The vertical magnetic component caused by a grounded wire source is sensitive to the earth and decays sharply with the increase of transmitting-receiving distance. It is very difficult to conduct the conventional CSAMT method which measures horizontal electric and magnetic components when the earth is hardly conductive or the topography is very steep. Measuring the vertical magnetism is an effective alternative. Based on the theory of wide field electromagnetic method, the authors analyzed the characteristics of vertical magnetic component and obtained full-zone apparent resistivety by using the method of iteration. This survey mode was applied to mineral deposit detection in a frozen ground area, with good result achieved.