Seismic Refraction Test
1. Concept of the Test
The Seismic Refraction Test is a geophysical method used to determine the subsurface geological structure and stratigraphy by measuring the travel times of seismic waves refracted at the interfaces between different subsurface layers. The test provides critical information on the depth and composition of soil and rock layers, helping engineers design foundations, assess earthquake hazards, and conduct geological surveys. Standards for the Seismic Refraction Test include ASTM D5777.
2. Descriptions and Procedure
Apparatus Required
- Seismograph or geophone array
- Seismic source (e.g., sledgehammer, weight drop, explosive charge)
- Geophones (typically 12-24 geophones arranged in a linear array)
- Cables and connectors
- Trigger mechanism (for synchronizing the seismic source and seismograph)
- Measuring tape or wheel
- GPS or surveying equipment (for precise location measurements)
- Data analysis software
Procedure
1.Site Selection and Preparation
- Choose a site with minimal surface obstructions and noise interference.
- Clear the test area of any large debris or obstacles that could interfere with the seismic waves.
2.Layout of Geophones
- Lay out the geophones in a straight line, typically at equal intervals ranging from 1 to 5 meters, depending on the desired depth of investigation and the subsurface conditions.
- Ensure the geophones are securely planted into the ground to maximize their sensitivity to seismic waves.
3.Seismic Source Placement
- Position the seismic source at one end of the geophone array (or both ends for more comprehensive data).
- Ensure the source is capable of generating sufficient energy to penetrate the subsurface layers to the desired depth.
4.Triggering the Seismic Source
- Use the trigger mechanism to synchronize the seismic source activation with the seismograph recording.
- Activate the seismic source to generate seismic waves. This can be done using a sledgehammer striking a metal plate, a weight drop, or controlled explosives, depending on the site conditions and required depth of investigation.
5.Recording Seismic Waves
- The seismic waves travel through the subsurface and are refracted at the interfaces between different layers.
- The geophones detect the refracted waves, and the seismograph records the travel times of these waves from the source to each geophone.
6.Data Collection and Analysis
- Repeat the test at multiple source points along the geophone array to ensure adequate coverage and data accuracy.
- Collect and download the data from the seismograph for analysis.
Data Interpretation
1. Travel Time Curves
- Plot the travel times of the seismic waves against the distances from the source to each geophone.
- Identify the distinct linear segments in the travel time curves, which correspond to different subsurface layers.
2.Determining Layer Velocities
- Calculate the seismic wave velocities for each layer by determining the slope of the linear segments in the travel time curves.
- Use the inverse of the slope (time/distance) to find the velocity.
3.Depth Calculations
- Determine the depth of each layer interface using the following formula:
ℎ=𝑡𝑖2𝑉𝑖2−𝑉𝑖+12h=2tiVi2−Vi+12
where ℎh is the depth of the interface, 𝑡𝑖ti is the intercept time, 𝑉𝑖Vi is the velocity of the upper layer, and 𝑉𝑖+1Vi+1 is the velocity of the lower layer.
4.Modeling Subsurface Structure
- Develop a subsurface profile by combining the depth and velocity data for each layer.
- Use software tools to create detailed geological cross-sections and interpret the subsurface conditions.
Summary Table
Step
Site Selection and Preparation
Description
Choose a site and clear the test area.
Description
Arrange geophones in a straight line at equal intervals.
Step
Seismic Source Placement
Description
Position the seismic source at the end(s) of the geophone array.
Step
Triggering the Seismic Source
Description
Synchronize and activate the seismic source.
Step
Recording Seismic Waves
Description
Detect and record seismic waves using geophones and seismograph.
Step
Data Collection and Analysis
Description
Repeat the test, collect data, and analyze travel time curves.
Description
Plot travel times and identify linear segments.
Step
Determining Layer Velocities
Description
Calculate velocities from the slopes of travel time curves.
Description
Determine layer depths using travel time and velocity data.
Step
Modeling Subsurface Structure
Description
Create geological cross-sections and interpret subsurface conditions.
Key Points to Remember
- Purpose: Determine subsurface stratigraphy and geological structure.
- Apparatus: Seismograph, geophones, seismic source, trigger mechanism.
- Procedure: Set up geophones, trigger seismic source, record data, analyze travel times.
- Data Interpretation: Calculate velocities, determine layer depths, model subsurface structure.
- Standards: ASTM D5777.
Final Summary
The Seismic Refraction Test is invaluable for identifying subsurface geological layers, aiding in construction, mining, and environmental projects. It provides detailed insights into the composition, depth, and structure of subsurface materials, essential for safe and efficient engineering practices.