Borehole and Underground Geophysics
Utilization of well logs in grade determination of ore beds in situ is expected to be a research topic with broad prospects, especially in exploration where coring is very difficult to conduct. Techniques currently used in China for grade determination in terms of well logs can be grouped into two categories, (1) nuclear log and (2) regression analysis. The nuclear log-technique has been used to determine fluorite grade by using a neutron-activation log and the regression analysis technique has been applied to calciomirabilite and phosphorus exploration with gamma-ray logs; both techniques have worked successfully. Based on the results we obtained, effective working procedures have been summarized.
Determination of ore bed grade by use of well logs proves superior to use of the conventional method, which basically consists of drilling and coring, visually sampling, and core assaying. The advantages of well logs can be seen from the following viewpoints. They conduct continuous measurement to obtain complete information on ore beds. The detecting-radius of well logs can be much larger than that of cores. Well logs are not usually affected by errors generated from chemical analysis due to poor core recovery and random sampling. Therefore, well logs provide more realistic data for grade determinations, as is clearly demonstrated in practical examples from three ore districts in China. Well logs can serve as a guide for reasonable sampling and can be used effectively to work out a program for the next exploration stage. When drilling exhibits poor core recovery or when certain ore beds are missed by drilling, well logs are used.