Geophysics Notes 2s6f26

Geophysical Exploration & Seismic Interpretation PE-309 Lecture Notes

Made by Maisam Abbas PE-038

Course Outline 1. 2. 3. 4. 5. 6.

Geophysical & Seismic Exploration Introduction Gravity and Magnetic Methods- description and correction factors Reflection and Refraction Seismic Fundamentals Seismic Methods- onshore and offshore Seismic Data Acquisition-2D,3D, VSP; offshore & onshore equipment, planning 7. Seismic Data processing 8. Seismic Interpretation basics 9. Interpretation pitfall 10.Sedimentary processes and depositional environment-stratigraphy, and structural features 11.Sequence and seismic stratigraphy- sea-level changes and depositional variations 12.Seismic Data Interpretation- 2D & 3D geophysical and geological aspects 13.Seismic data interpretation- mapping 14.Modern seismic exploration trends 15.Seismic facies analysis and reflection characteristics 16. GPS surveying, navigation and positioning methods

Books: Exploration Geophysics by Mamdooh Gadallah Elements of Petroleum Geology by Shelley

Prelude: Our task is to create a picture of subsurface layers so as to look for potential occurrence of oil and gas. Most common practice is performing operations, prior to drilling, to look for oil and gas prospects.

Geophysical & Seismic Exploration Introduction pg. 1

Geophysical Exploration & Seismic Interpretation PE-309 Lecture Notes

Made by Maisam Abbas PE-038

Geophysics is a field that integrates geology, mathematics, and physics in order to examine and understand the earth's structure . A geophysicist is someone who studies the Earth using gravity, magnetic, electrical, and seismic methods. Subsurface Rock Geophysical properties: 1. Density 2. Magnetic Susceptibility (constant that indicates the degree of magnetization of a rock in response to an applied magnetic field) 3. Electrical Conductivity 4. Radioactivity 5. Geothermal Properties (e.g. thermal conductivity) 6. Elasticity (Bulk and shear Modulus) Methods of Exploration: 1. Seismic methods (based on elastic property of rock, examined using seismic wave velocity, which changes due to difference in elastic properties of rock) 2. Gravity methods (based on rock’s density) 3. Magnetic methods (based on rock’s magnetic susceptibility) 4. Electrical methods (based on rock’s conductivity) 5. Radioactive methods (based on rock’s radioactivity) 6. Geophysical well logging 7. Geothermal methods Gravity and magnetic methods identify basins and after that subsurface structures, traps, layers and stratigraphy are identified by seismic which gives clue for hydrocarbon. We have to imagine subsurface indirectly. Geophysical explorations are performed on; 1. 2. 3. 4. 5.

Airborne Surface Subsurface Oceans Lakes

Main branches of geophysics: 1. Solid earth geophysics pg. 2

Geophysical Exploration & Seismic Interpretation PE-309 Lecture Notes

Made by Maisam Abbas PE-038

 Theoretical geophysics (like, study of seismology)  Applied geophysics 2. Atmospheric geophysics 3. Space Geophysics

Gravity and Magnetic Methods (Description and Correction factors) Gravity and magnetic exploration, also referred to “potential fields” exploration is used to give geoscientists an indirect way to “see” beneath the Earth’s surface by sensing physical properties of rocks (density and magnetization, respectively). Potential field surveys are relatively inexpensive and can quickly cover large areas of ground. The methods are relatively cheap, non-invasive and nondestructive environmentally speaking. They are also ive – that is, no energy needs to be put into the ground in order to acquire data. The small portable instruments (gravimeter and magnetometer) also permit walking traverses.

