approx_update

EnRML (IES) without the prior increment term.

approx_update

Approximate LM Update scheme as defined in "Chen, Y., & Oliver, D. S. (2013). Levenberg–Marquardt forms of the iterative ensemble smoother for efficient history matching and uncertainty quantification. Computational Geosciences, 17(4), 689–703. https://doi.org/10.1007/s10596-013-9351-5". Note that for a EnKF or ES update, or for update within GN scheme, lambda = 0.