Some considerations on shallow seismic reflection surveys

High-resolution shallow seismic reflection surveys require more attention t o the choice of source and configuration, receivers and recording geometry for optimizing data acquisition than conventional oil exploration surveys. Moreover, some standard processing techniques to increase signal/noise (S/N ) ratio need special accuracy (for example, surgically precise removal of e arly-time coherent noise and iterative, small rime shift static corrections ). This paper compares results obtained using different sources at two test sites: explosive, cap, shotgun, hammer and weight drop. Data from experime nts using geophones with different natural frequencies and using various ac quisition geometries are also compared. In data processing, it is demonstra ted how increasing the S/N ratio for high-resolution results requires speci al consideration in some common processing steps (F-K filter, first arrival s muting, elimination of air wave and static corrections). The comparison, based on shot gathers and stack sections, shows that attenuation of high fr equencies by the earth is the most significant influence on the spectral pr operties of the data, as expected the source itself also does have some inf luence on frequency content, depending to some extent on surface conditions . The high-velocity explosive sources produced the highest frequency reflec tions and best S/N ratio, because they have higher energy related to higher burn/blast velocity and source containment then the other sources and they are used in hole (i.e. below ground surface where the air wave energy is m ore attenuated) but the shotgun also an explosive source was reasonably com parable to high explosive when used in hole. Special care must be taken dur ing processing to insure artifacts are distinguished from real reflection e vents. (C) 2000 Elsevier Science B.V. All rights reserved.