LU500661B1 - Method for Extracting Abnormal Topography of Hydrothermal Vents - Google Patents

Abstract

The invention disclose a method for extracting abnormal topography of a hydrothermal vents, which comprise that following steps: creating a spatial point plane layer on the basis of topographic data; performing interpolation of layer with geostatistics approach of Universal Kriging method, and carrying out reclassification according to varying topography; exporting the interpolation results as topographic isosurface; editing the plane layer attribute table, adding the geometric area field, calculating the area delineated by isolines corresponding to different r values, and calculating log- log curves; judging segment number of the log-log graph; performing linear fitting for the straight lines at both ends of the dividing point respectively. The method has the beneficial effects that the extracting method of structural topography of hydrothermal vents is constructed, and the expectation of realizing hidden deposit prediction and hydrothermal resource prediction will provide a basis for the formulation of sulfide investigation plans in future ocean prospecting.

Description

DESCRIPTION Method for Extracting Abnormal Topography of Hydrothermal Vents

TECHNICAL FIELD The invention belong to that technical field of geology and relates to a method for extracting abnormal topographic characteristics of hydrothermal vents.

BACKGROUND It is of great scientific significance to interpret and study the geological characteristics of the ocean, especially the mid-ocean ridge, and to extract the structural parameter features for the prediction of the genesis and size of hydrothermal deposits, as well as for wider research and application. The mid-Atlantic ridge is the center of slow expansion. The research achievements of hydrothermal solution on the mid-Atlantic ridge show that the mid-Atlantic ridge has great potential for ore controlling and ore storage (for example, Rona, 1974 Hoffert et al, 1978; Fouquet et al, 1993; Cuvelier, et al, 2009). At present, although many discoveries and researches have been made on hydrothermal ore occurrences and geological background in the Atlantic Ridge, especially in the North Atlantic Ridge, the availability of geological characteristics in the Atlantic Ridge is not detailed and comprehensive enough. There 1s no detailed geological interpretation and research for hydrothermal areas with detailed investigation data, so it is impossible to fully and further analyze and utilize them. Geography evolution caused by crustal movement or substance eruption and filling not only shows tendency, but also inflects nonuniformity on local regions. The interpretation of traditional topography data is generally reflected by contour lines or gray-scale color systems. Because this method organizes the overall numerical statistics of regional data, it is easy to only reflect obvious topography tendency and ignore slight topography undulation abnormality. In complex metallogenic environment, slight topography abnormality may be the sign of deposits or ore occurrences. Therefore, it is necessary to adopt a more effective method to interpret the detailed linear structure in the oceanic ridge area, which is of great significance to analyze the background of hydrothermal deposits by using the linear structure information and to analyze the formation mechanism of deposits by integrating other geological background elements. There are many factors for the formation of hydrothermal occurrences, and geological structure is an important background factor. In addition, the formation of hydrothermal occurrences are also affected by the filling degree of pulp, magma chamber temperature and size, etc. Among these factors, in the uninvestigated areas, the only channels to obtain the information of geological structure and abundance of shell substance are through topographic data and geophysical information such as gravity and magnetic force which can reflect physical characteristics such as substance density. However, ROV, TVG, camera trailer, turbidimeter, methane abnormality detection (Zhou Huaiyang et al., 2007), heat flow abnormality (Sarrazin, 2009) and other means of hydrothermal information capture and acquisition used in hydrothermal investigation can only be measured in situ, and some of them must be captured at active vents, thus losing the significance of economic prospecting. Therefore, it is very necessary and meaningful to establish an effective modelling method, use the information that can be captured to predict the orebody area and narrow the future prospecting scope.

At present, as for the areas where hydrothermal points have been found, such as the famous TAG hydrothermal field (Rona et al, 1975), Snake Pit hydrothermal field (Fouquet et al., 1993), Lucky Strike hydrothermal field (Humphris et al, 1993) and Broken Spur vent field (Elders, 1993), etc., although there are very detailed survey data such as water depth, photography, gravity, magnetic force and earthquake, the interpretation of the structure can not inflect detailed fracture or linear characteristics, the interpretation method needs to be improved, and the data fusion and spatial analysis are also not fully carried out.

SUMMARY The purpose of the invention is to provide a method for extracting abnormal topography of hydrothermal vents, which solves the problem of stage change reflected by topographic characteristics of hydrothermal vents.

