Geology & Exploration

Moveout (seismic)

Moveout: Deciphering the Echoes of Oil and Gas

In the world of oil and gas exploration, seismic surveys play a crucial role in mapping the subterranean landscape. By sending sound waves deep into the earth and analyzing their echoes, geophysicists can identify potential reservoirs of oil and gas. One key concept in seismic data interpretation is moveout, which refers to the difference in arrival times of reflected seismic data at different detectors.

Imagine throwing a pebble into a pond. The ripples spread outwards, reaching different points on the pond's surface at different times. Similarly, seismic waves, upon encountering a geological interface (like a rock layer or a fault), reflect back to the surface. These reflected waves are picked up by a network of detectors called geophones.

The key takeaway is this: Moveout is directly related to the depth and position of the reflecting interface. Let's break it down:

  • Deeper reflections: Waves reflecting from deeper structures will take longer to travel back to the surface. Therefore, they will arrive later at the geophones. This results in a larger moveout.
  • Offset: The distance between the source of the seismic wave and the geophone is called the offset. For a given reflector, the moveout will be greater for geophones with a larger offset, simply because the wave has a longer distance to travel.

Understanding moveout is crucial for several reasons:

  1. Depth Estimation: By analyzing the moveout of reflections, geophysicists can estimate the depth of the reflecting horizon. This is essential for pinpointing potential oil and gas reservoirs.
  2. Velocity Determination: Moveout also helps determine the velocity of seismic waves in the subsurface. Velocity information is critical for accurate depth estimation and understanding the geological structure.
  3. Imaging: Moveout correction is a crucial step in seismic processing, helping to create a clear and accurate image of the subsurface.

Different Types of Moveout:

  • Normal Moveout (NMO): The most common type of moveout, NMO is the time difference between a reflection arriving at a geophone directly above the reflector and one arriving at a geophone with an offset.
  • Dip Moveout (DMO): This type of moveout accounts for the effects of dip (the angle of the reflector) on the arrival times. DMO correction is essential for obtaining accurate images of dipping structures.

In conclusion, understanding moveout is fundamental to seismic data interpretation in oil and gas exploration. By analyzing the time differences in seismic reflections, geophysicists can unlock vital information about the subsurface, leading to the discovery and extraction of valuable resources.

Test Your Knowledge

Quiz: Moveout - Deciphering the Echoes of Oil and Gas

Instructions: Choose the best answer for each question.

1. What does "moveout" refer to in seismic data interpretation?

a) The distance between the source of the seismic wave and the geophone. b) The difference in arrival times of reflected seismic data at different detectors. c) The depth of the reflecting interface. d) The velocity of seismic waves in the subsurface.

Answer

b) The difference in arrival times of reflected seismic data at different detectors.

2. How does the depth of a reflecting interface affect moveout?

a) Deeper reflections result in smaller moveout. b) Deeper reflections result in larger moveout. c) Depth has no influence on moveout. d) Deeper reflections result in faster arrival times.

Answer

b) Deeper reflections result in larger moveout.

3. Which of the following is NOT a reason why understanding moveout is crucial?

a) Depth estimation of potential oil and gas reservoirs. b) Determining the velocity of seismic waves in the subsurface. c) Identifying the type of rock formations. d) Creating a clear and accurate image of the subsurface.

Answer

c) Identifying the type of rock formations.

4. What is the most common type of moveout?

a) Dip Moveout (DMO) b) Normal Moveout (NMO) c) Lateral Moveout (LMO) d) Vertical Moveout (VMO)

Answer

b) Normal Moveout (NMO)

5. Which type of moveout accounts for the effects of the angle of the reflector?

a) Normal Moveout (NMO) b) Dip Moveout (DMO) c) Lateral Moveout (LMO) d) Vertical Moveout (VMO)

Answer

b) Dip Moveout (DMO)

Exercise: Moveout and Depth Estimation

Scenario:

Imagine a seismic survey where a reflection from a potential oil reservoir is detected at two geophones. Geophone 1 is directly above the reflector (offset = 0 meters), and Geophone 2 is 1000 meters away from the reflector (offset = 1000 meters). The reflection arrives at Geophone 1 after 2 seconds and at Geophone 2 after 2.5 seconds.

Task:

  1. Calculate the moveout: Find the difference in arrival times between the two geophones.
  2. Estimate the depth of the reflector: Assuming the seismic wave velocity in the subsurface is 2000 meters per second, estimate the depth of the oil reservoir.

Exercice Correction

1. **Moveout Calculation:** The moveout is the difference in arrival times: 2.5 seconds - 2 seconds = 0.5 seconds. 2. **Depth Estimation:** * **Time for the wave to travel to Geophone 1:** 2 seconds. * **Distance traveled by the wave to Geophone 1 (depth of the reflector):** 2 seconds * 2000 meters/second = 4000 meters. Therefore, the estimated depth of the oil reservoir is 4000 meters.

Books

  • "Seismic Data Processing" by Oz Yilmaz: A comprehensive textbook covering various aspects of seismic data processing, including moveout correction techniques.
  • "Exploration Geophysics: An Introduction to Geophysical Methods in Exploration" by Robert E. Sheriff: A standard text offering a broad overview of geophysical methods, with a dedicated chapter on seismic data processing and moveout.
  • "Seismic Data Analysis" by Jon Claerbout: This book delves into the mathematical foundations of seismic data analysis, providing a deeper understanding of the principles behind moveout.

Articles

  • "Normal Moveout (NMO) and Dip Moveout (DMO)" by Yilmaz & Doherty (SEG): A detailed explanation of NMO and DMO, focusing on their application and impact on seismic data processing.
  • "Velocity Analysis and Moveout Correction" by Lines & Treitel (SEG): An article discussing velocity analysis and its connection to moveout, highlighting the importance of accurate velocity determination.
  • "Moveout Correction and Seismic Imaging" by Claerbout (SEG): A research paper diving into the relationship between moveout correction and seismic imaging, explaining the impact on image quality and interpretation.

Online Resources

  • Society of Exploration Geophysicists (SEG): The SEG website offers a wealth of resources on seismic exploration, including technical papers, research articles, and presentations related to moveout.
  • OpenLearn - Open University: This online platform features a course on "Introduction to Seismic Exploration," providing a comprehensive introduction to the subject, including a section on moveout.
  • Stanford Exploration Project (SEP): The SEP website hosts various research projects and resources related to seismic data processing and interpretation, including information on moveout correction techniques.

Search Tips

  • "Seismic moveout tutorial": Find introductory materials and tutorials explaining the concept of moveout.
  • "NMO correction equations": Search for equations and mathematical formulas used in Normal Moveout correction.
  • "DMO correction methods": Look for articles and resources discussing different Dip Moveout correction methods.
  • "Moveout analysis software": Explore available software tools designed for moveout analysis and correction in seismic data processing.

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