Check Shot Survey (seismic)

Test Your Knowledge

Quiz: Unveiling Earth's Secrets: Understanding Check Shot Surveys in Seismic Exploration

Instructions: Choose the best answer for each question.

1. What is the primary purpose of a Check Shot Survey? a) To identify the location of oil and gas reservoirs. b) To measure the precise velocity of seismic waves through different rock formations. c) To map the boundaries of groundwater aquifers. d) To analyze the composition of subsurface rock layers.

2. Which of the following is NOT a key component of a Check Shot Survey? a) Downhole geophones b) Surface geophones c) Electromagnetic sensors d) Explosive charges

3. Why is accurate velocity determination important in Check Shot Surveys? a) To identify the type of minerals present in the subsurface. b) To calculate the depth of seismic reflections. c) To determine the age of rock formations. d) To analyze the magnetic properties of the subsurface.

4. How does Check Shot data contribute to the calibration of seismic models? a) By providing information about the density of rock layers. b) By refining the estimated velocities of seismic waves in different formations. c) By determining the direction of seismic wave propagation. d) By analyzing the amplitude of seismic reflections.

5. Besides oil and gas exploration, Check Shot Surveys can also be applied in which of the following fields? a) Archaeology b) Astronomy c) Meteorology d) Geotechnical studies

Exercise: Unveiling Earth's Secrets: Understanding Check Shot Surveys in Seismic Exploration

Instructions:

Imagine you are a geophysicist working on an oil and gas exploration project. You have conducted a Check Shot Survey in a borehole and obtained the following data:

| Depth (meters) | Travel Time (seconds) | |---|---| | 100 | 0.15 | | 200 | 0.28 | | 300 | 0.42 | | 400 | 0.55 | | 500 | 0.68 |

Task:

1. Calculate the average velocity of seismic waves in each rock layer between the specified depths.
2. Plot the velocity data against depth on a graph.
3. Based on the velocity profile, what conclusions can you draw about the geological formations encountered in the borehole?

Techniques

Unveiling Earth's Secrets: Understanding Check Shot Surveys in Seismic Exploration

This document expands on the provided text, breaking it down into separate chapters focusing on Techniques, Models, Software, Best Practices, and Case Studies related to Check Shot Surveys.

Chapter 1: Techniques

Check shot surveys employ a straightforward yet crucial technique for determining seismic velocities within boreholes. The process involves several key steps:

1. Borehole Preparation: A borehole is drilled to the desired depth. The borehole diameter and condition influence the geophone placement and signal quality. Proper casing and cementing are essential to maintain a stable environment for measurements.

2. Geophone Deployment: Downhole geophones, specifically designed to withstand the borehole environment, are lowered into the borehole at predetermined intervals. These geophones precisely record the arrival times of seismic waves generated by the shots. The spacing of geophones depends on the desired resolution and anticipated velocity variations.

3. Shot Generation: Small explosive charges (e.g., dynamite, shaped charges) are detonated at known depths within the borehole. Alternatively, some surveys use a seismic vibrator source placed at various depths within the borehole. The shot size and depth are carefully controlled to provide sufficient signal strength without causing damage to the borehole or surrounding formations.

4. Surface Geophone Acquisition: Surface geophones, arranged in a suitable configuration (e.g., a spread), record the arrival times of the upgoing seismic waves from the detonations. The precise locations of these geophones are recorded using GPS.

5. Data Acquisition and Processing: The arrival times of seismic waves are recorded by both downhole and surface geophones. This data undergoes processing to account for factors such as geophone response, timing errors, and wave propagation effects.

6. Velocity Calculation: The precise depth of each shot and the arrival times recorded by both the downhole and surface geophones are used to calculate the interval and average velocities of seismic waves in the traversed formations. Different techniques, like the time-depth relationship and ray tracing, can be employed for this calculation.

Chapter 2: Models

Several velocity models are used in conjunction with check shot data:

• Interval Velocity Model: This model directly represents the velocity of seismic waves within each layer between successive geophone depths. It is generated by combining the travel times between geophone levels and the known vertical distances.

• Average Velocity Model: This model represents the average velocity of seismic waves from the surface to a given depth. This is crucial for depth conversion of seismic reflection data. It is derived from the interval velocities.

• RMS (Root Mean Square) Velocity Model: This model represents the average velocity weighted by the travel time through each layer. It is particularly important for seismic processing and imaging, especially for depth migration.

• Seismic Velocity Models (integrated): Check shot data is often incorporated into broader 3D velocity models, which are typically created from seismic tomography, well log data and other geological information. This integration allows for better understanding of the subsurface and improved seismic interpretation.

Chapter 3: Software

Specialized software packages are used to process and interpret check shot data. These typically include:

• Data Acquisition Software: This software controls the instruments, records arrival times, and manages the data flow during the survey.

• Data Processing Software: This software performs corrections for timing errors, geophone response, and other instrumental effects. It calculates interval and average velocities. Examples include seismic processing packages like SeisSpace, ProMAX, and Kingdom.

• Velocity Modeling Software: This software builds velocity models, integrates check shot data with other geological and geophysical data, and assists in depth conversion and seismic imaging. Examples might be included within the larger seismic processing packages, or dedicated geological modeling software.

• Visualization Software: Allows for visualizing the results in various formats, such as depth-velocity plots, velocity profiles, and integrated with seismic sections.

Chapter 4: Best Practices

To ensure accurate and reliable results, several best practices should be followed:

• Careful Borehole Selection: The borehole should be appropriately located for the geological objective, stable, and provide good coupling for geophones.

• Precise Geophone Placement: Accurate depth measurements and proper geophone coupling are crucial to minimize errors in travel time measurements.

• Controlled Shot Parameters: Consistent shot size and accurate depth control are critical for repeatability and data quality.

• Environmental Considerations: Safety regulations and environmental protection measures should be adhered to throughout the survey.

• Quality Control: Regular checks and quality control procedures are essential to identify and correct any potential errors during data acquisition and processing.

• Data Validation: The processed data should be thoroughly validated and compared with other available geological and geophysical data to ensure accuracy.

Chapter 5: Case Studies

Case studies demonstrate the application of check shot surveys in various settings:

• Oil and Gas Exploration: Check shot surveys in a deep offshore environment provide critical velocity information for depth conversion and reservoir characterization, leading to more accurate delineation of hydrocarbon accumulations.

• Geotechnical Engineering: Check shot data aids in site characterization for large infrastructure projects, improving foundation design and stability assessment, for example, in dam construction or tunnel boring.

• Hydrogeology: In groundwater studies, check shot surveys contribute to constructing accurate velocity models for aquifer mapping and groundwater flow simulations.

These case studies highlight the value of check shot surveys in providing essential velocity information for a range of applications, ultimately contributing to improved subsurface understanding and informed decision-making.