The accuracy of Norton's empirical approximations for groundwave attenuation

In his papers on the numerical evaluation of the Sommerfeld surface wave attenuation, Norton gave the results primarily in graphical and tabular form, but he also included some empirical approximations. These approximate expressions are simple and readily programmed on hand calculators, while the exact expression involves the complementary error function with complex argument. The accuracy of the approximations is therefore of practical interest; the results of numerical checks of the accuracy of the various approximate expressions are reported here.