Between 1992 and 1997, under a project conducted by the Water Resources Bureau, more than 200 well loggings were measured by Taiwan Sugar Corporation Groundwater Center (TSCGC) in the Choshui and Pingtung areas. The purpose of this paper is to evaluate the workability of the long trend method in this hydrogeology investigation project. Previous studies used the Schlumberger formula to calculate the mud content and had a 40% error rate. The reason for this high bias is that mineral assemblage, casing, caves and borehole diameter variations shift the natural gamma readings. In the author¡¦s experience, borehole diameter variation is the major factor. The gradual shifting greatly reduces the exactness of the Schlumberger formula but isn¡¦t a seriouw problem when using the curve type method. In this research gamma logs in three boreholes of the Choshui fan-delta, Taiwan, are classified to basic log trend facies and compared with the core samples.