Abstract:In the area of Dagang oil field and west Bohai oil field, there exist three .major density interfaces: S1, interface between Cenozoic group and Mesozoic group (or upper Paleozoic group), S2 interface between Mesozoic group (or upper Paleozoic group) and lower Paleozoic group, and S1 interface-Moho (Mohorovicic discontinuity). Gravimetric Bouguer anomaly in this area is actually the sum of the anomalies which result from the three interfaces. Our object is to ascertain the depth of S2 interface (the top of lower Paleozoic group) by separating out S2 interface anomaly from exisitng Bouguer anomaly map. This is achieved in the following steps. First, gravity values △gs1 and △gs3 which result from S1 and S3 interfaces are calculated respectively by making forward modeling with the use of linear element method. Then, gravity value △gs2 corresponding to S2 interface is obtained by subtracting △gs1 and △gs3 from tire total observed gravity value. Finally, the depth of S2 interface can be ascertained by making inverse modeling of △gs2 with the use of Cordell's method. This calculated depth quite agrees with those which are determined respectively from seismic reflection survey and drilling.