Evaluación y aspectos externos vinculantes para el bucle educativo

It was mentioned in sections 3.2.1.1 and 3.2.1.2 as well as in section 4.0 that a new source of gravity signals in the medium-to-short wavelength spectrum of the gravity field was investigated in this work. This gravity field signal comes from the topographic den­

sity anomalies. Initially, the geological information was sourced from the Geological

Survey of Zimbabwe. The detailed geological map contained such information as the crustal topographic density assumed to the mean sea level, the different rock types and economic minerals abundant in this mineral-rich area. This detailed map was digitized using the Kern tablet digitizer. A DDM of the area is shown in figure 4.8. A more complete discussion and analysis of this signal is carried out in chapter 7.

To facilitate this investigation, it was necessary to create new software based on the ter­ rain correction software of the GRAVSOFT package [Forsberg,R., pers. comm.] The software GEOTOPODENC necessitated the creation of a Digital Density Model (DDM) in addition to a DEM in the computation area. The DDM was created at the same spatial specifications as the DDM.

A Digital Density Model was necessary to compute the topographical gravitational den­ sity anomaly effects. The DDM just like a DEM is a gridded set of data, but with topographic density information. The DDM created for the area surrounding and includ­ ing Zimbabwe covered the area 14°S to 24°S & 24°E to 34°E. The original data was sourced from a 1:1000000 geological map of Zimbabwe produced by the Geological Survey of Zimbabwe (GSZ) As no DDM of Zimbabwe existed before, it was found nec­ essary to create one based on the available data set. The original map was digitized and using the ARCINFO software package was then gridded into a 1km grid and stored on database for future use.

The geological map was quite detailed. The most striking feature is the famous Great Dyke, which is a ridge of intrusive igneous rocks consisting mainly of the serpentinites, pyroximites, norites and gabbro type of mineral rocks. The Great Dyke splits the country from the northeast cutting across the country down to the south-west It is a mineral rich area exploited for platinum, nickel, copper, chromites, asbestos and magnesites. The to­ pographic density given is assumed to be for the crust down to the mean sea level. It is widely accepted that the long wavelength gravity signal originates from the density anom­ alies in the mantle (down to ~ 1000km from the earth’s surface). Conditions in the crust give rise to the high degree harmonics of the observed power spectrum of the earth’s

Chapter 4: Geoid determination Data Sources

Zimbabwe Digital Density Model (gem-**3)

3.2-

3.0.

2.0.

I Ï 1 2 L 0 S V

Zimbabwe Digital Density Model (gcm-=‘“"3)

<o -19 -20 A B O V E 3.a- 4.0 B ELO W 28 29 30 31 Longitude (deg. E)

Chapter 4: Geoid determination Data Sources

gravity field. [Lambeck,K., 1976]. However, the lateral density changes also give rise to the medium wavelength signals of the power spectrum of the earth’s gravity field. The topographic density anomaly signal is therefore regarded as a medium-to-short wave­ length signal of the spectrum of the gravity field.

Table 4.1 Density Variation in Zimbabwe

detailed elevation grid and density anomaly grid:

-gridlab -24.0000-14.0000 24.0000 34.0000 grid spacing:0.0083 0.0083 stations: 13948, grid points: 1201 x 1201 = 1442401 hmin hmax mean std.dev. rhomin rhomax rhomean std.dev.

11 2250 863.01 340.08 1.03 5.60 2.77 0.35

Digitizing the map was not an inconsiderable task. This detailed map took quite some time to complete. The whole procedure involved checking for gross errors and re-digitizing incomplete polygons and lines and rechecking the new polygons against the original data. It was found necessary to carry out some coordinate transformation to the geographical system to be consistent with the rest of the gravity data reference frame. The gravity data geographical coordinates are based on the modified Clarke 1880 ellipsoid. [Rens,J. & Merry,C.L., 1990]. The digitised map was in local digitizer coordinate system. The com­ pleted digitized data set was then gridded to form a uniform 30" grid covering the whole area of interest. This was the same spatial resolution as the rest of the gravity field data, i.e. gravity data and DEM.

Table 4.1 shows the typical range of rock densities in Zimbabwe. Zimbabwe has a large inland water body - the Lake Kariba, which is formed by the largest man made dam in the world. This is located on the North-Western border of Zimbabwe (ZW) with Zambia (ZA) and runs roughly in a NE-SW direction.This is where the low density values of 1.03 are to be found in the DDM shown in figure 4.8. Generally the crustal density is taken to be 2.67gcm'^. A mean value of 2.77 was obtained in Zimbabwe. Unusually large values such as 5.60gcm'^ were obtained based on minerals and rock types as obtained in some small localised patches from the geological map of Zimbabwe. In reality, however, these large values are unlikely to be truly representative of densities of crustal blocks down to the mean sea level.

Chapter 4: Geoid determination Data Sources