1. Gravity Method: Introduction: Gravity surveying measures variations in the Earth’s gravitational field caused by differences in the density of sub-surface rocks. Gravity methods have been used most extensively in the search for oil and gas, particularly in the twentieth century. Theory: The basis on which the gravity method depends is encapsulated in two laws derived by Newton, namely his Universal Law of gravitation, and his Second Law of Motion. The first of these two laws states that the force of attraction between two bodies of known mass is directly proportional to the product of the two masses and inversely proportional to the square of the distance between their centers of mass. Consequently, the greater the distance separating the centers of mass, the smaller the force of attraction between them is. If an object of mass m is placed on the surface of earth, then; F=G

Mem R2

Where G=gravitational constant= 6.67x10-11 Nm2/kg2, Me=mass of earth=6x1024 kg, R=radius of earth=6371km.

pg. 3

Geophysical Exploration & Seismic Interpretation PE-309 Lecture Notes

Made by Maisam Abbas PE-038

Newton's Second Law of Motion states, “The force acting on an object is equal to the mass of that object times its acceleration.” Considering the object’s case, force of gravity, F, is tending to accelerate the object by acceleration g. F=mg Equations 1 and 2 can be combined to obtain another simple relationship: mg=G g=G

Me m R2

Me R

2

This shows that the magnitude of acceleration due to gravity on Earth (g) is directly proportional to the mass (Me) of the Earth and inversely proportional to the square of the Earth’s radius (R). Theoretically, acceleration due to gravity should be constant over the Earth. In reality, gravity varies from place to place because the earth has the shape of a flattened sphere, rotates, and has an irregular surface topography and variable mass distribution. Principle of Operation: What one weighs depends on the force of gravity at that spot and the force of gravity varies with elevation, rock densities, latitude, and topography. Mass, however, does not depend on gravity but is a fundamental quantity throughout the universe. When a mass is suspended from a spring, the amount the spring stretches is proportional to the force of gravity. This force, F, is given by F = mg, where g is the acceleration of gravity. Since mass is a constant, variations in stretch of the spring can be used to determine variations in the acceleration of gravity, g.

pg. 4

Geophysical Exploration & Seismic Interpretation PE-309 Lecture Notes

Made by Maisam Abbas PE-038

Figure 2.1 illustrates the principle of gravity exploration. On the left the surface elevation is moderate but there is a thick sedimentary section overlaying the basement complex; let’s say it is some igneous/metamorphic rock of higher density. At the center the surface elevation is near sea-level and the subsurface has a sedimentary section of normal thickness and density overlaying an “average” basement complex. On the right the surface elevation is also moderate but there is a thin sedimentary section resulting in the basement complex being close to the surface. The center part of Fig. 2.1 represents the “normal” earth situation and the suspended mass stretches the spring a “normal” amount here. On the left, the thick sedimentary section has lower density than the basement rocks so the “pull” of the earth is reduced, resulting in the suspended mass stretching the spring less than the “normal” amount. The situation on the right is the opposite. The higher density basement rocks closer to the surface causes the “pull” of the earth to be greater, stretching the spring more than the “normal” amount. Gravity Units: The normal value of g at the Earth’s surface is 980 cm/s2. The word “normal value” refers to the value of g measured at sea-level, which is 980 cm/s2. In honor of Galileo, the c.g.s. unit of acceleration due to gravity (1 cm/s2) is Gal. Modern gravity meters (gravimeters) can measure extremely small variations in acceleration due to gravity, typically 1 part in 109. The sensitivity of modern instruments is about ten parts per million. So, cm/s2 is a bit large for measuring the variations. Such small numbers have resulted in sub-units being used such as the:

pg. 5

Geophysical Exploration & Seismic Interpretation PE-309 Lecture Notes

Made by Maisam Abbas PE-038

1 milliGal (1 mGal = 10-3 Gal); 1 microGal (1 μGal = 10-6 Gal); and 1 gravity unit = 1 g.u. =0.1 mGal [10 gu =1 mGal] The unit used for measuring residual gravity is the milligal (mgal), or onethousandth of a gal, where a gal is 1cm/s2. The gal is named for Galileo. So, in problems of gravity corrections, since we are determining the variation, the variation be better expressed in mGal.

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