The lumped elements recently underwent some improvements, which are explained in detail in the following thesis
https://research.utwente.nl/en/publications/finite-element-methods-for-seismic-modelling Sjoerd Geevers, Finite element methods for seismic modelling (English), University of Twente Doctoral Theses, Enschede: University of Twente (2018), ISBN 978-90-365-4613-3.
