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A microgravity investigation was conducted on an excavated foundation located in the Technical University of Tehran. The aim was the detection of subsurface cavities or other anomalous conditions that threaten the stability of the foundation. The survey consists of 420 gravity stations. The positive coarse grain alluvium saturated by surplus water and negative cavity anomalies have been detected based on the Bouguer and second vertical derivative maps. The depth of the sources have also been determined. To confirm the results an inversion 2D-modelling is also applied to residual anomalies. Some of the shallow accessible anomalies have been confirmed in the field by excavation.