Abstract:

The downward continuation process is inherently unstable and any high frequency noise present in the data gets strongly magnified in the transformed map in such a way to mask any useful signal. How to accurately calculate the any order vertical derivative of total field magnetic anomaly (Bm) is a crucial matter of implementing downward continuation. In this paper, we studied the properties of harmonic function and proved that Bm is a pseudo-harmonic function. Based on the use of stable vertical derivatives, a stable algorithm was presented to perform downward continuation applying improved Taylor series approximation. Furthermore, the problem of edge effects could be settled out using grid extension to the four directions with half a cosine function before the vertical derivative calculation. The effectiveness of the suggested algorithm has been illustrated by simulated sphere/prismatic examples and real magnetic data from an airborne and seaborne magnetic survey. The conclusion shows that the presented technique can be employed to perform stable downward continuation of total field magnetic data and provide better results than other techniques based on Fast Fourier Transformation (FFT) or on normal Talor’s series or integral-iteration method when the data is noise-free, and almost the same as integral-iteration method if the data contains certain noise.

Key words: total field magnetic anomaly, downward continuation, improved Taylor series, Laplace equation, vertical derivative, grid extension processing