Introduction
Shallow refraction wave measurement is a geophysical method used to measure seismic velocity of subsurface layers. Originally developed in the 1920s, the method involves the use of a seismic source to generate a localized wave that is refracted when it reaches an interface between two layers of different seismic velocity. Shallow refraction wave measurements are a time-consuming process but are effective for determining the depth, lateral extent, and thickness of subsurface layers and for creating high-resolution maps.
History
The use of refraction wave measurements for seismic exploration has been around since the 1920s. It gradually gained acceptance in the geophysical community, but remained relatively low-key until the advent of digital recording technology in the 1970s. Digital recording allowed for more precise measurements and analysis, leading to a surge in popularity for shallow refraction wave measurements.
Theory
Shallow refraction wave measurements involve the use of a seismic source to generate a localized wave that is refracted when it reaches an interface between two layers of different seismic velocity. The refracted wave travels with different traveltime and amplitude in the two layers and these differences are used to identify the subsurface layers and to calculate their velocity.
Process
The process of shallow refraction wave measurements typically involves four steps:
1. Seismic Source: A seismic source (typically vibration, dynamite, or air gun) is used to generate a localized refraction wave.
2. Recording System: A recording system consisting of receivers (e.g. geophones or hydrophones) is set up in a grid pattern for the purpose of receiving the refracted waves. Data from the recording system is then analyzed to determine the velocity of the refracted waves.
3. Data Acquisition and Analysis: Data from the receivers is collected and analyzed and the results are used to create a velocity profile for the subsurface layers.
4. Mapping and Interpretation: Results of the velocity profile are mapped to produce a high-resolution visual representation of the subsurface layers.
Applications
Shallow refraction wave measurements are often used in geological and geotechnical exploration projects. This method is particularly efficient in areas with difficult geological conditions due to its ability to image near surface layers. Examples of applications of shallow refraction wave measurements include investigating soil properties, detecting landslides and cavities, locating buried pipelines, and delineating dewatering systems.
Conclusion
Shallow refraction wave measurements are a geophysical method used to measure seismic velocity of subsurface layers with the use of a seismic source to generate a localized wave that is refracted when it reaches an interface between two layers of different seismic velocity. The method is often used in geological and geotechnical exploration projects and is known for its ability to create high-resolution maps of subsurface layers.
