Inverse Synthetic Aperture Imaging of the Ground‐Airborne Transient Electromagnetic Method with a Galvanic Source

Abstract

AbstractThe ground‐airborne transient electromagnetic method with a galvanic source has such advantages as high efficiency, great exploration depth, high signal to noise ratio and good performance in mountainous areas. However, so far its interpretation system has not been established, which restricts the development of this method. This paper aims at building up a complete ground‐airborne electromagnetic detection system to enrich the theory of this method. This paper consists of three parts: (1) full field apparent resistivity definition; (2) Kirchhoff immigration of TEM virtual wave field; and (3) inverse synthetic aperture imaging. Magnetic field intensity is selected for full field apparent resistivity definition. The results of theoretical models show that the first and last part of the apparent resistivity curve can reach the resistivity of the first and the last layer, respectively. Thus, full space and full time apparent resistivity definition is achieved. Based on previous study, Kirchhoff migration of ground‐airborne transient electromagnetic method with a long wire source is realized. A correlation method is adopted for multiple survey point synthesis, which pushes the traditional single point processing forward point‐by‐point for multi‐coverage inverse synthetic aperture imaging.The results of layered models show that full field apparent resistivity can reveal the underground electric change smoothly, completely and gradually. When the resistivity of the middle layer changes, the differentiation of full field apparent resistivity is obvious, which can easily and intuitively identify the electric layer. As receiver altitude, offset and time are considered simultaneously, full field apparent resistivity can be calculated in full space and full time. The results of the complex model with a water‐bearing gob shows that the further the offset from the gob is, the weaker the resistivity anomaly on full field apparent resistivity section is. There are two interfaces from the Kirchhoff immigration results, of which the upper one indicates surface. As the electrical difference between earth and air is large, the wave field signal of this interface is strong, which spreads over the whole area. While the anomalies of the lower interface are mainly distributed in the middle area and weaken outwards, which indicate the water bearing gob. The upper interface changes little after inverse synthetic aperture imaging, while the gob anomaly narrows and has a clear boundary. The anomaly boundary matches well with the designed model. Inverse synthetic aperture imaging of ground‐airborne transient electromagnetic survey with a long wire source is realized on the basis of inverse synthetic aperture radar (ISAR). Full field apparent resistivity is put forward with inverse function and iteration, which can realize calculation of this variable in the full zone and full time. Pseudo‐seismic migration of transient electromagnetism is used in 3D transient field imaging. Inverse synthetic aperture radar is introduced in inverse synthetic aperture imaging of the ground‐airborne transient electromagnetic method with a long wire source, which can help enhance resolution. The results of designed models show that correlation can strengthen useful signal and improve signal to noise ratio and resolution, which prove the efficiency of inverse synthetic aperture imaging of the ground‐airborne transient electromagnetic method with a long wire source.