大地电磁法是一种天然源电磁测深方法，具有勘探深度较大，分辨能力强，观测效率高的特点，是探测岩溶及研究地质构造和寻找隐伏断层的 有效方法。尤其在大中型隧道的前期勘探中将会发挥重要的作用。该方法与常规的同类物探方法相比，在勘探深度和分辨率均有较大的提高。介绍了大地电磁法在沪蓉国道隧道比选勘 探中的应用效果，查明了两条测线的溶洞发育情况、断层分布及其特征，为今后在隧道、桥址地质勘察中快速探测溶洞及断层等地质构造，探索了一条新途径。

The magnetellurics(MT) is a natural source electromagnetic technique. It has many advantages, such as more deep exploration, higher resolving power and survey efficiency. In exploring karst, studying geological structure and looking for hidden faults, especially in exploring tunnel of earlier stage, the MT is an effective method. Compared with the other geophysical meth ods, the exploration depth and resolving power of this method have a greater imp rovement. An application of the MT in the tunnel exploration of Hu-Rong nationa l road is presented. It is verified that this method can ascertain the geologica l status of some tunnel and has received good effect.