Karst is one of the hidden dangers in the construction of bridge pile foundation. It is particularly important for the safety of the bridge to prove the spatial distribution characteristics of karst in the scope of the bridge. At present

the exploration of karst is mostly based on the combination of drilling and geophysical prospecting methods. But the specific karst exploration needs to choose the appropriate geophysical method according to the specific situation. Commonly used geological radar and other geophysical prospecting methods often can not accurately determine the boundary of the local area of the cave due to their limited scope of application. The cross hole electromagnetic wave CT technique reconstructed the distribution image of the medium absorption coefficient of the wells base on the difference of the absorption of electromagnetic wave in different media. The image visually shows the internal structure of geological bodies. It has the characteristics of high resolution and small exploration distance. It is especially suitable for detecting the distribution of karst cave in the local area such as bridge pier. This paper introduces the principle

working method and application effect of the electromagnetic wave method CT detection technology by an example of detecting the karst of Shuixi bridge pier of highway S119. The results show that cross hole CT technology has high precision and reliability. It can fully and clearly reveal the spatial distribution of the karst

so as to provide reliable geological data for engineering design.