The technical scheme adopted by the invention is as follows: (1) creating a spatial point layer on the basis of topography data in ArcGIS; (2) performing interpolation of layer with geostatistics approach of Universal Kriging method and carrying out reclassification according to varying topography; (3) exporting the interpolation results as topographic isosurface; (4) editing the plane layer attribute table, adding the geometric area field of area id, r, area, lgr, lgarea, calculating the area id, r to record data during calculation; wherein area id is the area of each equivalence, r is the initial boundary value of isoline, lgr is the common logarithm of r, lgarea is the natural logarithm of the area of each equivalence, and area is the area; (5) importing the plane layer as Feature Class into the newly created Personal Geodatabase; (6) copying the isosurface initial value column and area column into Excel;; (7) then calculating the area delineated by isolines corresponding to different r values, and calculating lgr and Igarea;

(8) taking advantage of lgr and lgarea columns to draw log-log curves; (9) judging the segment number of the log-log curves; (10) performing linear fitting on the straight lines at both ends of the dividing point respectively, and when the sum of square sum of residuals formed by the two straight line fitting models is the smallest, taking the ler value at this time, and restoring it to the r value; (11) delineating the abnormal topography area in ArcGIS with the help of an abnormal lower limit. Furthermore, in (2), the topography layer is interpolated by the geostatistical universal Kriging approach, and is reasonably divided into 30 grades according to the topography change. Furthermore, the area in (4) is the area delineated as for r > ri. The method has the beneficial effects that the hydrothermal vent structure extraction method is constructed, and the expectation of realizing hidden deposit prediction and hydrothermal resource prediction will provide a basis for formulating sulfide investigation plans in future ocean prospecting investigations.

BRIEF DESCRIPTION OF THE FIGURES Fig. 1 is a schematic diagram of TAG active mountain; Fig. 2 is a fractal graph of TAG active mountain.

DESCRIPTION OF THE INVENTSION The present invention will be described in detail with reference to specific embodiments. In the late 1980s, the discovery of TAG hydrothermal zone (26 °N), Snakepit (23 °N), Broken Spur and Lucky Strike hydrothermal zone on the mid-Atlantic ridge showed that the expansion rate was not the primary condition to determine the formation of hydrothermal activity. In recent years, with the intensive investigation efforts in various countries around the world, some new hydrothermal areas are continuously being discovered, such as Logachev (14°45'N), Ashadze (12°58'N), Krasnov (16°38'N), Semenov (13°31'N) and Zenit-Victory (20°08'N) in the mid-Atlantic ridge and new recently discovered hydrothermal area in south Atlantic have been more than 50 so far. Although the discovery of hydrothermal activity in the Atlantic slowly expanding ridge suggests that there are still dynamic factors to promote the formation of hydrothermal activity in the slowly expanding ridge environment, in view of the ultra-slow expanding ridge (full expansion rate<2cm/yr) accounting for more than 40% of the global ridge system, with crust-mantle being cold and without volcanic and tectonic movements, it is still an important scientific issue that is controversial whether ultra-slow expanding ridge is conducive to the large-scale formation of hydrothermal activity. Fractal study of volcanic bodies in hydrothermal area: phenomena in nature have characteristic of self-similarity and generalized self-similarity, with statistical data being singular. Fractal method does not take the distribution of data as the premise, and can describe the local characteristics of the field, highlighting the superimposed abrnormality and weak abnormality. The information obtained by singularity analysis method is about the fractal density and multifractal dimension of the field, while the traditional statistics is about the normal area density or non-singularity data. Topography data can reflect continuously multi-scale spatial variation pattern of abnormality. If the fractal density dimension (singularity index) changes in space, the spatial field is multifractal distribution. On the basis of ArcGIS platform, the original grid data was converted into a plane layer according to the water depth grade increment and the earth coordinate was converted into the projection coordinate of North Pole Lambert Azimuth Equal area to calculate the geometric area of each plane and its logarithm. Herein, the area parameter was area, and the upper limit water depth parameter of each grade was r, and lgr and lgarea were used as log-log curves to judge the number of segments, that was, the fractal dimension of multifractal distribution. According to the log-log graph, there were 18 different linear distributions including small mutations, that was, the fractal dimension of volcanic body is 18. By calculating the linear simulation parameters and residuals distributed in every two intersecting segments, the boundary water depths were determined according to the principle of minimum residuals of two intersecting segments, and 17 boundary water depths were determined here.

