The paper deals with a solution of the problem on non-stationary gaseous filtration for a semi-infinite solid with non-conductive inclusion by the method of near-boundary element. The behavior of filtration flow is studied for the case, when a non-conductive inclusion has the form of a rectangular trapezoid. The interpretation formulae are introduced which allow one to determine an acute angle of the rectangular trapezoid by the known values of filtration flow and immersion depth of the source a nd inclusion.