From Oman Drilling Tests to the Mohole to Mantle (M2M)

To understand the Moho discontinuity, first identified in 1909 as a downward step increase in seismic wave velocity, it is necessary to drill and sample the oceanic crustal-mantle transition zone (MTZ). This original goal of scientific ocean drilling, articulated by Harry Hess in 1959, has come closer via an M2M site survey offshore Hawaii (Sep-Oct 2017), and new technology tests during drilling the crust-mantle transition of the Samail ophiolite by the Oman Drilling Project (Nov 2017 to Apr 2018).

Following the International workshops (2010), M2M proposal writings and initial feasibility studies for drilling (IODP/Blade, 2011), JAMSTEC and the University of Hawaii made site survey over 1000 km northeast of Hawaii (Yamashita et al, 2018). In parallel, the Oman Drilling Project provided an opportunity to evaluate M2M strategies at low risk by making observations in drill core, that can be related to the surrounding three-dimensional geology at any desired scale (Kelemen et al., 2014). During coring at Sites CM1 and 2 drilling parameters were recorded. These records were supplemented by core scratch tests, newly developed borehole magnetometer measurements, and the most advanced geochemical logging technology ever used in scientific drilling (Moe et al., 2018).

Seeking instruments that operate in hostile environments and provide the most advanced technology for future Mohole drilling, we worked with Schlumberger to acquire high-resolution and continuous measurements of density, porosity, natural and spectral gamma, resistivity, sonic velocity, and geochemistry estimates (elements & minerals) via wireline logging in Oman CM boreholes. Data quality was excellent. Preliminary results match observations of core and scratch test results. More extended data integration is underway. Oman results will aid in planning for offshore M2M operations.

Distance from the coast, water depth, drilling target depth, crustal age and expected Moho temperature are different for the 3 identified, potential sites for M2M. Further scientific investigation, cost estimates, and technology assessments will supplement the IODP-MI/Blade Report. Now that 4000 m water depth riser drilling capability and 200°C tools have become standard, together with the new results presented here, realization of M2M drilling is closer than ever.