Scientists have developed a method of directly measuring rock strength beneath the seafloor by using drilling performance parameters.
This study project was led by Dr. Yohei Hamada at the Fault Mechanics Research Group of the Japan Agency for Marine Earth Science and Technology (JAMSTEC).
The depth-continuous in situ rock strength is a critical parameter for numerous studies in earth science particularly for seismology and tectonics at plate convergence zones. To measure rock strength below the seafloor, laboratory experiments have been common. However, due to limited amounts of rock samples and uncertain conditions below seafloor such as pressure and temperature, the depth profile of the rock strength has not been able to determine.
To identify continuous rock strength, the team has focused on performance of a drill-bit model such as drill string rotational torque, bit depth, and string rotational speed. Using data recorded during drilling operations of the Nankai Trough Seismogenic Zone Experiment (NanTro SEIZE) by the deep water drilling vessel, Chikyu, they constructed a conversion equation from the parameter into the strength.
Their results have identified gradual increase in the strength from 1,500 meter-below-seafloor (mbsf), while no significant strengthening is found at the major lithology boundary between the forearc basin sediments and the accretionary prism. From that depth to the deepest part of a hole at 3,000 mbsf, the drilling-parameter-based measurement has indicated that the rock strength has been larger than expected from the pressure condition. It suggests that sediments above the megathrust zone, which caused the 1944 Tonankai Earthquake, have been strengthened by horizontal compression due to the plate conversion.
According to JAMSTEC, this new method offers significant advantages in measurement of rock strength below seafloor even when core samples are not available as it is highly practical for non-core or technically challenging drilling projects at hydrothermal areas and future mantle exploration.
Though further study including laboratorial drilling experiments is necessary for higher accuracy of the conversion method, it is expected to help identify how strong the rocks are and how the elastic strain energy accumulates around the plate boundary fault.