Seismic Reflection and Refraction Survey

Seismic Reflection
Reflection seismic techniques share similarities with refraction methods, as they involve recording the travel times of induced seismic waves that are reflected from subsurface interfaces to an array of geophones positioned at known distances from the source. Reflection of the transmitted energy occurs only when there is a difference in acoustic impedance (the product of seismic velocity and density) between these interfaces. Since we are capturing reflections, the seismic waves traverse a significantly shorter distance within the subsurface. As a result, these seismic waves exhibit higher frequencies, which can lead to improved resolution. Seismic energy is introduced using the same methods employed in refraction surveys, and the reflected travel times to various interfaces are documented on a seismograph.

Travel times are influenced by the seismic velocities of all subsurface materials between the surface and a specific interface, but do not account for their differences; thus, reflection techniques can be utilized to determine the depths of less dense materials beneath denser strata. Additionally, because reflected waves cover a shorter horizontal distance, the reflection method can achieve greater depths with less energy compared to the refraction method. However, it is important to note that seismic reflection is more sensitive to interference, making it less suitable for use in noisy environments.

Seismic Refraction
Seismic refraction is a widely utilized technique for mapping the depth to bedrock, capable of reaching depths of up to 40 meters below the surface. The variations in material density and acoustic velocity of the subsurface, collectively known as acoustic impedance, facilitate the mapping of geological layers and the interpretation of material types. This process involves measuring refracted seismic waves in relation to the established geometry of the seismic wave path.

A seismic event is generated by an acoustic source, typically initiated at the surface, with the arrival times recorded at geophone sites located at predetermined distances from the source. Acoustic energy is delivered to the subsurface through a sledgehammer striking a metal plate. An instrument such as a seismograph is then utilized to measure the return of the refracted waves.

The seismic refraction method is a non-invasive and straightforward technique that provides numerical data regarding the spatial variability of the depth to bedrock. The depth to bedrock is crucial during geotechnical investigations, as it influences the stability of structures built above it. Shallow bedrock can be advantageous when solid foundation support is needed, minimizing the necessity for soil removal.