The Parallel Seismic (PS) test is a non-destructive test that is used to determine the depth of foundations. This thesis explores the use of the PS method to detect flaws in in-situ constructed foundation piles as an alternative to existing testing methods. An acoustic signal is introduced to the head of the pile with a hammer. An elastic wave propagates through the pile, and vibrations are radiated out into the soil. Neighbouring the pile, geophones are lowered or pushed into the ground such that the vibrations in the soil can be measured. Theoretically, the vibrations originating from the pile shaft should provide a blueprint of the pile's condition. Flaws will affect wave arrivals, damping, and cause reflections. However, difficulties arise if the soil should be considered heterogeneous or when vibrations of the shaft are masked by other vibrations originating from the surface or the pile toe. A computer model was made using an analytical approach to determine what vibrations may develop. This was followed by two field tests with the PS on an experimental pile field. From the results an understanding of vibrations in the pile and soil was developed and an attempt was made to detect any flaws. The procedure did not sufficiently measure the vibrations radiated from the pile shaft while avoiding interference from other vibration sources or soil heterogeneity. The field test did however provide a better understanding of the vibrations that develop in the soil. This provided improvements on the model, allowing for more extensive simulations. Based on these, possible improvements on the PS procedure were realised.