Patent ID: 8731888

Claim:
A method comprising: representing an investigation domain using a plurality of discrete radiation source volumes, the investigation domain comprising at least one layer of a geological structure and each radiation source volume being characterized by a constant number of gamma emissions per second per unit volume; modeling a gamma ray (GR) tool response by computer simulating an intensity of gamma ray particles emitted by each radiation source volume of the plurality of discrete radiation source volumes which would impinge upon a point GR detector without scattering in the propagation path between an emitting one of the radiation source volumes and the point GR detector, the gamma ray particles impinging upon the point GR detector without scattering being zero-scatter gamma ray particles, wherein the modeled gamma ray intensity is a sum of contributions from all discrete radiation source volumes that make up at least one layer of the investigative domain, and wherein said modeling a gamma ray tool response comprises beginning with an expression for intensity dJ of unscattered gamma rays of a selected energy at a point detector emitted from a radiation source volume dv, in which expression the intensity falls off as the inverse square of the distance between dv and the detector and also decays exponentially with distance due to absorption attenuation of gamma rays along a path between dv and the detector, then summing the expression for dJ over all source volumes making up at least one layer; and wherein, in said modeling, input quantities include spatial location of the detector, which is known, and model parameters including one or more medium-dependent constants representing absorption of gamma rays per unit distance, one or more medium-dependent constants representing number of gamma emissions per second per unit volume, and structure information in the form of location values defining layer boundaries, where the model parameters are unknown and are represented by assumed values in the simulation; and using the modeled gamma ray tool response to interpret actual gamma ray logs measured by the GR tool being modeled for information regarding media and structure of the investigative domain.