1前言 在进行石油勘探、人为污染调查及其它环境研究的同时,人们对海水中的轻烃踪迹进行了长期监测。所有这些应用都需要对在水体中所观察到的烃类分布和其源地特征之间的关系进行很好理解。本文我们提出了一个烃类从海底向成层水体扩散的定量混合模式,并用于解释存在的异常中层水。我们使用的数值处理方法是极一般的,易于推广用于模拟由水体中任意点源或长条源产生的烃迹混合水的动力学。这就使得它可用于模拟烃迹释放实验、生物成因的烃迹分布和其它相关的过程。 在菲律宾拉盖湾观察到的一个甲烷垂向剖面(Evans等,1992)如图1所示。热成因的饱和轻烃(甲烷、乙烷、丙烷)异常在中心为约150m深度处出现浓度极强值。巳报导的位于海底附近的几个异常的横向范围大约是3～30km。与约10ppm的背景值相比,峰值处的总烃浓度达到200PPm。考虑该区海底附近沉积岩中的断裂特征和构造活动,烃类很可能作为局部烃苗进入海水。 在稳定状态的纯混合情况(也就是缺乏流动和上浮力)下,不管混合剖面如何,起源于海底烃苗的扩散烃的深度剖面可显示出烃浓度随深度的减小而稳定地减小。这是因为由混合机制所驱使的烃迹分子总是使浓度梯度减小的缘故。因而在稳定状态和没有中层水源情况下, 1 Introduction While carrying out oil exploration, man-made pollution surveys and other environmental studies, long-term monitoring of light hydrocarbon traces in seawater has been conducted. All of these applications require a good understanding of the relationship between hydrocarbon distribution observed in the body of water and its source characteristics. In this paper, we propose a quantitative mixed model of hydrocarbon diffusion from seafloor to stratiform water and to explain the presence of anomalous mid-water. The numerical methods we use are extremely general and easily generalized to simulate the kinetics of a mixture of hydrocarbon traces from an arbitrary point source or strip source in a body of water. This allows it to be used to simulate hydrocarbon release experiments, biological genesis of hydrocarbon traces and other related processes. A vertical section of methane observed in Laguna Bay, Philippines (Evans et al., 1992) is shown in Figure 1. Thermally-induced anomalies of saturated light hydrocarbons (methane, ethane, propane) show extreme concentrations at a depth of about 150 m at the center. Some of the anomalous lateral extents that have been reported near the seabed have been around 3 to 30 km. The total hydrocarbon concentration at the peak reaches 200 ppm as compared to the background value of about 10 ppm. Considering the fault features and tectonic activities in the sedimentary rocks near the seafloor in this area, it is probable that hydrocarbons will enter the seawater as local hydrocarbon seams. Under pure mixing conditions at steady state (ie, lack of flow and buoyancy forces), the depth profile of the diffusible hydrocarbons originating from subsea hydrocarbon seedlings may show a steady decrease in hydrocarbon concentration as the depth decreases, regardless of the mixed profile. This is because the hydrocarbon molecules driven by the mixing mechanism always cause the concentration gradient to decrease. Thus in the steady state and without the middle level of water,