Modeling and Inversion of Airborne and Semi-Airborne Transient Electromagnetic Data with Inexact Transmitter and Receiver Geometries

Tao Chen,Dikun Yang

doi:10.3390/rs14040915

Abstract

Airborne and semi-airborne transient electromagnetic (TEM) surveys have high efficiency but may suffer from systematic errors due to the inexact shape, position, and orientation of the transmitter and receiver, which can deviate from the nominal design because of complex terrain, platform instability, or external forces. Without considering actual survey geometry, modeling and inversion can bias the interpretation of results. We develop a universal approach to layered earth capable of modeling arbitrarily complex transmitter and receiver geometry used in airborne and semi-airborne surveys. Our algorithm decomposes an airborne loop or grounded wire source to a set of x-, y-, or z-oriented electric dipoles. An arbitrarily oriented receiver coil is simulated by projecting three-component data to the actual direction of receiving. In airborne TEM, the transmitter loop and receiver coil are often bound together on a rigid frame and tilt during the flight. Our simulations and synthetic inversion show that such a tilt may reduce responses relative to the data obtained with the nominal geometry; an inversion without considering the tilt can underestimate near-surface conductivity. In semi-airborne TEM, the transmitter wire on the surface can be crooked, and the airborne receiver coil can also tilt. Our modeling shows that the simulated data can change significantly if the actual transmitter and receiver geometry does not exactly follow the nominal survey design; if not appropriately accounted for, such an error may distort the recovered conductivity model. Finally, the benefit of our algorithm is demonstrated by an airborne TEM field data inversion of groundwater problems with the tilt angle of the transmitter–receiver frame accurately modeled. Our work provides a tool for improving the resolution of airborne and semi-airborne TEM in near-surface conductivity characterization.

Highlights

Transient electromagnetics (TEM), a time-domain electromagnetic remote sensing technique, has proven to be an effective approach for near-surface geophysical surveys in the past few decades
Remote Sens. 2022, 14, 915 (SATEM), the transmitter is a long wire, or a large closed loop fixed to the ground, whereas the receiver coil flies in the air taking magnetic field measurements at a series of locations
We carefully studied the effect of inexact survey geometry

Summary

Introduction

Transient electromagnetics (TEM), a time-domain electromagnetic remote sensing technique, has proven to be an effective approach for near-surface geophysical surveys in the past few decades. (SATEM), the transmitter is a long wire, or a large closed loop fixed to the ground, whereas the receiver coil flies in the air taking magnetic field measurements at a series of locations. Because of the airborne platform, ATEM and SATEM have high efficiency and mobility compared to ground TEM. Previous works studied the inexact geometry problem of ATEM and ground large loop TEM and treated them using different dipole source models or data correction methods. Such a problem in SATEM still needs further investigation, as the combination of a grounded long wire source and a drone-borne coil receiver has gained popularity, but the study of such a system is lacking. We show that our methods can make a difference to the inversion and interpretation of field data acquired with inexact survey geometries

Inexact Survey Geometry

Forward Modeling and Analysis

Inversion

Findings

Discussion

Conclusions