This study presents the impact echo method to estimate the depth of inverted T-shape foundations at fields. Effects of different impact sources and effects of using different accelerometers and geophones were investigated to optimize the testing system. the foundation depth was computed from the resonant frequency value and the P-wave velocity of the foundation. In addition, such presence of the multiple modes of longitudinal vibration in the inverted T-shape foundations were also confirmed in the numerical modeling results. Last, the field test results on more than 50 towers were compiled to examine the reliability of this method.