The “conversion log” is a new approach to quantify seismic scattering in situ in terms of PS conversion in transmission along a vertical seismic profile (VSP): The amount of converted seismic energy is determined by slant-stacking and plotted as a function of depth, thus forming a borehole log of seismic conversion. We investigated seismic scattering of crystalline crust at the Continental Deep Drilling Site (KTB) in southern Germany where detailed knowledge exists of crustal parameters down to 9 km depth. In 1999 a deep VSP was acquired in the KTB main borehole. The experiment yielded high quality seismic data in terms of signal bandwidth, signal-to-noise ratio and stability of the source signal. The seismic data show varying levels of PS conversion along the borehole. The dip of layering and foliation is about 45° to 75° along the KTB drill hole. Under these conditions the conversion amplitudes depend only weakly on the angle between the incident seismic wave and the impedance contrast surface. The conversion log method was used to quantify energy loss by forward scattering. Field data were compared with finite-difference computations and with petrological and structural borehole information. It turned out that only 10–50% of PS forward scattering originates from conversion at lithological interfaces and structural complexity whereas 90–50% is due to velocity heterogeneity caused by fractures. The conversion log is correlated with the depth function of fracture density, and it is inversely correlated with the depth function of chlorite content, that seems to ‘heal’ the influence of cracks and fissures.