Downhole Seismic Test
1. Concept of the Test
The Downhole Seismic Test is a geophysical method used to measure the velocity of seismic waves traveling through subsurface materials. This test is typically conducted to determine the dynamic properties of soil and rock layers, which are crucial for earthquake engineering, site classification, and foundation design. The test involves lowering geophones or other sensors into a borehole and using a seismic source at the surface to generate waves. The travel times of these waves are then recorded at different depths to calculate the seismic wave velocities. The test is standardized under ASTM D7400 and ISO 18649.
2. Descriptions and Procedure
Apparatus Required
- Borehole (drilled to the required depth)
- Downhole geophone or tri-axial geophone array
- Seismic source (e.g., sledgehammer, weight drop, pneumatic hammer)
- Seismograph or data acquisition system
- Trigger mechanism
- Borehole casing (optional, to stabilize the borehole)
- Measuring tape or depth indicator
- Cables and connectors
- Software for data analysis
Procedure
1. Site Selection and Borehole Preparation
- Select a site with minimal surface obstructions and noise interference.
- Drill a borehole to the required depth, ensuring the borehole is stable. If necessary, install a casing to prevent collapse.
2. Geophone Installation
- Lower the downhole geophone or tri-axial geophone array into the borehole to the first measurement depth.
- Ensure the geophones are securely positioned at the desired depth and are properly oriented to detect seismic waves.
3. Seismic Source Placement
- Position the seismic source at a fixed location near the top of the borehole.
- Ensure the source is capable of generating sufficient energy to penetrate the subsurface layers.
4. Triggering the Seismic Source
- Use the trigger mechanism to synchronize the activation of the seismic source with the recording of seismic waves.
- Generate seismic waves by activating the source (e.g., striking a metal plate with a sledgehammer).
5. Recording Seismic Waves
- The seismic waves travel through the subsurface and are detected by the downhole geophones.
- Record the travel times of the seismic waves at each geophone depth using the seismograph.
6. Incremental Depth Measurements
- Move the geophones to the next measurement depth (e.g., in increments of 1 to 2 meters).
- Repeat the process of generating seismic waves and recording travel times at each depth.
7. Data Collection and Analysis
- Collect data for multiple depths to ensure comprehensive coverage and accuracy.
- Download the recorded data from the seismograph for analysis.
Data Interpretation
1. Travel Time Analysis
- Plot the travel times of the seismic waves against the depths of the geophones.
- Identify the arrival times of different wave types (P-waves and S-waves).
2. Velocity Calculations
- Calculate the seismic wave velocities (Vp for P-waves and Vs for S-waves) using the following formula: 𝑉=Δ𝑑Δ𝑡 where V is the velocity, Δd is the change in depth, and Δt is the change in travel time.
3. Layer Identification
- Determine the boundaries and properties of different subsurface layers based on changes in wave velocities.
- Identify key geotechnical properties, such as stiffness and density, from the calculated velocities.
4. Site Classification
- Use the measured Vs values to classify the site according to standard codes and guidelines (e.g., NEHRP, Eurocode 8).
Summary Table
Key Points to Remember
- Purpose: Determine dynamic properties of subsurface materials.
- Apparatus: Borehole, geophones, seismic source, seismograph.
- Procedure: Install geophones, trigger seismic source, record data, analyze travel times.
- Data Interpretation: Calculate velocities, identify subsurface layers, classify site.
- Standards: ASTM D7400, ISO 18649.
Final Summary
The Downhole Seismic Test provides essential information on subsurface properties by measuring the velocities of seismic waves. It is crucial for engineering applications, including foundation design, earthquake site classification, and subsurface characterization. Proper execution and interpretation of the test ensure accurate and valuable geotechnical data.