There appear to be some mistakes in the subject paper.
The authors seem not to have realized that the anti-symmetric contact anomaly represents one of the flanks of a symmetrical dike anomaly of infinite width, and that the symmetric contact anomaly belongs to the family of anti-symmetric dike anomalies. This explains the peculiar looking curve for contacts (R = ∞ curve) in their Figure 2 where the contact curve is seen to intersect the dike curves. To be correct the R = ∞ curve has to be turned around a vertical axis so that the value for θ = 90 ends up at θ = 0 and vice versa. It may have added to the authors' confusion that they used two different sources for the mathematical expression of contact and dike anomalies without realizing that the angle θ has different definitions in the papers cited. In the formulas given by Åm (1972) the angle θ has a value of 90 degrees for the symmetric and 0 degrees for the antisymmetric dike anomaly, whereas in Gay (1963) the angle θ has a value of 0 degrees for the symmetric and −90 degrees for the antisymmetric case.