
Explosive Source:
· Dynamite
· Flexo-Tir
· Maxi-pulse
Non-explosive Source:
· Sparker
· Boomer
· Air Gun
· Aqua Pulse
SPARKER
Here the shock wave is generated by suddenly discharging
a current between a pair of electrodes in the sea. The electrodes are built into an assembly which is towed behind the vessel.
A bank of capacitors on board are charged by a high voltage generator. When the shot is to be fired, i.e., at the instant
a contact closure is obtained which discharges the capacitors through the electrodes and the salt water which acts as the
path for the current between the electrodes. This electrical discharge produces an intense heat and vaporizes the water in
the immediate vicinity creating a shock wave in the process. The signal return from the sea bed and also the sub-surface layers
is picked up by a set of hydrophones and recorded, usually, on a single channel recorder. The energy can be increased by increasing
the number of electrode pairs and the generator capacity.
A particular application of this method is its use in shallow penetration surveys using
multiple channels.
The Sparker is generally used for sea-bed surveys, studies of underwater sound propagation
and geological studies in the sub-bottom.
The technique has recently been improved and the penetration
increased to about 1000 meters, using high energy sparker rays, multichannel streamers and digital recording. This technique
is routinely used to detect high pressure shallow gas pockets and faults which are potential drilling hazards. It can also be used for exploration for hydrocarbons
at shallow depths.
HYDROPHONE
The pressure geophones are commonly used for receiving signals in marine seismic survey.
In the hydrophone piezo-electric crystals or comparable ceramic elements are used as pressure sensors. They generate a voltage
proportional to the instantaneous water pressure associated with seismic signal. This pressure is proportional to the velocity
of the water particles set into motion by the signal.
MARINE SHALLOW SEISMIC INVESTIGATION
For
a majority of this work we use an EGG
Uniboom sub-bottom profiling system.
For
high resolution surveys not requiring deep penetration we use an ORE Pinger and for projects requiring
deep penetration where high resolution is not as essential, we use a Sparker.
The boomer system consists of an insulated metal plate
and rubber diaphragm adjacent to a flat wound electrical coil mounted on a towed catamaran.
A short duration, high power electrical pulse, generated
by the shipboard power supply and capacitor banks, discharges to the electrical coil. The resultant magnetic field explosively
repels the metal plate generating a broad band acoustic pressure pulse in the water column.
The frequency of this pulse is in the range 400Hz to 14Hz
with the majority of the energy being directed vertically downward at a maximum output of 300 joules per pulse.
A percentage
of the acoustic energy is reflected from the sea floor. This percentage is dependent upon the composition
of the seabed materials. The remaining energy penetrates the seabed and is reflected from layers of contrasting acoustic impedance.
Acoustic impedance is a product of the density and seismic velocity of the material.
The
reflections are detected by a multi-element
hydrophone which is towed parallel to the source catamaran,
astern of the vessel. This configuration is used in order to minimize the direct source-receiver signal.
The reflections detected by the hydrophone are converted
to an electrical signal which is amplified, filtered and displayed in graphic form on the seismic recorder.
The
character of the sub-bottom records are therefore dependant upon the way in which the acoustic signal is reflected. This is
used to interpret the condition present. An Ultra 120 series, 3 channel thermal recorder will be
used to display the data graphically on board with the incoming data being enhanced by the use of an in-line TSS 360 Shallow Seismic Processor.
Processed
seismic data will be produced on an Isopach and for pipe route investigations we would also provide a longitudinal section.
How can data derived from seafloor samples be
used?
· To study past climate change for environmental prediction.
· To understand the impact of benthic habitat on fisheries
and other biological communities.
· To study offshore pollution patterns and mechanisms
to help sustain healthy coasts.
· To find sources of dredged material for beach replenishment.
· To evaluate the impacts of proposed offshore waste
disposal.
· To learn about and estimate the impacts of events
such as gas hydrate releases related to slope stability.
· To locate strategic offshore mineral resources.
· To determine sites for seabed communications cables,
drilling platforms, & other structures.
· To provide groundtruth values for remotely sensed/satellite
data, helping refine new techniques for environmental assessment and prediction.
· To learn more about how the Earth and its environmental
systems function.
