基于最小反演拟合差的重磁场源深度计算方法

Depth estimation of potential field by minimum inversion fitting error

以等效源及位场物性反演为基础,本文提出一种新的求取重磁场源深度的方法.该方法将一层等效源以一定的间隔从浅部向深部移动,并将等效源作为初始模型进行反演,当反演拟合差最小时,停止反演,此时的等效源底深即为所求场源的中心深度.由于仅需要反演一层等效源,比传统的物性反演计算时间大大减少,并且不需要进行深度加权约束.理论模型数据处理结果表明该方法能够获得较准确的场源深度：以长宽比为7.5的薄板模型为例,深度计算误差约为1个点距（25 m）;以长宽比为0.5-1.5的厚板模型为例,深度计算误差小于1个点距（25m）.将该方法应用于实测航磁梯度数据,计算的磁源中心深度在200-250m之间,钻井资料显示该异常由埋藏深度在200-300m的闪长岩引起,计算结果与钻井资料较吻合.

Estimation of source depth plays an important role in quantitative interpretation of magnetic and gravity anomaly data.Various methods have been developed to conduct this estimation,especially for magnetic source depth.They include slope,Naudy,Werner deconvolution,Euler deconvolution,analytical signal,source parameter imaging（SPI）,the continuous wavelet transform（CWT）and tilt-depth approaches.We present a new method to estimate the depth of a field source,which is based on equivalent source technology and potential field inversion.A single layer of 2.5Dcuboids model is established as an equivalent source with initial physical property parameters.This single equivalent source layer moves from shallow to deep at certain intervals and is used as the initial model to invert the data.Then we estimate the field source depth by the inversion fitting error.From shallow to deep,the inversion fitting error usually becomes smaller.The minimum inversion fitting error matches the corresponding field source depth.Because only one equivalent source layer is necessary to invert,the inversion is faster than the traditional inversion methods and does not require depth weighting.Calculation on theoretical models data shows that this method can obtain accurate depth of the field source.The data processing of a thin plate with 7.5times aspect ratio shows that thedepth calculation error is about one measured point（25m）.The data processing of a thick plate with an aspect ratio from 0.5to 1.5shows that the depth calculation error is less than one measured point（25 m）.Processing of measured aeromagnetic gradients data indicates that the center depth of the magnetic source is between 200 mto 250 m.Drilling data show that such anomalies are caused by the diorite at depths from 200 mto 300m,in agreement well with the estimation.These tests demonstrate that the depth estimation method suggested in this paper is applicable to both isolated anomalies and combined anomalies.