Abstract: Gravity changes are the comprehensive reflection of density changes and deformation in Earth's medium.Gravity gradient changes reflect the change of gravity anomaly,with a resolution higher than the gravity anomaly changes.In this paper,using the gravity anomaly data during 1992—1994 and leveling data during 1988—1991,we compute the gravity anomaly caused by stratum density changes by eliminating Bouguer anomaly with Bouguer correction formula,then inverse the stratum density changes by using genetic finite element method.The gravity gradient changes are also computed in order to analyze the main contribution to the gravity change in the Xi'an area.The results show that the excessive extraction of confined groundwater has led to the confined water level drop,much water was lost and clay layer was compacted,and the resulting land subsidence is the main reason for gravity change.Densification of clay layer in most parts of the aquifer system can generate surplus stratum density,but for several local areas,such as Xiaozhai and Tielumiao areas,excessive extraction of ground water has lowered the groundwater level,resulting in water loss but not densification of clay layer.This is the major cause for some negative gravity changes.But we cannot rule out the contribution of possible activities of fissures in this area.

Key words: gravity changes, ground subsidence, genetic finite element method, Xi'an area

摘要： 结合1992—1994年的重力变化资料和1988—1991年的水准资料,运用遗传有限元方法计算西安地区的重力变化和对应的地层密度变化,然后计算重力变化梯度场进一步分析西安地区重力变化的原因。结果表明,过量抽取承压地下水引起承压水位下降、黏土层释水压密和两者共同作用导致的地面沉降是西安地区的重力变化的主要原因。大部分地区承压含水层系统中黏土层的压密产生正的剩余地层密度,而在如小寨、铁炉庙区的局部区域,抽取潜水导致潜水位下降、水体流失是产生负的地层密度变化的主要原因,但不排除其与地裂缝活动相关的可能性。

关键词: 重力变化, 地面沉降, 遗传有限元法, 西安地区