Bayesian Spatial Modelling of Gamma Ray Count Data

Gamma ray logging is a method routinely employed by geophysicists and environmental engineers in site geology evaluations. Modelling of gamma ray data from individual boreholes assists in the local identification of major lithological changes; modelling these data from a network of boreholes assists with lithological mapping and spatial stratigraphic correlation. In this paper we employ Bayesian spatial partition models to analyse gamma ray data spatially. In particular, a spatial partition is defined via a Voronoi tessellation and the mean intensity is assumed constant in each cell of the partition. The number of vertices generating the tessellation as well as the locations of vertices are assumed unknown, and uncertainty about these quantities is described via a hierarchical prior distribution. We describe the advantages of the spatial partition modelling approach in the context of smoothing gamma ray count data and describe an implementation that may be extended to the fitting of a more general model than a constant mean within each cell of the partition. As an illustration of the methodology we consider a data set collected from a network of eight boreholes, which is part of a geophysical study to assist in mapping the lithology of a site. Gamma ray logs are linked with geological information from cores and the spatial analysis of log data assists with predicting the lithology at unsampled locations.     

3D behaviour of contamination in landfill sites using 2D resistivity/IP imaging: case studies in Portugal

Aiming at defining a valid spatial contamination model, resistivity and induced polarization (IP) measurements were used to investigate contamination plumes in the vicinity of two municipal landfills (Ovar and ílhavo). Previous geophysical surveys and underground water samples confirmed the contamination. However 2D resistivity/IP surveys enabled in obtaining a more accurate spatial model. The Ovar survey consisted of two profiles with nine Wenner soundings each; the ílhavo survey was carried out along two individual lines using a Wenner standard pseudo-section. In both situations, negative IP values were found associated with positive IP values, which can be explained mainly by 2D or 3D geometric effects caused by the presence of the conductive plumes. The data were modelled using a 2D inversion program (RES2DINV) and the resulting resistivity and chargeability distributions were displayed as pseudo-sections. The resistivity and chargeability pseudo-sections define the contamination plumes and the sedimentary structure. These case studies illustrate the advantages of 2D resistivity/IP surveys for the mapping of shape and dimension of contamination associated with landfills.     

交互的初至波反演及静校正

一种人机交互式的初至波反演方法是在反演过程中加入人为的干预,并利用人的经验控制反演的方向,使结果更加符合实际的地质情况。再将反演的结果与其它的短波长静校正方法相结合,就能够有效地提高静校正的精度。实例证明,这种方法是有效且可行的。     

岩石复电阻率Dias模型及其参数求取方法

Dias模型是众多描述岩石复电阻率的频散特性模型中的一种,它不但参数少,且可由这些参数导出具有特定物理意义的参数。针对Dias模型参数的求取及反演解的不确定性评价,这里提出了一种模拟退火阻尼最小二乘联合反演方法。该方法首先将参数求解空间离散化,利用模拟退火找到全局最优解区间,然后利用阻尼最小二乘法在解空间进行求解。该方法有效地克服了原来两种方法的反演速度慢、反演结果依赖于初值的缺点,并实现了对反演结果多解性的评价。研究结果表明,对于岩石复电阻率数据的处理解释,Dias模型的使用比传统的Cole模型及其变形更具有潜在的优势。     

不等电极距情况下高密度电阻率法的反演结果分析

通过高密度电阻率法的模型实验,得出不等电极距情况对直立高阻薄板和低阻球体反演结果的影响:缺失电极对它正下方的ρ_s影响最大,而异常形态主要在水平方向有明显的变化,异常体随电极缺失的位置有趋势变化,电极缺失的数目越多,异常形态变化越大,但是从整体来看反演剖面图基本相似。