Methods, systems, and related computer program products for the non-invasive spectrophotometric monitoring of a biological volume having multiple tissue layers are described. Aggregate absorption and scattering properties are measured for each of a plurality of predetermined source-detector separation distances along a surface of the biological volume, the measurement being based on a model of the biological volume as a single-layer, semi-infinite, homogeneous volume. A predetermined multi-layer tissue model is retrieved that characterizes a mathematical relationship among (a) absorption and scattering properties of each layer of a multi-layer tissue structure, and (b) aggregate absorption and scattering properties of the multi-layer tissue structure as would be measured at selected source-detector separation distances along a surface thereof. The measured aggregate absorption and scattering properties are processed in conjunction with the predetermined multi-layer tissue model to compute therefrom a deep-layer-specific absorption property corresponding to the relatively deep tissue layer.