What is Seismic tomography transmission (STT) testing?
STT testing is a cutting-edge non-destructive test method that uses seismic waves to map the internal structure of concrete elements.
Through measuring the time taken for these waves to move through a concrete structure detailed 3d images can be produced to identify the locations of defects such as cracks, voids or honeycombing.
The technique is extremely useful for probing deep into massive concrete structures such as dams, bridge supports or foundations.
How does Seismic tomography transmission (STT) testing work?
Seismic tomography transmission testing in concrete structures uses an array of multiple receiving transducers to capture the travel time of compression waves, the fastest wave type, and maps the velocity of these waves across the different parts of a concrete structure to create a 2D or 3D contour maps for defect localisation.
The compression or P-waves are generated by mechanical impacts on the concrete surface with accelerated weight drop devices preferred to standard sledge hammer impacts due to the greater consistency and reproducibility of force.
This approach closely aligns with classical ultrasonic velocity transmission techniques, providing detailed insights into the material's internal characteristics based on wave speed variations.
What is Seismic tomography transmission (STT) testing used for?
Deterioration process | Defects | Control of repairs |
To correlate the deterioration process | Grouting voids and voids in sound concrete Loss of concrete strength Fracture of pre-stressing steel | Quality control |
How do I carry out Seismic tomography transmission (STT) testing?
Several methodologies have been developed for carrying out STT testing including ASTM D7400-14 and ACI 228.2R-13 but so far, no standards documents have been released by BSI or ASTM specifically concerning STT for concrete structural testing.
The general process outlined for carrying out STT testing is summarised below:
Determine the geophone layout most suitable for capturing comprehensive data across the structure, with spacing adjusted based on the expected depth of investigation. A linear array or a grid pattern are commonly used, depending on the element of interest.
Use a sledgehammer or an accelerated weight drop device to create energy pulses at the surface. Weight drop devices are preferred as they ensure a controlled generation of consistent seismic waves.
Create an initial, hypothetical model of the subsurface based on known or assumed properties to serve as a baseline for interpreting seismic data.
Compare the actual seismic data with the model, adjusting the model to reduce discrepancies and repeat the process to refine the velocity model of the concrete structure and reveal details about its internal characteristics.
What equipment and expertise are required for Seismic tomography transmission (STT) testing?
Seismic Tomography Transmission (STT) testing for concrete structures involves using seismic waves to detect internal defects such as voids or cracks. This requires specialized seismic testing equipment, such as accelerometers and geophones, to capture wave reflections and transmissions through the concrete and expertise in interpreting seismic data to differentiate between structural anomalies and intact areas is crucial.
No commercial systems specifically designed for STT testing of concrete structures are available, however companies like Olson Instruments, Geomatrix Earth Science Ltd., and Proceq offer all components required for bespoke device creation.
What are the advantages of Seismic tomography transmission (STT) testing?
Capable of detecting a wide range of defects, including voids, cracks, and delamination.
Provides detailed internal images of concrete structures, enhancing understanding of internal conditions.
Can cover large areas quickly, making it efficient for extensive structures.
2D and 3D images can be created of defects for assessment.
What are the disadvantages Seismic tomography transmission (STT) testing?
Requires specialized equipment and expertise for data acquisition and interpretation.
May be less effective on very thick or highly reinforced concrete structures due to wave attenuation.
Data analysis can be complex and time-consuming.
High initial cost for equipment and training.
No off the shelf systems are currently available.
Slow measurements.
How accurate is Seismic tomography transmission (STT) testing?
The accuracy of STT testing for defect detection in concrete structures can vary significantly with the quality of the equipment used, the operator's expertise, and the specific conditions of the concrete being tested.
Modern STT equipment, combined with advanced analysis software, can provide detailed insights into the internal structure of concrete, identifying voids, fractures, and other anomalies with high precision. However, factors such as the presence of reinforcing materials, the concrete's heterogeneity, and environmental conditions can impact the accuracy of the results.
What are the limits of Seismic tomography transmission (STT) testing?
STT testing requires access to both sides of the concrete element where receivers must be coupled to the surface; if access to the structure is limited then this test type may be unsuitable. Where large quantities of rebar are present then this may provide a preferred path for the waves generated and affect test results.
Due to the significant cost and requirements of STT testing and the length of time it takes to receive results this may not be suitable for short, small scale, low budget projects that require rapid results.
This test may also not be suitable in areas with extreme environmental vibration from local industry, high speed train lines or busy roadways.
Ancillary information
Maturity of test: > 10 years
Qualification & interpretation : Specialised lab
Service disruption: No
Preliminary works: Yes
Time consumption Medium (one day)
Cost High
Access to element 2 faces
References and further information