Microgravimetry is indeed a powerful method for the detection of many small geologic and anthropogenic bodies (Arzi, 1975). Fajklewicz essentially advocates a particular procedure for microgravimetric surveys. Whereas gravity measurements are usually performed at the ground surface, Fajklewicz simultaneously performs measurements also at a surface situated 3 m above ground, using a portable tower. His data are presented as a map of the difference between the bottom and the top measurements. This difference (which may be measured in gradient units after being divided by the tower height) is called the Gravity Tower Vertical Gradient (GTVG). Similar procedures have been occasionally employed for various purposes (Thyssen‐Bornemisza, 1976). My main comments on the paper by Fajklewicz are as follows.

One of the problems encountered in a variety of near-surface investigations is detecting and mapping localized inhomogeneities. Typical examples of such inhomogeneous sources are cavities, caves and tunnels. Different methods for detecting shallow subsurface sources utilizing seismic waves diffracted by these sources were proposed by many researchers in the last three decades. Most of these methods suggest that every subsurface point is a possible location of a point diffractor. Imaging of the diffractors is based on a spatial summation of the diffracted wavefield along diffraction time surfaces (defined by source-receiver geometry) in 2D or 3D space. The summation is performed with a fixed velocity value estimated from velocity analysis of the diffraction data. In this study, we present a path integral summation approach, where for every subsurface point the wavefield is stacked together along all possible diffraction time surfaces having a common apex at a given time. The result of the imaging is a 3D volume in which prominent diffraction anomalies appear at spatial locations close to the imaged sources. This path integral summation approach has been successfully tested on synthetic data and further applied at several sites with known subsurface sources.

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