The first order operator

The approximate operator with elements that are first order polynomials in the slowness has four new elements, these are anti-symmetric terms in the operator:

$$\left[ \begin{array} {ccccc} \cos(\phi)/2 & & \sin(\phi)/2 ... ... p_x + b_3 p_y & & a_4 p_x + b_4 p_y & & 1/2\end{array}\right].$$