The electrical conductivity of the upper mantle is estimated from low‐latitude magnetic field variations (magnetic storms) caused by large fluctuations in the equatorial ring current. The data base is derived from magnetic field magnitude data measured by satellites OGO 2, 4, and 6, which offer better global coverage than land‐based observatories. The procedures of analysis consist of 1) separation of the disturbance field into internal and external parts relative to the surface of the earth, 2) estimation of an electromagnetic response function Q(ω) which relates the internally generated magnetic field variations to the external variations due to the ring current, and 3) interpretation of the estimated response function using theoretical response functions for assumed conductivity profiles. Special consideration is given to possible oceanic and ionospheric effects. Best estimates of the geomagnetic response function Q(ω) for 0.2 to 2.0 cpd indicate an upper mantle conductivity of the order of 10−2 S/m. This conductivity is reasonable, especially when laboratory measurements of conductivity versus temperature for olivine are considered.

As part of the effort to develop On-Site Inspection (OSI) techniques for verification of compliance to a Comprehensive Test Ban Treaty (CTBT), a computer code was developed to predict the interaction of an electromagnetic (EM) wave with an underground cavity. Results from the code were used to evaluate the use of surface electromagnetic exploration techniques for detection of underground cavities or rubble-filled regions characteristic of underground nuclear explosions. The interaction of EM radiation originating on the earth's surface with a subterranean cavity is calculated using an integral formulation of the problem. Given a primary source located on or above an interface separating a dielectric (air) from a dissipative half-space (earth), the primary and secondary (scattered from the cavity) fields are calculated. Specific attention is given to the vertical magnetic dipole source which is the characteristic field of a horizontal loop source, commonly used in exploration geophysics. Since experiments were to be carried out at NTS where the water table is very deep, the underground cavity is assumed to be empty, hence, is treated as an air-filled void. Computer models were calculated for ground conductivities .001 to .01 S/m which represent values found in a granitic rock to those in the alluvium encountered on the Flats. In t^is range of .001 to .01 S/m, the cavity is more easily detected at the higher ground conductivities of .01 S/m. This situation allows better contrast with the anomaly and is limited only by EM attenuation, which also increases with ground conductivity. For OSI applications, this report evaluates the situation encountered at NTS; namely, that the water table is deep and the cavity-chimney structure is a resistive anomaly. In other regions of the world, it is expected that the iv cavity would be water filled, hence, the cavity-chimney structure may be a conductive anomaly. This is a situation which should be studied in the future. However, it is the opinion of the authors that control source EM methods show promise as candidate OSI technologies. This code also can be applied to applications other than the detection of voids. Any three-dimensional anomaly can be considered; the anomaly can be characterized by its electrical permittivity and conductivity which is included in the complex permittivity used in the code. A suggested application is for evaluating EM methods for use in site characterization. > v n-*!fe c6 .i +B fe +, w SBOA Case pH jam L2 i!]2 J. 8H 1

A direct potential measurement in the photoelectric effect experiment Am.

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