Plus–minus method

It can be used to calculate the depth and velocity variations of an undulating layer boundary for slope angles less than ~10°.

The method is based on the analysis of the so-called 'plus time'

Assuming that the layer boundary is planar between A'' and B'' and that the dip is small (<10°), the plus time

over which the slope is estimated should be chosen according to data quality.

results in more stable velocity estimates but also introduces stronger smoothing.

Like in classical refraction analysis, the thickness of the upper layer can be derived from the intercept time

:[1][2] This requires an estimation of the velocity of the upper layer

which can be obtained from the direct wave in the traveltime diagram.

[2] Furthermore, the results of the plus–minus method can be used to calculate the shot-receiver static shift

The plus–minus method was developed for shallow seismic surveys where a thin, low velocity weathering layer covers the more solid basement.

The thickness of the weathering layer is, among others, important for static corrections in reflection seismic processing or for engineering purposes.

An important advantage of the method is that it does not require manual interpretation of the intercept time or the crossover point.

This makes it is also easy to implement in computer programs.

However, it is only applicable if the layer boundary is planar in parts and the dips are small.

These assumptions often lead to smoothing of the actual topography of the layer boundary.

However, the plus–minus method is still used for real-time processing in the field because of its simplicity and low computational costs.