摘要
三维勘探一般是在已作过二维勘探的勘探成熟区进行的。二维勘探的结果已给我们提供了基本的地层参数,如目的层深度、倾角、频率响应、大致形态及速度信息等。三维勘探的主要任务是使地震波正确归位,弄清真实的地质构造形态和位置,从而提高勘探精度。这里指的是主要目的层的勘探精度,因为事实上,在一次勘探过程中,要把浅、中、深所有的反射层都同时兼顾得很好是困难的,有时甚至是不可能的。因此所有措施的着眼总都是围绕着主要目的层,其次才尽量兼顾其它反射层。在采集阶段,影响勘探精度的主要因素是采样网格密度,网格点太密,增加工作量,提高成本;网格点过疏,则会漏掉一些小构造或构造的细微变化。合适的网格点密度既要满足勘探精度的要求,又要符合经济的原则。如何利用上述二维助探的信息来设计网格点密度是数据采集设计的首要问题。 网格点密度的确定取决于反射波视波长。视波长的计算,在平面波前假设前提下和球面波前的假设前提下是有差别的。当目的层深度H与最大炮检距X之间的关系不满足H》X时,采用球面波前的假设是较合理的,这种情况正是现阶段数据采集中最普遍的一种情况。由于球面波前的假设,视波长沿测线方向而变化,并与激发方向有关,同时也随深度而变化,因此视波长在垂向和横向都是变化的。
Abstract
Generally speaking,3-D survey is applied where the 2-D survey has been performed. By 2-D survey, the basic parameters of the strata, such as the depth of the horizon interested, its dip, its frequency response,the rough configuration and velocity information,etc., could be obtained beforehand. The role of the 3-D survey is mainly to have a correct migration, make certain the real figuration and the correct location of the geological structure, and then, to improve the accuracy of the exploration. The "accuracy" we mean here is concerned with the accuracy of the exploration of the horizon interested. As is well known to geophysicists, to take account of all the reflection horizons in different depths on one: occasion is very difficult. Sometimes, it is well-nigh impossible. Hence, all the measures takeu arc aimed at the main horizon of interest first and then, considerations are given to the other horizons. During data acquisition, the main factors affected to the accuracy of exploration are the grid density of data acqusition points. Dense net points will be a burden to the work and the cost of the work will also be increased. Conversely, if the net points are excessively thin,some minor structures or the details of the structure will possibly slip through the net. Proper density of the net points should meet the requirements of the accuracy of exploration and tally with the principle of economics. How to choose a density of the net points properly by dint of the information obtained from the 2-D survey is supposedly the most important problem In the determination of data acquisition.The grid density set up depends on the apparent wave length. The calculation of the wave length varied with hypotheses, say, the plane wave, front and the spherical wave front. When the depth H of the hor- izon of interest and the maximum offset xraax does not satisfy the relation H>>x, it seems to be better and reasonable to consider the wave front to be spherical. At present, this is the case commonly seen in data acquisition. On the assumption that the wave front is spherical, the apparent wave length varies along the profile and is related to the direction of explosion. Of course, the apparent wave length also varies with the depth. As a matter of fact, it varies lateraly and vertically.Considering the accuracy of exploration as well as the principle of economics, we firstly choose the apparent wave length of the horizon of interest vertically, then, the minimum apparent wave length of the horizon of interest on the acquisition surface. Apparent wave length is the function frequency. The requirements for frequency are related to resolution. The consideration of resolution combined the accuracy of exploration and the density of net points. For simplicity, the determination of of density of 3-D net points is divided into two steps, it is in essence to transform the 3-D problem into a 2-D ones. The lateral resolutions and the dips of the horizon of interest should be both considered. Besides, the other parameters are to be considered too. Of course, the discussion is not detail and deep enough. Undoubtedly, the knowledge about 3-D survey will be deepened and tends to be complete with the accumulation of the experience of the 3-D survey.
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