Geophysical Lectures

Introduction
The exact age of the earth is not known, but it is thought to be at least 4.5 billion years old.
Rocks and fossils (the remains of plants and animals preserved in the rocks) can be dated by
measuring the decay rate of radioactive material that they contain. The number of radioactive
particles given off by a substance during a certain time period provides a surprisingly
accurate estimate of the age of the substance. The geologic past is measured by means of a
geologic time chart. Each interval of time has been given a name so that a particular time in
the past can be referred to more easily. Periods in history are referred to in terms such as “the
ice age” “the iron age” and “the atomic age”. These time periods are measured in centuries or
millennia at most. Intervals of geologic time, by contrast are measured in millions of years.
For example, the dinosaurs became extinct about 70 million years ago. Another way to
express it is “dinosaurs died at the end of the Cretaceous period.”
Over the nearly 5 billion years of earth’s , history mountains have been risen, eroded away
and extreme environmental changes have occurred. For example, palm tree fossils have been
found near the north pole, indicating that a warm climate prevailed there in the geologic past,
shark teeth have been found hundreds of miles from the nearest modern sea. When rivers
flow into a large body of water, suspended and dissolved sediments settle to the bottom. The
coarsest sediments, such as sand, are deposited first and nearest to the river’s mouth. Lighter
sediments, such as mud and silt, are deposited farther out and in deeper water. Lime (calcium
carbonate), produced by tiny life forms living in warm, shallow water, is deposited on the
water bottom. This deposition of sediments has occurred throughout geologic times that
surface water has been present. Deposited sand is compacted and cemented to form
sandstone. Lime hardens into limestone. These two sedimentary rock types are the rocks most
important to petroleum accumulation and production.
Parts of the earth are constantly rising (being uplifted) or falling (subsiding) because of
forces acting within the earth’s upper layers. These are called tectonic forces. The uplifted
parts cause the ocean shore to extend farther out. Sand transported in rivers gets deposited
farther out in the ocean and covers the organic-rich layers of mud and silt. Later, the land
subsides again and layers of mud and silt rich in organic material are deposited on top of the
sand. The sequence of uplift, deposition, subsidence and more deposition has been repeated
over and over again throughout the earth’s history. Since the organic material deposited on
the ocean bottom is so quickly covered a process called anaerobic decay (without oxygen)
occurs. The end products of this decay include the molecules of hydrogen and carbon
(hydrocarbons) that make up oil and gas. Over a period of about one million years, the
organic material is converted to petroleum. According to experts in the field, it takes 2003
feet of dead organisms to make one cubic inch of oil. Many people think that oil and gas are
found in huge, cave-like caverns beneath the surface. This concept is completely wrong. In
order to understand where oil and gas came from, how it is accumulated in place, and how to
look for it, it is important to realize how very, very old the earth is and how many changes
have taken place.
Oil was first found in oil seeps where it accumulates on the surface of streams and lakes.
Surface geology methods of petroleum were used for a while but it became increasingly
harder to find oil in these ways. More powerful and reliable techniques employing gravity,
magnetic and seismic measurements have replaced the older methods, as well as the use of

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“water witches” to locate oil drilling sites. Magnetic exploration for minerals, including,
petroleum, is based on finding anomalous measurements of the earth’s magnetic field.
Similarly, measured anomalies in the earth’s gravity can indicate the presence of subsurface
geologic situations conducive to the accumulation of petroleum.
The most reliable and most used technique of petroleum exploration is the seismic
method. This involves recording “earthquake waves” produced artificially by explosives or
some other energy source. Downward-traveling energy produces “echoes” at boundaries
between rock layers. Determination of the times at which these “echoes” return to the surface
supplemented by other information such as seismic propagation velocities allows an
interpreter to develop a picture of the subsurface below the area investigated.

Geophysical methods
Geophysics is a subject of natural science concerned with the physical processes and
physical properties of the Earth and its surrounding space environment, and the use of
quantitative methods for their analysis, then we can see the Geophysics is the application of
physical principles and methods to problems in Earth Sciences.

What do we mean by applied geophysics?

Remote sensing that is constraining the Earth’s sub-surface with observations at the surface.

