Vasheghani, F. and Lines, L.R., 2012. Estimating heavy oil viscosity from crosswell seismic data. Journal of Seismic Exploration, 21: 247-266. The reliable estimation of oil viscosity is crucial for simulating fluid flow in heavy oil fields. While there are methods for measuring heavy oil viscosity from borehole samples, it would be very useful to reliably estimate fluid viscosity between wells using seismic data. This paper applies a methodology for estimating viscosity from crosswell seismic data, by using seismic traveltime tomography, seismic attenuation tomography and rock physics. Heavy oil sands are viscoelastic and the oil affects the attenuation of seismic waves, which can be measured in terms of the seismic quality factor, Q. We relate seismic Q to fluid viscosity in a two stage process. Q tomograms are estimated from crosswell seismic data by applying a tomographic technique proposed by Quan and Harris (1997). Q is then related to fluid viscosity by utilizing BISQ (Biot Squirt Theory), a poroviscoelastic model that couples the simultaneous Biot and squirt flow mechanisms. The classical BISQ equations of Dvorkin and Nur (1993) are modified to allow for two phase flow of bitumen and water. By applying Q tomography and BISQ, we estimate viscosity tomograms between boreholes. The estimated viscosity tomograms show ambiguity because for every Q value, more than one viscosity value can be calculated. Despite this ambiguity, our technique demonstrates that seismic data have the potential to be used for estimation of fluid viscosity in heavy oil reservoirs, especially if some constraints can be placed on the viscosity values. We apply our methodology to reservoir characterization of the Grand Rapids formation in the Athabasca oil sands. Future applications to time-lapse monitoring of heavy oil reservoirs are recommended.