Abstract: The invention is concerned with electromagnetic exploration of the earth's surface. In a method proposed by the invention, a primary coil is powered to generate a primary field and the primary field is applied to the earth and a receiver, used to detect a secondary field generated by the earth in response to the primary field, is moved over the surface of the earth. According to the invention, Helmholtz coils (14.1, 14.2) are arranged in a predetermined array and are powered in such a manner that they generate, in a volume accommodating the receiver, a magnetic field which serves at least partially to null the magnetic field of the earth. The invention also provides apparatus for carrying out the method.