Geotechnical Applications of Downhole Sonic and Neutron Logging for Surface Coal Mining

At BHP-Utah Coal Ltd minesites, sonic and neutron logs are obtained in about twenty percent of exploration drill holes to identify the geotechnical properties of the overburden rocks for blast design and diggability assessment. Applications in blast design range from the simple identification of massive, high-strength rock units through to the semi-quantitative measurement of rock strength from correlation with sonic log transit time. At Goonyella Mine a bucketwheel excavator is used to remove overburden, and the overburden diggability is determined from sonic logs. The onset of hard digging conditions is identified by a sonic transit time of 150 μs/ft (equivalent to 492 μs/m), and marginal digging conditions occur at 125 μs/ft (equivalent to 410 μs/m). Direct correlation between geophysically determined digging resistance and monitored production data from the bucketwheel excavator shows that the sonic log method of digging resistance measurement is superior to more conventional geomechanical testing of rock cores.