ZTEM is an airborne electromagnetic survey in which the vertical magnetic field from natural sources is recorded. The data are transfer functions that relate the local vertical field to orthogonal horizontal fields measured at a reference station on the ground. The transfer functions depend on frequency and provide information about the 3D conductivity structure of the Earth. Since a 1D conductivity structure produces no vertical magnetic fields, the ZTEM technique is not very sensitive to the background conductivity. In order to increase sensitivity to the background conductivity, and greatly improve the depth of investigation, MT and ZTEM data can both be collected. The combination of sparse MT data, with the economical and rapid spatial acquisition of airborne ZTEM data, creates a cost effective exploration technique that can map large-scale structures at depths that are difficult to image with other techniques. We develop a Gauss-Newton algorithm to jointly invert ZTEM and MT data. The algorithm is applied to a synthetic model and to a field example from the Reese River geothermal property in Nevada.
Natural source electromagnetics have an important role in understanding the electrical conductivity of upper regions of the Earth. Their primary advantage, compared to controlled source methods, is the depth of penetration that is a consequence of the plane wave excitation. The magnetotelluric (MT) method uses ratios of electric and magnetic fields as data and it has played a significant role in crustal studies as well as in mining and hydrocarbon exploration. A practical limitation of the MT technique however, is that surveys are costly and time consuming because many expensive stations must be installed to measure all of the field components at the surface of the earth. It would be preferable to collect MT data in an aircraft but this goal has not yet been achieved because of the difficulty in measuring the electric fields. In an effort to continue to use the penetration advantage of natural sources, it has long been recognized that tipper data, the ratio of the local vertical magnetic field to the horizontal magnetic field, provide information about 3D electrical conductivity structure. It was this understanding that prompted the development of AFMAG (Audio Frequency Magnetics) Ward (1959). However, because the direction and strength of the inducing field varies with time, AFMAG results were not always repeatable. Many of the AFMAG problems can be removed by using improved signal processing and instrumentation. This has resulted in the Z-Axis Tipper Electromagnetic Technique (ZTEM) Lo and Zang (2008). In ZTEM, the vertical component of the magnetic field is recorded above the entire survey area, while the horizontal fields are recorded at a ground-based reference station. MT processing techniques yield frequency domain transfer functions typically between 30-720 Hz that relate the vertical fields over the survey area to the horizontal fields at the reference station. By taking ratios of the two fields (similar to taking ratios of the E and H fields in MT), the effect of the unknown source function is removed.
