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TYPES OF GEOPHYSICAL INVESTIGATIONS
An appreciation of the physical properties of rocks has been used to
create the four primary geophysical investigation techniques:
 Gravity
 Electrical
 Magnetic
 Sonic
Gravity methods are based on the fact that changes in density of parts of
the Earth can be detected because of their effect on the gravity field. So,
measurements require accuracy of better than one part in a million.
Gravitational effects associated with density changes decline with
distance. Small density changes that occur near the observation point
(usually, the Earth's surface) can have as much impact on the observed
gravity field as a much larger density change that occurs at a larger
distance. In practice, gravity surveys are used primarily for detecting large
rock bodies that have a significant density anomaly.
READING
Read the text and make its diagram
TYPES OF GEOPHYSICAL INVESTIGATIONS
An appreciation of the physical properties of rocks has been used to
create the four primary geophysical investigation techniques:
 Gravity
 Electrical
 Magnetic
 Sonic
Electrical methods are based on the fact that rocks have varying
resistivities. In most cases, the resistivity is actually a "bulk" value
representing the resistive rock framework and the (often lower) resistivity
of the contained pore fluids. In practice, an electrical potential (voltage) is
applied across widely-separated electrodes located at the ground surface,
and another electrode pair is used to determine the electrical potential at
intervening sites. Source rocks, because of their high carbon content, can
have lower resistivities. Sometimes a survey is undertaken to assess the
regional extent of a source rock layer. However, electrical techniques are
not commonly used for reservoir studies.
READING
Read the text and make its diagram
TYPES OF GEOPHYSICAL INVESTIGATIONS
An appreciation of the physical properties of rocks has been used to create the
four primary geophysical investigation techniques:
 Gravity
 Electrical
 Magnetic
 Sonic
Magnetic methods are based on the fact that rocks can alter the observed
magnetic field of the Earth. We all know about the main field of the Earth (e.g.
compass needles pointing to the magnetic poles). Rocks that have their own
magnetic fields can add or subtract from the main field. Magnetic methods are
not widely applied in Petroleum Engineering.
READING
Read the text and make its diagram
TYPES OF GEOPHYSICAL INVESTIGATIONS
An appreciation of the physical properties of rocks has been used to create the
four primary geophysical investigation techniques:
 Gravity
 Electrical
 Magnetic
 Sonic
Seismic investigations play a major role in Petroleum Geophysics. Seismic waves
in rocks are periodic, oscillatory motions; they are a form of acoustic wave.
Seismic waves can be created naturally (e.g. by an earthquake), or they may be
intentionally induced as part of an investigation.
There are two main categories of seismic techniques. One is termed (seismic)
refraction. In this method, the goal is to determine the position, and velocity and
density, of rock bodies.
The other seismic method is called reflection seismology, or often just "seismic".
In this method, sonic (vibrational) energy is deliberately propagated into the
ground, and the approach is to detect the sonic energy that is reflected back to
the surface, and to use these data to create an interpretation of the subsurface. The
returned seismic data is processed and presented in a form suitable for
interpretation. The usual approach is to display the processed data in a way that
looks similar to a geological cross section.