Abstract: An apparatus and method for logging information concerning the identity and/or condition of materials present within a wellbore is based on gamma ray spectroscopy. The apparatus includes a gamma ray source and at least one spectral gamma ray detector, with a controller capable of commanding or effecting digitization of the spectral data obtained by the detector. The data can be stored in a memory system and downloaded for analysis. The invention eliminates the problem of instrument “gain” and, because it generates spectral data, provides downhole qualitative and quantitative information beyond that of conventional non-spectral downhole imaging means and methods. Applications include, but are not limited to, reliably determining gravel pack integrity.