Given the values of a component of secondary magnetic<br />field on the plane surface of the earth, an attempt has been made to approach<br />the sources by finding the values of the component on a plane at<br />a depth h below the surface, the plane being above the sources.<br />To this end a Green's Function for the wave equation, appropriate<br />for a semi &mdash; infinite space was determined. The integral obtained was<br />expanded into series suitable for numerical calculations.<br />The average values of the component on the earth plane were assumed<br />to satisfy a double series and the coefficients were determined by the least<br />square method.<br />Formulae were ultimately developed to calculate the values on a plane<br />at a depth h, in terms of average values on six concentric circles around<br />each point.