激电效应对AMT正演的影响及其在砂岩型铀矿中的数值模拟

Influence of induced polarization effects on AMT forward modeling and its numerical simulations for sandstone uranium deposits

音频大地电磁测深法应用比较广泛,已成为固体矿产深部地球物理探测的主要方法之一,但在处理解释中,往往只考虑电磁效应,忽略了激电效应,这与实际地质情况不相符。本文在实现带激电效应的二维音频大地电磁有限元正演基础上,模拟了激电效应各参数对二维正演响应的影响大小及规律,同时对二连盆地砂岩型铀矿的地电模型进行了数值模拟研究。结果表明:①基于Cole-Cole模型实现的带激电效应二维AMT正演,随着极化率、频率相关系数及时间常数取值的增加,主要降低二维正演视电阻率异常响应值,增大阻抗相位异常响应值,这对探测低阻目标体有利,探测高阻目标体影响较大;②激电效应中的零频电阻率和极化率对二维正演响应影响较大,频率相关系数与时间常数对正演的影响大小主要取决于极化率,当极化率较大时,二者对正演响应同样具有较大的影响;③在砂岩型铀矿中,当砂体中存在浸染状及含硫化物的岩性时,较大的激电效应会对频域电磁测深法探测目标砂体产生较大影响,因此在探测前需进行正演建模,了解激电效应的影响大小。

The extensively applied audio magnetotellurics(AMT)has become a primary method for deep geophysical exploration of solid mineral resources.However,its data processing and interpretation often only consider electromagnetic effects but ignore induced polarization(IP)effects,which is inconsistent with actual geological conditions.Based on the two-dimensional AMT finite-element forward modeling with IP effects,this study simulated the magnitude and regularity of the influence of various parameters of IP effects on the two-dimensional forward response.Moreover,this study conducted a numerical simulation on the geoelectric model of sandstone uranium deposits in the Erlian Basin.The results show that:(1)With an increase in the values of polarizability,frequency correlation coefficient,and time constant,the two-dimensional AMT forward modeling with IP effects based on the Cole-Cole model primarily reduced the abnormal response value of two-dimensional forward modeling apparent resistivity and increased the abnormal response value of impedance phase.This is beneficial for detecting low-resistivity targets rather than high-resistivity targets;(2)The zero-frequency resistivity and polarizability in IP effects exhibit a significant influence on the two-dimensional forward response.The influence of both frequency correlation coefficient and time constant on the forward response primarily depends on the polarizability.High polarizability suggests their significant influence on the forward response;(3)In the case of disseminated and sulfide-bearing lithologies in the sand bodies of sandstone uranium deposits,greater IP effects will significantly influence the detection of target sand bodies using frequency-domain AMT.Therefore,forward modeling is necessary before detection to determine the magnitude of IP effects.