On: “A new insight into the reciprocity principle” (Børge Arntsen and José M. Carcione, GEOPHYSICS, 65, 1604–1612).

Reciprocity is an important property of elastodynamic, electromagnetic, and acoustic wavefields. Combined with optimization techniques, reciprocity theorems can be regarded as providing the basic ingredients to imaging and inversion methods in geophysical exploration and remote sensing (de Hoop, M. V. and de Hoop, A. T., 2000). Furthermore, reciprocity serves as the basis for the elimination procedures of surface‐related multiples in marine seismic data processing (Fokkema and van den Berg, 1993). In view of all this, a thorough and elucidating discussion on the configurations to and the conditions under which reciprocity applies, and what reciprocity leads to, like the recent paper by Arnsten and Carcione (2000), serves a useful purpose. In this paper, also a number of interesting applications are worked out in detail. The aim of this discussion is to indicate briefly how some of the results obtained in that paper are related to the ones that the present author has presented in de Hoop (1995), a reference that does not appear in Arntsen and Carcione (2000).