The specific implementation step of the invention are as follows: (1) creating a spatial point layer on the basis of topography data in ArcGIS; (2) performing interpolation of layer with geostatistical approach of Universal Kriging method and carrying out reclassification according to varying topography( 30 grades in this application); (3) exporting the interpolation results as topographic isosurface; (4) editing the plane layer attribute table, adding the geometric area field of area id (the area of each equivalence), r (set as initial value of isoline) to record data during calculation; (5) for convinence of calculation and edit, importing the plane layer as Feature Class into the newly created Personal Geodatabase (alternatively, exporting the attribute table to generate Excel file, and further calculating (6)-(7)), namely, the remaining parameter values lgr (taking the common logarithm for r), lgarea (taking the natural logarithm for the area of equivalence), and area (the delineated area as for r > ri); (6) copying the isosurface initial value column and area column into Excel; (7) then calculating the area delineated by isolines corresponding to different r values, and calculating lgr and Igarea; (8) taking advantage of lgr and lgarea columns to draw log-log curves;

(9) judging the segment number of the log-log curves; (10) performing linear fitting on the straight lines at both ends of the dividing point respectively, and when the sum of square sum of residuals formed by the two straight line fitting models is the smallest, taking the lgr value at this time, and restoring it to the r value, namely, dividing point of abnormal topography; (11) delineating the abnormal topography area in ArcGIS with the help of an abnormal lower limit.

Fig. 1 shows the TAG active mountain, and Fig. 2 shows the fractal graph of TAG active mountain obtained according to the method of the present invention.

Fractal results showed that the top tectonic and topography of volcanic body were complex, which was related to the frequent activity of volcanic body and the direction of fault distribution.

The newest and largest volcanic vent was located in the north-west direction at the top of the volcanic body, and its eruptive substance all flocked to the southwest of the vent, and finally spread southward.

This distribution existed at least four times and substance accumulated at the southeast corner of the volcanic peak as for the earliest distribution, forming two hilltops, and between the two hilltops and the crater is a low-lying zone for the top of the mountain, forming low-lying strip.

Combined with the previous geological characteristics analysis, hydrothermal vents may occur in these places.

In addition to the four-grade new geological characteristics, other fractal terraces are distributed annularly around the mountain, reaching the foot of the mountain.

This difference may be related to the direction of tectonic activity around the volcano, and its early fractal shape is the result of substance balance.

According to the method, with dominant elements (topography), recessive elements (gravity, magnetic force), dynamic genetic elements (earth stress field) and the like of geological bodies being fully utilized, several typical hydrothermal areas in the North Atlantic being taken as typical examples and on the basis of the known positions, topographical features, element geochemical features and the like of the hydrothermal ore bodies, various methods such as geostatistics are combined to analyze the control function of different structural parameters on the hydrothermal ore bodies under the conditions of different regions, different topographical features and different earth stress fields, to extract multi-dimensional ore-controlling structure and ore-controlling structure classification, and to carry out hydrothermal mineralization correlation research.

Finally, this method is applied to the investigated mining areas in the South Atlantic Ridge (12°S—26°S), which are seldom studied, and compared and verified, so as to improve the method system, and provide a basis for future qualitative or quantitative evaluation of sulfides in the South Atlantic and for the purpose of ore enclosure.

The expectation of realizing hidden deposit prediction and hydrothermal resource prediction will provide a basis for the formulation of sulfide investigation plan in future ocean prospecting investigation.

The above is only a preferred embodiment of the present invention, and does not limit the present invention in any form.

Any simple modifications, equivalent changes and modifications made to the embodiments above according to the technical essence of the present invention shall be subjected to the scope of the technical scheme of the present invention.

Claims (3)

1. A method for extracting abnormal topography of hydrothermal vents, which is characterized by: (1) creating a spatial point layer on the basis of topography data in ArcGIS; (2) performing interpolation of layer with geostatistics approach of Universal Kriging method and carrying out reclassification according to varying topography; (3) exporting the interpolation results as topographic isosurface; (4) editing the plane layer attribute table, adding the geometric area field of area id, r, area, lgr, lgarea, calculating the area id, r to record data during calculation; wherein area id is the area of each equivalence, r is the initial boundary value of isoline, lgr is the common logarithm of r, lgarea is the natural logarithm of the area of each equivalence, and area is the area; (5) importing the plane layer as Feature Class into the newly created Personal Geodatabase; (6) copying the isosurface initial value column and area column into Excel; (7) then calculating the area delineated by isolines corresponding to different r values, and calculating lgr and Igarea; (8) taking advange of lgr and lgarea columns to draw log-log curves; (9) judging the segment number of the log-log curves; (10) performing linear fitting on the straight lines at both ends of the dividing point respectively, and when the sum of square sum of residuals formed by the two straight line fitting models is the smallest, taking the ler value at this time, and restoring it to the r value; (11) delineating the abnormal topography area in ArcGIS with the help of an abnormal lower limit.

2. The method for extracting abnormal topography of hydrothermal vents according to claim 1, characterized in that, in (2), the topography layer is interpolated by the geostatistical universal Kriging approach, and is reasonably divided into several grades according to the topography change.

3. The method for extracting abnormal topography of hydrothermal vents according to claim 1, characterized in that the area in (4) is the area delineated as for r > ri.