Optimising the computational performance of high degree lithospheric field models

The British Geological Survey (BGS) World Magnetic Anomaly Model (WMAM) code calculates spherical harmonic models of the natural magnetisation of the rocks of Earth’s crust. These models allow us to estimate the value of the full magnetic field vector at any location, based on scattered pointwise marine or aero-magnetic measurements of only the scalar magnetic field. Modelling the magnetic field in this way serves many important purposes, such as geological research, navigation and safe resource extraction. Global spherical harmonic models of degree and order 1440 (28 km spatial resolution) have been successfully computed on the HPC facilities local to BGS, but such runs require nearly the full compute capacity for multiple days. Further, the available resolution of the scalar field measurements is too high to be fully exploited by the WMAM code, limiting models of the crustal magnetic field to a resolution of 28 km. To overcome these issues, we refactored the WMAM code such that models of spherical harmonic degree 1440 and 2000 (20 km resolution) can be produced in hours rather than days. For example, a degree 2000 model was calculated using 64 HPE Cray EX nodes (8 192 cores) in 3 h and 44 mins. The resulting model power spectra and magnetic field maps showed excellent agreement with the existing degree 1440 model and the original input data. The performance of the WMAM code was further improved via offloading to GPU. We show the improvements due to GPU acceleration in terms of energy consumption as well as runtime. This fruitful collaboration between experts in the fields of Geoscience (BGS) and HPC (EPCC) has created the opportunity for the WMAM code to be used to gain new knowledge about crustal magnetic fields.