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Remote sensing

The techniques we’ll talk about are used in the fields of:-

• Environmental geophysics

• Engineering geophysics

• Exploration geophysics
Applied geophysics infers property contrasts inside the earth from surface (borehole)
measurements

Figure.2 Contrasts in physical properties from different geological targets give rise to the target, when
there is no contrast, the target is undetectable geophysically

The term geophysics sometimes refers only to the geological applications: Earth's shape; its
gravitational and magnetic fields; its internal structure and composition; its dynamics and
their surface expression in plate tectonics, the generation of magmas, volcanism and rock
formation. However, modern geophysics organizations use a broader definition that includes
the water cycle including snow and ice; fluid dynamics of the oceans and the atmosphere;
electricity and magnetism in the ionosphere and magnetosphere and solar-terrestrial relations;
and analogous problems associated with the Moon and other planets.
There is no clear distinction between general and applied geophysics, but in general
geophysics methods are typically applied to solve academic questions, whereas applied
geophysics can be characterized by the application of geophysical methods for commercial
purposes (making money). Examples:oil, gas, coal, minerals, groundwater, geo-engineering
(tunnels, dams) ,archaeology ,pollution.

Various geophysical surveying methods have been used on land and offshore. Each of these
methods measures something that is related to subsurface rocks and their geologic
configurations. Rocks and minerals in the earth vary in several ways. These include:
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• Density – mass per unit volume. The gravity method detects lateral variations in density.
Both lateral and vertical density variations are important in the seismic method.
• Magnetic susceptibility – the amount of magnetization in a substance exposed to a magnetic
field. The magnetic method detects horizontal variations in susceptibility.
• Propagation velocity – the rate at which sound or seismic waves are transmitted in the earth.
It is these variations, horizontal and vertical, that make the seismic method applicable to
petroleum exploration.
• Resistivity and induced polarization – Resitivity is a measure of the ability to conduct
electricity and induced polarization is frequency-dependent variation in resistivity. Electrical
methods detect variations of these over a surface area
• Self-potential - ability to generate an electrical voltage. Electrical methods also measure this
over a surface area.
• Electromagnetic wave reflectivity and transmissivity – reflection and transmission of
electromagnetic radiation, such as radar, radio waves and infrared radiation, is the basis of
electromagnetic methods.

There is a broad division of geophysical surveying methods into those that make use of
natural fields of the Earth and those that require the input into the ground of artificially
generated energy. The natural field methods utilize the gravitational, magnetic, electrical and
electromagnetic fields of the Earth, searching for local perturbations in these naturally
occurring fields that may be caused by concealed geological features of economic or other
interest. Artificial source methods involve the generation of local electrical or
electromagnetic fields that may be used analogously to natural fields, or, in the most
important single group of geophysical surveying methods, the generation of seismic waves
whose propagation velocities and transmission paths through the subsurface are mapped to
provide information on the distribution of geological boundaries at depth. Generally, natural
field methods can provide information on Earth properties to significantly greater depths and
are logistically more simple to carry out than artificial source methods.
The latter, however, are capable of producing a more detailed and better resolved picture
of the subsurface geology. Several geophysical surveying methods can be used at sea or in
the air. The higher capital and operating costs associated with marine or airborne work are
offset by the increased speed of operation and the benefit of being able to survey areas where
ground access is difficult or impossible. A wide range of geophysical surveying methods
exists, for each of which there is an ‘operative’ physical property to which the method is
sensitive.
The methods are listed in Table 1.1. There will be no further discussion of electrical or
electromagnetic methods. This course provide brief introductions to the gravity, magnetic,
and seismic methods.

To select the most appropriate geophysical method to investigate a certain task/problem,

What spatial scales are relevant?

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Which acquisition geometries are optimal? (e.g. 2D vs 3D seismics)
Is there useful a priori information?
Is there a cheaper alternative?

Classification of geophysical methods

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 Figure 3. Interpreted Ground penetration radar profiler (left) and seismic profiler

The influential factors in choosing of the geophysical methods should be used:

- Scientific or technical aspects

- Working speed (execution time)

Planning a survey

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 Figure 4. Seismic field work array

Figure 5. Marine Seismic work array

Figure 6. 3D Seismic reflection