The plus minus method is a geophysical principle attributed to J. G. Hagedoorn for statics solution derivation in which two emergence points on a refractor and a common midpoint (cmp) on the surface are considered. It is used to interpret seismic refraction profiles over irregular layers whose slope angles are less than 10°. Both forward and reverse shooting allow for matched time-distance graphs to be plotted.
The 'Plus' component of the method allows for the perpendicular depth to the refractor to be calculated. The 'Minus' component helps determine the seismic velocity of the refractive medium.
Examples:
t+ = tABCD + tDEFG - tABFG
t- = tABCD - tDEFG + tABFG
see the figure in the Seismic Data Processing text by Ozdogan Yilmaz on pg 227
References
Plus minus method Wikipedia(Text) CC BY-SA