Up-down wavefield separation is often performed with dualsensor summation, where collocated hydrophone and geophone responses are summed after application of an appropriate scalar. While this technique can be used in both land and marine surveys, scalar selection can be more challenging for land applications since the receiver may be placed in heterogeneous media. Here we present a method for scalar estimation using buried receiver data. This approach utilizes the autocorrelation of a summed geophone and hydrophone. We show that the correct calibration scalar delivers minimum to this autocorrelation at specific locations. We develop an algorithm for estimation of this summation scalar and test it on a realistic horizontally layered media.