A method is proposed for obtaining reflectivity magnitudes from seismic record sections that are densely sampled in the horizontal dimension. The technique consists of finding the spectral ratio of subcritical (target) to postcritical (reference) reflection amplitudes. The method's novelty is that both target and reference amplitude spectra are estimated from slant (also known as ray parameter-intercept time, p-tau velocity) stacks of the record sections. The source level is determined because the reference amplitude is totally reflected; source level estimation does not require knowledge of the impedance structure of the reference reflector. The technique functions well for reflections having low S/N ratios, little or no lateral continuity, poor near-surface velocity control and steep dips. Application of the method to the Consortium for Continental Reflection Profiling (COCORP) survey in the Rio Grande supports the interpretation that its large-amplitude midcrustal reflection is a magma body.