Fig. 6. Plot of prediction root mean square error (RMSE) and mean error (ME) for the various prediction methods used to validate the survey data in Field a) 18 and b) 20. Co-K = cokriging, OK1 = ordinary kriging of raw EM_0,H data, OK2 = ordinary kriging based on EC_e estimates, RK = regression kriging, 3-DK = three-dimensional kriging