1. In a two-dimensional seismogram depicting at least one trace having amplitude variations representing the variations in the value of a property of rock layers beneath the earth's surface as a function of horizontal and vertical dimensions of the earth; said amplitudes varying in one dimension on both sides of a null axis extending in the other dimensions; said null axis intersecting said trace at null points; the portion of the trace between each pair of consecutive null points forming a lobe; and a pair of consecutive lobes forming a wavelet, characterized in that only portions of the areas under at least the significant wavelets on said trace, on both sides of the null axis, are shaded, and each shaded area originates substantially at a null point.