The results of drill core observation are among the most important basic data for geological survey; however, these results tend to include subjective aspects, because the core sample is qualitatively evaluated through macroscopic observation by engineers with different skill levels. Similarly, the classifications of strata, rock hardness and degree of weathering, which are derived from the results of drill core observation, tend to be subjective and tend to depend on the abilities of the engineers involved in determining them. These subjective aspects sometimes result in differences in evaluations that make it difficult for others to understand the basis for evaluation.

This study aimed at adding quantitative indexes to core observation. Measurements of magnetic susceptibility and color were done for drill cores of igneous rocks and sedimentary rocks. To examine the validity of the quantitative indexes, the results of classification of strata, rock hardness and weathering from the quantitative method were compared with those from conventional core observation.

Magnetic susceptibility and color were measured using portable meters, which enabled onsite measurement.

Examinations revealed that the differences in measured magnetic susceptibility and color correspond to the differences in strata, lithofacies, rock hardness and degree of weathering. In measurements using a magnetic susceptibility meter and a color meter, it was possible to qualitatively determine the borders between different types of rock, even when it was difficult to determine the border by macroscopic observation because the changes in the borders were gradual. Using measurements of magnetic susceptibility and color in combination made it possible to examine the results by comparing and supplementing the results from the two measurement methods.

Measurements of magnetic susceptibility and color of drill cores showed that it was possible to use these measurement results as useful quantitative indexes for classifying strata, lithofacies, rock hardness and degree of weathering, for comparing multiple cores, which is conventionally done by visual inspection, and for determining the landslide slip surface.

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