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Gravity Field Imaging by Continued Fraction Downward Continuation: A Case Study of the Nechako Basin (Canada)

the study of geologic structure and resource exploration in the deep part of the earth, like the lower crust, the upper mantle (Lü et al., 2013, 2019). The gravity anomaly reflects the lateral resolution of the underground mass distribution. So gravity data has the advantage of identifying the horizontal positions of the geologic structure and ore bodies. But both horizontal distributions and vertical distributions, also called depth variations, are important and interesting for interpreters and geoscientists (Hartman et al., 1971). Several gravity methods of source depth estimation have been proposed and developed. Li (2003) summarized and compared the most common classical methods, including Euler deconvolution (Thompson, 1982), Werner deconvolution (Hartman et al., 1971), analytic signal-based methods (Nabighian, 1972), local wavenumber methods (Thurston and Smith, 1997), and wavelets methods (Cooper, 2006). These methods are very useful and effective in the depth interpretation of gravity data. With the development of airborne and satevllite techniques of gravity measurement, combined with the ground or the ship measurement, multi-height data from different ways are possible to be accessed over one area. Multi-scale methods, like imaging methods, which use the data of different levels are popular. In the study areas without multi-height data from different ways, continuation is a common and effective strategy to implement these. Because of the stability of upward continuation, many imaging methods have been proposed using upward continuation, such as DEXP, ScalFun, the reduced and multiridege Euler, summarized by Florio and Fedi (2018). As upward continuation reduces the resolution of the gravity data with the increasing of the upward height, Fedi et al. (2009) pointed out that these methods can recover the resolution that is lost with the increased altitude by combining the transformation with high order vertical differentiations. However, it is still difficult to distinguish the information with great burial depths (Zhou et al., 2021). Conversely, downward continuation enhances the resolution of gravity data (Zhang et al., 2018), so it can be used in imaging methods. However, downward continuation has an inherently unstable feature that has limited its widespread applications. Imaging methods based on downward continuation, such as the normalized total gradient method and the singular (quasi-singular) point method, usually require additional calculations which are similar to the basic normalized theory. More accurate and stable results are obtained through modified methods (Fedi and Florio, 2011). But these mainly rely on the accuracy and stability of the downward continuation method used in imaging methods. The error is very obvious when the spatial measured distance between the different geologic sources is small (Zhou et al., 2021). Although numerous methods have been presented to improve the accuracy and stability of downward continuation, they are nowhere near enough for improving the imaging of gravity data. In this paper, we used a fourth-order method of continued fraction downward continuation to image gravity data. This downward continuation method is developed in the space domain, and for imaging the gravity data, only downward continuation calculations and a median filtering are needed from the measurement upper level to an expected maximum lower level. For general methods, the calculations of downward continuation are very sensitive to the vertical distributions of the geological sources, which leads the downward continuation to fail to go deeper than the shallow point of these sources. But the positions of singular points’ horizontal and vertical distribution, which usually can be used to indicate the positions of geological sources, can be identified by the presented imaging method. So, the geologic source depths were directly obtained without any normalized calculations. First, we used two sets of model data without noise to test the imaging method by continued fraction downward continuation. Secondly, we applied the method to real airborne gravity data over the Nechako basin (Canada). For the synthetic model, we consider three cubic prisms all sized 10 m × 10 m × 10 m with different burial depths and density contrast: the first prism’s depth to the top is 15 Gravity Field Imaging by Continued Fraction Downward Continuation: A Case Study of the Nechako Basin (Canada)