f “SwyphoneTM”: A New seismic sensor with increased response to SH-waves

It is well known that reflection seismic is an extremely powerful technique of investigation but, at the same time it is also quite expensive. The related costs, mainly as far as the shallow high resolution surveys are concerned, limit an extensive application of it. This is the reason why today research is mainly focussed to a technological improvement so that data could be acquired more and more quickly and with a signal to noise ratio more and more favourable. Although several authors have demonstrated that certain situations could be addressed more effectively through the use of SH-waves rather than the more commonly used P-waves, SH-waves are far from being a widely used exploration tool since they involve higher costs than P-waves. When suitable circumstances occur, and the appropriateness of the method is verified, the key to the successful use of shallow seismic reflection is its cost-effectiveness, that is, a quick data-acquisition and minimal data processing. Usually, SH-wave data acquisition involves the use of polarized sources. SH-wave records require two shots, striking the source in opposite directions perpendicular to the seismic line, which must be subtracted to eliminate P-wave contamination. This means an increase of survey costs with respect to similar P-wave survey. Also, it is common experience that effective P-wave removal requires the equalization and the removal of time break variations of shot records prior to subtraction. This means additional processing time and, as a consequence, additional costs. In order to avoid the above drawbacks we have designed and realized (Sambuelli and Deidda, 1998) a new receiver that enables to acquire SH-wave data without the abovementioned additional costs. Using this receiver, SH-wave records, without or with minimal P-wave contamination, can be achieved performing single shots and avoiding extra processing. Consequently, SH-wave survey can be performed as well as P-wave survey, without involving traditional extra costs. In an attempt to quantify the significant characteristics and field performances of the new receiver, some laboratory (electrical and mechanical) and field (with different acquisition parameter settings) tests with consistent testing procedures and equipment were carried out. Besides, in order to have a comparison between this receiver and the traditional one, all tests were carried out on two sets of receivers: 24 SwyphoneTM type receivers (prototypes) and 24 Mark Product (L-40 A2) 100 Hz horizontal geophones. All results prove that the SwyphoneTM has better performances and so, it is an improvement in SH-wave reflection seismology that could be of great assistance to engineering seismologists. In particular, with respect to traditional geophones, it has a higher sensitivity to SH-waves, higher energy efficiency and allows cost-effectiveness data acquisition and data processing. The higher sensitivity to SH-wave and higher energy efficiency of the SwyphoneTM type receiver, if it is mounted together with a vertical geophone, allow performing P- and SH-wave simultaneous data acquisition. Figure 1 shows a P- plus SH-wave record acquired with a 48- channel ABEM Mark VI seismograph. Odd channels recorded P-wave signals while even ones recorded SH-wave signals. The P- and SH-wave records can be obtained by traces sorting.