中文摘要：

坑道空腔的存在使其周围电场分布不同于全空间的电场分布．以无限大板状体为例求解全空间电场分布，提出探厚比概念以讨论超前探测极限问题．通过对比全空间与坑道内超前探测异常形态可知，坑道空腔只会影响极距较小时的视电阻率幅值，不会掩盖迎头前方地质体的异常响应．大量模型计算结果表明，电阻率异常极值位置与板状体位置之间的差异在5-10倍之间，且同时受到异常体厚度、电性以及板状体倾角的影响．仅利用电阻率极值位置准确解释超前探测异常的难度较大．

英文摘要：

The distribution of the electric field around a tunnel is different from that in full space because of the tunnel-cavity effect. Taking the infinite plate for instance, the distribution of electric field in full space is solved and detection-thickness ratio is presented to discuss the limit of advanced detection. The apparent resistivity can be influenced by on small spacing of electrodes, but the anomaly of geologic-bodies ahead of the tunnel is not screened by the tunnel-cavity＇ s effect. The calculating results of many models show that the place of the anomalous extreme is 5 - 10 times the plate and influenced by thickness and electric property of the plate. It is difficult to determine exactly the geologic body only using the place of the anomalous extreme.