The forward calculation shown that when providing current through whole casing pipe , the electrical potential on surface is mainly effected by shallow resistivity anomaly , whereas when providing current only at gun perforation position , the electrical potential is on surface mainly effected by corresponding resistivity of gun perforation position 正演结果表明：在全井套管供电时，地表观测电位主要受浅层电阻率不均匀性的影响；在射孔段供电时，地表观测电位主要反映射孔段的电阻率分布。
That causes new problems of construction on making use of or abandon old basis structure . based on the characteristics of the bridge foundation , this paper in detail introduces the design of construction procedures , construction methods and technological process for large temporary structures in foundation construction and driven or pull out steel casing pipe etc , which are seldom meet in common bridge construction . about digging and con struction of foundation piles , dynamic detection method of pile foundation etc . are also introduced and analysized 由于设计的变更，前期已施工的基础结构与修改后的结构有较大差异，给施工带来旧基础利用和废弃处理等新问题，本文针对该桥的基础特点，详细介绍了基础施工中大型临时结构、打拔钢护筒等一般桥梁施工中很少遇到的施工工序的设计、施工和工艺问题，对于基桩的开挖和施工、桩基的动态方法检测等问题也作了详细介绍和分析。
