The conditions for applicability of the reciprocity theorem, commonly invoked in seismic data gathering, are rarely met in usual practice.A field test was conducted in an area characterized by remarkable inhomogeneities of the surface layers to assess quantitatively the discrepancies between direct and reciprocal traces; the energy sources used were both of vibratory and impulsive type.The results show that the coherence between the two traces is good, except for the case of short offsets and early times with explosive sources. The vibratory source, even if theoretically 'more reciprocal' than the explosive one, yields coherencies in the same range, and is less uniform along the frequency axis.

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