سجلت محطات الشبكة الوطنية للرصد الزلزالي زلزال في وسط البحر الأحمر يبعد حوالي 175 كم من مدينة أسمرة بأريتريا

  • Date: 2024-02-15

  • Time: 06:51:00 am

  • The power: Richter scale 4.40

  • Depth: 32 km

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Principles of Magnetic Surveying

The complete magnetic anomaly maps of 2002 are considered a new collection of aerial magnetic data owned by the Saudi Geological Survey for the central and western areas of the Kingdom of Saudi Arabia.

Most areas in the western part of the Kingdom of Saudi Arabia are underlain by the Cambrian rocks of the Arabian Shield, and were covered by five aerial surveys during the sixties with terrestrial separators at a distance of 800 m, a height from 150 to 300 m, and with a total line of around 768,000 kilometers.

Aerial surveys are conducted for smaller areas in the Arabian Shield during the eighties. These areas are covered by abundant basalt or "Harrat" that date back to the Triassic Period.

Due to the importance of the data in the regional kinetic analysis, the central part of the Red Sea was surveyed from 1976 to 1977 as a part of the works of the Saudi-Sudanese Organization for the resources of the Red Sea. The duration of sampling for all data was one second, or around 60:70 m along the sector.

Previously, an aerial survey was conducted from 1982 to 1983 along a total distance of 490,000 km for the Neoproterozoic caprocks of the Arabian Shield and along the coastal plain of the Red Sea. The distance of the line spacing from the aircraft was 2 km with a 120-m azimuth height above the earth’s surface. The survey was overlapped with a small area of the southeastern survey network of the Shield, which was previously surveyed.

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A map of the surveyed areas used in collecting the new aerial magnetic data.

The original geological data of the survey was normalized and integrated for the first time with the data gathered from the central and western parts of the Kingdom of Saudi Arabia that had been carried out in 2002. The integration of the data lowered the noise in the data, leading to an improvement in the quantity of the recorded data. The integration was carried out to the collinear network, not to the data of the line. It included the local distortion of networks in order to reduce the contraventions within the survey limits. The previous processing of the data on the Arabian Shield that was carried out in the sixties and seventies led to the fact that the amendments of the international geomagnetic field was erroneous with a remarkable regional declination that reduced the accuracies of some magnetic maps previously published about the western areas of the Kingdom of Saudi Arabia. As for the data processing of the cap rocks that was carried out in the eighties, a more accurate international geomagnetic field basis was utilized, and it did not cause any estimated deviation and resulted in a balanced appearance of the survey maps of the cap rocks. The integration of the Arabian Shield data and the cap rocks data networks led to the matching of the international geomagnetic field data on the cap rocks with the Arabian Shield data and the removal of the deviation that previously existed in the Arabian Shield map.

The dimensions of network reach 200 m. It is considered sufficiently good for reserving the accurate data details, and are appropriate for the spacing line for most surveys conducted in the Arabian Shield. Contrary to some magnetic data that were formerly published, any continuation has not been applied, because the processing removed all the normalization errors in a scientific manner that resulted to a few interruptions within the survey limits. Therefore, the new data collection has more accurate details than the previous one.

The Reduction to the Pole (RTP) has been worked on by using the Fourier procedures. The RTP network plots the anomalies directly on the causing object instead of the bipolar anomalies being shifted within a Total Magnetic Intensity (TMI) network. The bipolar impacts are crystal clear in the low magnetic slopes. As for the isolated bipolar anomaly in the Kingdom of Saudi Arabia, it has a minimum TMI to the north of the object and a maximum to the south, and the apex in the north was shifted to the pole until it directly coincides with the object. This makes the analysis of the data easier, as RTP anomalies can be directly related to geology. The deviation of the magnetic field varied from around 40o in the north to less than 20o in the south. The RTP has allowed for this, and avoided any issues related to the low latitudes. The First Vertical Derivation (FVD) has been calculated for RTP network by using the Fourier methods. It provided a derivable survey network that improved the top spatial frequencies in the data as those caused by the cracks, rock interrelations, or contact zones, and produced information about the structures that may not be clear in the RTP network.

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The map showing the reduction of the total fields of pole (Zahran, 2002).

The use of the traditional colorimeter or spectrocolorimeter shows the low magnetic values in blue and the high values in red on the RTP maps developed by the Saudi Geological Survey. Their shape was improved by applying the artificial hatch system as the light direction is from the northeastern area. In the unicolor map of the First Vertical Derivation (FVD), the low values (negative) are dark and the high values (positive) are light. It does not display an FVD meter because the unicolor map is preferred in improving the image, and the set values are not usually important.

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First Vertical Derivation for the RTP set (Zahran, 2002).

The Saudi Geological Survey uses the magnetic anomaly maps with a regional scale, which were produced recently for a single coordinate system in order to simplify the use of information. The most available magnetic data were planned by using the coordinate system that is based originally on the International Spheroid. To make a set of complex information, all of the coordinate systems were transformed to the (WGS84) spheroid used by the Global Positioning System (GPS) that is generally used in modern surveys and exploration. The transformations used the constants of Ein Al Abd. The Lambert Conic Consistent (LCC) Projection was used to survey the network and to develop the maps. This Cartesian system is applicable to a wide set of longitudes and is appropriate for the intended scales of the regional maps. The central longitude of LCC Projection that was used falls at 48o to the north, and crosses very close to the center of the Arabian Peninsula. For this reason, it is used to create the regional maps. The two standard parallels fall at 17o and 33o to the north. The latitude of the original projection is considered as the equater, i.e., its origin is (zero, zero) at 48o to the east and 0o to the north. The UTM projection is generally used for the maps with higher scales than those shown here. It is not consistent for the large areas as those maps drawn from a set of reproduced regional information as projection may have distortions in the areas outside of any scale with the 6o latitude.

The Saudi Geological Survey uses the magnetic anomaly maps with a regional scale, which were produced recently for a single coordinate system in order to simplify the use of information. The most available magnetic data were planned by using the coordinate system that is based originally on the International Spheroid. To make a set of complex information, all of the coordinate systems were transformed to the (WGS84) spheroid used by the Global Positioning System (GPS) that is generally used in modern surveys and exploration. The transformations used the constants of Ein Al Abd. The Lambert Conic Consistent (LCC) Projection was used to survey the network and to develop the maps. This Cartesian system is applicable to a wide set of longitudes and is appropriate for the intended scales of the regional maps. The central longitude of LCC Projection that was used falls at 48o to the north, and crosses very close to the center of the Arabian Peninsula. For this reason, it is used to create the regional maps. The two standard parallels fall at 17o and 33o to the north. The latitude of the original projection is considered as the equater, i.e., its origin is (zero, zero) at 48o to the east and 0o to the north. The UTM projection is generally used for the maps with higher scales than those shown here. It is not consistent for the large areas as those maps drawn from a set of reproduced regional information as projection may have distortions in the areas outside of any scale with the 6o latitude.