Geophysical prospecting of underground caves being performed on a construction site is often still a challenging procedure. Estimation of a likelihood level of an anomaly found is frequently a mandatory requirement of a project principal due to necessity of risk/safety assessment. However, the methodology of such estimation is not hitherto developed. Aiming to put forward such a methodology the present study (being performed as a part of an underground caves mapping prior to the land development on the site area) consisted of application of electrical resistivity tomography (ERT) together with statistical analysis utilized for the likelihood assessment of underground anomalies located. The methodology was first verified via a synthetic modeling technique and applied to the in situ collected ERT data and then crossed referenced with intrusive investigations (excavation and drilling) for the data verification. The drilling/excavation results showed that the proper discovering of underground caves can be done if anomaly probability level is not lower than 90 %. Such a probability value was shown to be consistent with the modeling results. More than 30 underground cavities were discovered on the site utilizing the methodology.