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The Chemistry and Technology of Petroleum
unstable and hence very sensitive to slight variations in gravitational attraction. Deflections of the
boom from a central (zero) position are measured by observing the change in the tension in the
spring system required to bring the boom back to that position. Readings are taken from a graduated
dial on the head of the instrument that is attached to the spring system through a screw. There must
be an accurate calibration of the screw, reading dial, and spring response for the readings to have
gravitational significance.
Gravimeters can also be employed for use in shallow water. Thus, use of watertight housings
with automatic leveling and electronic reading devices allows gravimeter surveys to be carried out
in aqueous environments. Other gravimeters have been developed for use in submarines and on
gyro-stabilized platforms on surface ships as well as in aircraft.
6.2.2 mAgnetIC metHods
Magnetic methods are based upon measuring the magnetic effects produced by varying concentrations of ferromagnetic minerals, such as magnetite. Instruments used for magnetic prospecting vary
from the simple mining compass used in the seventeenth century to sensitive airborne magnetic
units permitting intensity variations to be measured with an accuracy greater than 1/1000 part of
the Earth’s field.
The magnetometer is a specially designed magnetic compass and detects minute differences
in the magnetic properties of rock formations, thus helping to find structures that might contain
oil, such as the layers of sedimentary rock that may lie on top of the much denser igneous, or
basement, rock. The data give clues to places that might conceal anticlines or other oil-favorable
structures. Of even more value is the determination of the approximate total thickness of the
sedimentary rock, which can save unwarranted expenditures later or more costly geophysics or
even the drilling of a well when the sediment may not contain sufficient oil to warrant further
investigation. Most magnetometer surveys used now are performed by the use of aircraft, which
permits very large scale surveys to be made rapidly and surveys over regions that may be otherwise inaccessible.
One of the most widely used magnetic instruments is the Schmidt vertical magnetometer. It
consists of a pair of blade magnets balanced horizontally on a quartz knife edge. The balance is
oriented at right angles to the magnetic meridian. The deflection from the horizontal is observed,
giving the variation in magnetic vertical intensity with gravity. The torsion fiber magnetometer
is also a vertical component instrument but has a greater operating range than the Schmidt
instrument. It also has an advantage in that it is easier and quicker to read. The instrument values
are referred to a base, corrected for temperature and diurnal variation, and corrected for the normal geographic variation of the Earth’s magnetic field. The nuclear precession magnetometer is
another continuous recording magnetic instrument that measures the Earth’s total magnetic field
by observing the free precession (progressive movement) frequency of the protons in a sample
of water.
The interpretation of magnetic measurements is subject to the same fundamental drawbacks as
noted for gravity measurements. The drawbacks are as follows:
1. The contrast in physical properties of the formations
2. The depth of origin and integrated contributions from many sources
3. The changes in strength and direction of the Earth’s field with location
4. Canceling effect related to proximity of opposite induced poses at the boundaries of finite
geological bodies
However, the method has proven valuable in exploration for magnetic mineral deposits, in the determination of geological structural trends, and in estimating the probable depth of the crystalline rock
floor beneath sedimentary rock areas.
The Chemistry and Technology of Petroleum
unstable and hence very sensitive to slight variations in gravitational attraction. Deflections of the
boom from a central (zero) position are measured by observing the change in the tension in the
spring system required to bring the boom back to that position. Readings are taken from a graduated
dial on the head of the instrument that is attached to the spring system through a screw. There must
be an accurate calibration of the screw, reading dial, and spring response for the readings to have
gravitational significance.
Gravimeters can also be employed for use in shallow water. Thus, use of watertight housings
with automatic leveling and electronic reading devices allows gravimeter surveys to be carried out
in aqueous environments. Other gravimeters have been developed for use in submarines and on
gyro-stabilized platforms on surface ships as well as in aircraft.
6.2.2 mAgnetIC metHods
Magnetic methods are based upon measuring the magnetic effects produced by varying concentrations of ferromagnetic minerals, such as magnetite. Instruments used for magnetic prospecting vary
from the simple mining compass used in the seventeenth century to sensitive airborne magnetic
units permitting intensity variations to be measured with an accuracy greater than 1/1000 part of
the Earth’s field.
The magnetometer is a specially designed magnetic compass and detects minute differences
in the magnetic properties of rock formations, thus helping to find structures that might contain
oil, such as the layers of sedimentary rock that may lie on top of the much denser igneous, or
basement, rock. The data give clues to places that might conceal anticlines or other oil-favorable
structures. Of even more value is the determination of the approximate total thickness of the
sedimentary rock, which can save unwarranted expenditures later or more costly geophysics or
even the drilling of a well when the sediment may not contain sufficient oil to warrant further
investigation. Most magnetometer surveys used now are performed by the use of aircraft, which
permits very large scale surveys to be made rapidly and surveys over regions that may be otherwise inaccessible.
One of the most widely used magnetic instruments is the Schmidt vertical magnetometer. It
consists of a pair of blade magnets balanced horizontally on a quartz knife edge. The balance is
oriented at right angles to the magnetic meridian. The deflection from the horizontal is observed,
giving the variation in magnetic vertical intensity with gravity. The torsion fiber magnetometer
is also a vertical component instrument but has a greater operating range than the Schmidt
instrument. It also has an advantage in that it is easier and quicker to read. The instrument values
are referred to a base, corrected for temperature and diurnal variation, and corrected for the normal geographic variation of the Earth’s magnetic field. The nuclear precession magnetometer is
another continuous recording magnetic instrument that measures the Earth’s total magnetic field
by observing the free precession (progressive movement) frequency of the protons in a sample
of water.
The interpretation of magnetic measurements is subject to the same fundamental drawbacks as
noted for gravity measurements. The drawbacks are as follows:
1. The contrast in physical properties of the formations
2. The depth of origin and integrated contributions from many sources
3. The changes in strength and direction of the Earth’s field with location
4. Canceling effect related to proximity of opposite induced poses at the boundaries of finite
geological bodies
However, the method has proven valuable in exploration for magnetic mineral deposits, in the determination of geological structural trends, and in estimating the probable depth of the crystalline rock
floor beneath sedimentary rock areas.