Smart and effective SH waves acquisition in shallow seismic reflection surveys

The generation of a seismic signal where compressional waves are completely absent is known to be a challenging task with shallow seismic surveys carried out in SH waves. This is true even when using appropriate source systems and taking the appropriate measures to prevent this from occurring. There are several factors that contribute to such issues including inclined energizing surfaces, oblique impact angles and the burial of the source system due to repetition of the shots. The presence of compressional waves is generally considered a serious problem in all SH waves investigations but it becomes a severe problem with reflection seismic surveys. In fact, amplification of the signal becomes a necessity with significant depths of investigation. This also occurs because attenuation of compressional waves is smaller than that of shear waves in saturated porous media. This paper presents an acquisition method that can prevent the conditions of amplitude non-equilibrium that may occur when operating with the well known cross-over procedure. This method makes use of signal stacking by calculating the difference between the opposite shot signals. Initially, signals from shots of the same polarity are summed together. Subsequently, the signals from a series of shots with reverse polarity are subtracted one by one. Due to the real time visualisation of the changes of the signal during the different phases, it is easy to identify how many reverse polarity shots are required to cancel the energy of the compressional waves within the seismogram.