The Parallel Seismic (PS) method is used for length determination of unknown foundation depths for bridges and buildings. The strengths of this method are that it can be used even in cases where the foundation itself is not accessible, and can be used to determine the tip depths of foundations with complex geometries such as piles under spread footings.

The PS test involves impacting the foundation or the structure connected to the foundation to "listen" for subsequent signal arrivals with a receiver in an adjacent cased borehole. The cased borehole is typically located within 5-7 feet of the edge of the foundation and is drilled to a bottom depth well below the suspected/required foundation depth. The borehole can often be the same borehole used for a geotechnical boring at the site.

A variation of the PS method is the Parallel Seismic / Cone Penetrometer Test (PS/CPT) method, which uses an instrumented cone pushed into the soil as the receiver. With this system, no borehole is required.