The spatial variation of S-wave coda attenuation () in the Galeras volcano region is analyzed. The study region is at present an active magmatic system situated in the southwestern Colombian Andes. is estimated using seismograms from 435 volcano-tectonic earthquakes recorded at 31 stations of the Galeras seismograph network. is then imaged using a 3D spatial stacking procedure. The technique is based on the assumption of uniform distribution of over a spheroidal shell with its volume determined by the associated source–receiver distance. The resultant tomograms also show frequency dependence, which is interpreted in terms of the scale of the heterogeneities producing the scattering. High-attenuation anomalies are detected at high frequencies. Synthetic tests indicate the validity of the inversion technique.