Terrain correction in the gradient calculation of spontaneous potential data

Terrain correction is to eliminate the effect of terrain relief around measurement points on observation results in gravitational and geoelectrical prospecting.. Because the microscopic mechanism of spontaneous potential anomalies. formed by terrains is rather complex, it is difficult to obtain an analytic formula, whereas the most visualized method can be adopted instead to find out the fitting formula between these two variables. According to the study, in spontaneous potential gradient calculation, spontaneous potential anomalies caused by topography can be divided into two kinds: one is that anomalies have a mirror-image relation with terrain relief, which accounts for the majority of cases; the other is that anomalies have an inverted mirror-image relation with terrain relief. The relation between a majority of anomalies and terrain relief could be linear, quadratic or exponential, among which linear relation is the primary. As for which fitting relation should be adopted specifically, it can be decided by selecting a typical terrain and spontaneous potential curve first and judging through its change rule; and trial calculation method can be adopted as well to choose the appropriate fitting formula by comparing the correction effects. The Ordos Basin was selected to be the research site in this paper and the terrain correction method which is applicable to spontaneous potential gradient calculation was put forward. Due to many ridges and valleys, the relative elevation in this region is up to 250-300 m. Slopes are steep and regular, so that it is one of the best places within China to study terrain correction. Five measuring lines were deployed in east-west direction and seven measuring lines were deployed in north-south direction, respectively. Through the comparison of the correction results of three kinds of fitting formulas, it was concluded that the linear fitting formula can better describe the relationship between relative height and spontaneous potential correction amount in the surveyed area. Because of the great impact of the terrain of this region, the spontaneous potential flat contour map before terrain correction shows significant positive anomalies that are associated with concave terrains and negative anomalies that are associated with raised terrains. Terrain correction can effectively eliminate the correlation between relative height and spontaneous potential. The corrected spontaneous potential curves can reflect subsurface geological structure better, and the anomalies shown by the spontaneous potential contour map can be more prominent. The quality of terrain correction is evaluated by whether the correlation between terrain fluctuation and spontaneous potential could be eliminated. Terrain correction is effective when there is no significant mirror or anti-mirror relationship between terrain fluctuation and the spontaneous potential after terrain correction. Since terrain relief can influence the appearance of anomalies in spontaneous potential gradient calculation, especially when the area of rugged terrain is comparable to the area of oil and gas reservoirs, which may make it difficult to distinguish these two anomalies. Thus the study on terrain correction has great importance in practice.