大港—渤海西部地区重力异常数据的处理和解释

Processing and interpretation of gravity anomaly data in Dagang-west Bohai area

曾华霖,杨华在大港油田和渤海油田西部地区存在三个主要密度界面,即新生界与中生界或上古生界之间的界面（S1）、中生界或上古生界与下古生界之间的界面（S2）、莫霍面（S3）。此区的重力布格异常实质上是上述三个界面引起的异常之和。本文的目的是依据现有的重力布格异常图从中分离出下古生界顶面（S2）引起的异常,求取S2的埋藏深度。首先采用线无法进行正演模拟,分别计算S1和S3界面引起的重力值△gs1和△gs3,再将实测值减去△gs1和△gs3,即可求得S2界面引起的重力值△gs2。然后应用Cordell法对△gs2重力值进行反演模拟,求得下古生界顶面（S2）的埋藏深度。此法的计算结果与该区的钻探和地震反映的下古生界顶面的深度符合较好。

In the area of Dagang oil field and west Bohai oil field, there exist three major density interfaces: S_1 interface between Cenozoic group and Mesozoic group (or upper Paleozoic group), S_2 interface between Mesozoic group (or upper Paleozoic group) and lower Paleozoic group, and S_3 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 S_2 interface (the top of lower Paleozoic group) by separating out S_2 interface anomaly from existing Bouguer anomaly map. This is achieved in the following steps. First, gravity values △gs_1 and △gs_3 which result from S_1 and S_3 interfaces are calculated respectively by making forward modeling with the use of linear element method. Then, gravity value △gs_2 corresponding to S_2 interface is obtained by subtracting △gs_1 and △gs_3 from the total observed gravity value. Finally, the depth of S_2 interface can be ascertained by making inverse modeling of △gs_2 with the use of Cordell's method. This calculated depth quite agrees with those which are determined respectively from seismic reflection survey and drilling.