Master curves, computed IP, resistivity, or electromagnetic response plotted against electrode interval, source-receiver spacing, or frequency for various models. Type curves are used for interpreting field data where the conditions of the models appear to hold. Abscissa and ordinate are usually normalized so that the curves are dimensionless and plotted on log-log paper; data may be compared by using transparent overlays. Type-curve derivations often employ the method of images. Models are sometimes given ‘‘type’’ names as in Figure T-7.