Introduction to Geophysics Lecture Notes PDF

## Introduction to Geophysics ### Basics of Geophysical methods #### Lecture 9: Seismic Method - Seismic Interpretation - Seismic Stratigraphy - Fluid Prediction - Al & El Inversion - AVO & Attribute Calibration - Well Calibration - Porosity Modeling - Al Inversion ## Seismic Reflection Survey...

## Introduction to Geophysics ### Basics of Geophysical methods #### Lecture 9: Seismic Method - Seismic Interpretation - Seismic Stratigraphy - Fluid Prediction - Al & El Inversion - AVO & Attribute Calibration - Well Calibration - Porosity Modeling - Al Inversion ## Seismic Reflection Survey The concept of the seismic reflection technique is to measure the time taken for a seismic wave to travel from a source down into the ground where it is reflected back to the surface and then detected at a receiver. This time is known as the two-way travel time (TWT). - The seismic method is used to obtain important details, not only about the geometry of the structures underground, but also about the physical properties of the materials present. ### Seismic Reflection Principles A diagram illustrates the concepts of depth, velocity, and two-way travel time. - Depth: The distance from the source to the reflector - Velocity: The speed of the seismic wave traveling through the medium - Time: The time taken for the seismic wave to travel from source to receiver. #### Seismic Reflection Method A diagram illustrates the concept of a seismic reflection method. - Energy Source: A device used to generate seismic waves. - Detectors: Devices used to detect the reflected seismic waves. - Overburden (Soil): the ground layer above the bed rock. - Reflecting Surface: The boundary between two materials with different acoustic properties that causes the reflection of seismic waves. - Bedrock: The solid rock layer beneath the soil. ### General Reflection Principles - For a seismic wave to be reflected back to the surface, there has to be a subsurface interface across which there is a contrast in acoustic impedance (Z), which is the product of the seismic velocity (V) and the density (p) of each layer (i.e. Z = Vi pi for the the layer). - The amplitude of the reflected wave is described by the reflection coefficient. - If a seismic source is discharged at a given shot point S and the reflected waves are detected at geophone locations laid out in a line each side of the shot, the resulting data can be used to create a seismic profile. A diagram illustrates the key concepts related to the reflection of seismic waves: #### Key Concepts - Offset: The distance between the source and a geophone. - Geophone Spacing: The distance between individual geophones within a seismic survey line - First Fresnel Zone: The area around the reflecting point where the seismic waves are most likely to be recorded. - CMP Spacing: The distance between common midpoint (CMP) locations, which represents the depth of the subsurface. - Subsurface Coverage: The area of the subsurface that is covered by a seismic survey. ## Seismic Refraction Survey ### General Reflection Principles - At each point of incidence on a subsurface interface, over an area corresponding to the first Fresnel zone the incident waves are reflected back. - The point of reflection is halfway between the source and the detecting geophone. - Consequently, the total subsurface coverage of an interface is half the total geophone spread length. The distance from the source to any geophone is known as the offset. A diagram illustrates the concepts related to seismic refraction: - Source: The location where seismic energy is generated. - Reflection: The bouncing back of seismic energy from a subsurface interface. - Refraction: The bending of seismic energy as it passes from one layer or material to another. - Buoy with GPS: A device used to mark the position of the seismic vessel. - Streamer Cable and Hydrophones: A cable towed behind the seismic vessel containing a series of hydrophones that detect seismic waves. #### Acoustic Energy sent into the Subsurface A diagram illustrates the concepts related to seismic refraction: - Seismic Vessel: A boat or ship equipped with airguns and a streamer cable. - Airguns: Devices used to generate compressed air that is released into the water to create seismic waves. - Water column: The layer of water between the sea surface and the seabed. - Seabed: The bottom of the ocean. - Geological Boundary: The subsurface interface between different rock layers. ## Seismic Reflection Survey ### General Reflection Principles - If more than one shot location is used, reflections arising from the same point on the interface will be detected at different geophones. This common point of reflection is known as the common midpoint (CMP). - Sometimes the terms common depth point (CDP) are used. A diagram illustrates the concept of common midpoint (CMP) #### Key Concepts - Shot-geophone mid-point: The point halfway between the shot and the geophone. - Common Offset: The distance between the shot and the geophone. - Spread: A group of geophones used in a seismic survey. - Distance: The distance between two shot locations. - Common Midpoint: The point on a reflecting surface that is equidistant from the source and all geophones. - Surface: The top of the Earth’s crust. - Reflecting Layer: A layer of rock or sediment that causes seismic waves to reflect back. ## Survey Layout A diagram illustrates two different survey layouts - **(a) Split spread, or straddle spread.** In this configuration, the source is located in the center of the geophone spread and reflections from the same point on the interface are recorded by different geophones on opposite sides of the shot. - **(b) Single-ended or on-end spread.** In this configuration, the source is located at one end of the geophone spread and the other end of the spread is used to record reflections. ## Single Horizontal Reflector A diagram illustrates the concept of a single horizontal reflector - **(a)** A shot is fired at a point S generating a seismic wave which travels through the earth with a velocity V. The wave is reflected back at a horizontal reflector. - **(b)** Data are recorded by a spread of geophones. Typical data are displayed on a time section. The data are presented in a graph with time on the vertical axis and distance on the horizontal axis. #### Key Concepts - t0: The time taken for the seismic wave to travel to and from a horizontal reflector at the zero offset. - V: The seismic velocity of the layer. - z: The depth to the horizontal reflector. ## Single Inclined Reflector A diagram illustrates the concept of a single inclined reflector - **(a)** A shot is fired at a point S generating a seismic wave which travels through the earth with a velocity V. The wave is reflected back at an inclined reflector. - **(b)** Data are recorded by a spread of geophones. Typical data are displayed on a time section. The data are presented in a graph with time on the vertical axis and distance on the horizontal axis. #### Key Concepts - t: The time taken for the seismic wave to travel to and from an inclined reflector at an offset distance x. - t0: The time taken for the seismic wave to travel to and from a horizontal reflector at the zero offset. - V: The seismic velocity of the layer. - z: The depth to the horizontal reflector. - x: The offset distance. - theta: The dip angle of the inclined reflector. - ∆T: The difference in two-way travel time between a horizontal reflector and an inclined reflector at the same offset.