SUMMARY
During the last 70 to 80 years, innumerable aerial and terrestrial gravimetric and magnetic surveys have been carried out in South, Central and North America. The Geophysical Institute of Peru (IGP), the Agustín Codazzi Geographical Institute of Colombia (IGAC), the National Hydrocarbons Agency (ANH) of Colombia, the Simón Bolívar Geographical Institute of Venezuela (IGSB), former National Cartography (DCN), Petroecuador , The Military Geographic Institute (IGM) of Argentina, the National Observatory in Brazil and the National Institute of Statistics, Geography and Informatics of Mexico (INDESI), are some of the institutions responsible for the NATIONAL GRAVIMETRIC NETWORKS and the Second Order Networks along the Leveling Points and Geodesic Triangulation in each country. Additionally, these organizations are repositories of all the potential information that is acquired in their territories.
Based on the NATIONAL GRAVIMETRIC NETWORKS, the oil and mining companies that carry out exploratory activities in the different sedimentary basins and areas of mining potential of these countries have carried out aerial and terrestrial gravimetric and magnetic surveys. Similarly, and especially in Venezuela, the Simón Bolívar University, through the Department of Earth Sciences and the Geophysics Section of Laboratory "D", has organized a compilation and evaluation of all the terrestrial gravimetric surveys carried out in Venezuela and part of Colombia.
It should also be noted that there is currently access to SATELLITE information. In the Geosoft Database it is possible to download, by applying the "SEEAKER", the corresponding meshes, not only of the (SRTM World Elevation 3 arc-secons.GRD), but also of the BATIMETRY (SRTM 30 Plus V7.GRD) and of the Free Air Anomaly (AAL) (Free Air Gravity (mgal) Sandwell v23.1.GRD). Properly using this information, it is possible to determine the Bouguer Anomaly values of the marine areas corresponding to the Caribbean Sea and the Pacific and Atlantic Oceans. Lastly, in the continental zone where there is no terrestrial or aerial survey, there is the coverage of the "South America Gravity Project of Leeds University, SAPG" and/or the described Satellite Bouguer. Additionally, the "National Center for Environmental Infrastructure (NOAA)" has published and released its gravimetric data, as well as the gravimetric data that for many years was raised and collected by the "Defense Mapping Agency (DMA), through the "Inter America Geodetic Survey (IAGS)”. This work also includes gravimetric data from Guatemala, Uruguay and part of Brazil provided by the "International Gravimetric Bureau (BGI)".
Although all the potential information (gravimetric and magnetic) is available, both in analog and digital form, the search for information to obtain regional or local maps of BOUGUER ANOMALY and/or TOTAL MAGNETIC INTENSITY is currently very slow and difficult. On the other hand, the individual surveys deserve to be integrated, evaluated and their parameters recalculated to be able to build and feedback the Gravimetric Databases: (i) GROUND and (ii) AERIAL, (iii) GROUND-AIR and (iv) GROUND- AIR-SATELLITE of South, Central and North America. These will allow interested companies, institutions, and researchers to accelerate many of their objectives:
- A.- In a fast and economical way, add value to all those areas that have gravimetric data, that are interested in PERUPETRO, the ANH, PETROECUADOR and PDVSA as well as associated oil companies.
- B.-Quickly detect zones and/or areas that present important possibilities for the industry and that until now have gone unnoticed.
- C.-In areas where additional geology, seismic and other earth science data are available, integrate with gravimetry and magnetometry to improve geological-structural knowledge.
- D.-Generate the Bouguer Anomaly Map of South, Central and North America, individual countries and/or specific areas of exploratory interest.
THE GRAVIMETRIC DATABASE OF SOUTH, CENTRAL AND NORTH AMERICA AND SURROUNDING AREAS
Introduction
All the works that allowed to elaborate the DATABASES that are presented in this work, were executed with the help of the OASIS MONTAJ Graphic System of GEOSOFT and computer programs developed by Ing. Victor R. Graterol G. Figure 1-A and 1-B show the existing Terrestrial and Aerial Gravimetric Potential data, which could be evaluated in this part of South, Central and North America. The marine areas where the Bathymetry and Free Air Anomaly are known are clearly observed. The terrestrial areas where there is no coverage exist the SAPG and/or the Bouguer information that can be obtained from the digital terrain and the Satellite Free Air Anomaly allowed to complete the entire area proposed in this work.
Digitization of gravimetric data from the "South America Gravity Project of Leeds University", aeromagnetogravimetric data from Perupetro, Petroecuador and the National Hydrocarbons Agency (ANH), terrestrial gravimetric data from the Simón Bolívar University Database and Anomaly data of Free Air and Satellite Bathymetry, allow to obtain the TERRESTRIAL GRAVIMETRIC DATA Base, the AIR GRAVIMETRIC DATA Base, the TERRESTRIAL-AIR GRAVIMETRIC DATA Base and finally the TOTAL GRAVIMETRIC DATA Base (Terrestrial, Aerial, Satellite) of South, Center and North America.
Gravimetric Databases. Initially given the origin in gravimetric acquisition, TWO different DATABASES were created: one for Ground Surveys and one for Aerial Surveys. Subsequently, the terrestrial and aerial data were joined and finally the terrestrial-air-satellite union.
The Terrestrial Gravimetric Database is made up of information from the following sources: Geophysical Institute of Peru (IGP) – INGEMET. Agustín Codazzi Geographical Institute (IGAC), Simón Bolívar Geographical Institute (IGSB), Simón Bolívar University (USB), NOAA, BGI), National Hydrocarbons Agency (ANH), National Observatory of Brazil and Petroecuador, National Institute of Statistics, Geography and Informatics of Mexico (INDESI) the Mexican Petroleum Institute (IMP) of Petróleos Mexicanos (PEMEX). Some of these organizations, with the advice of the IAGS, not only carried out the measurement of the first absolute observation of gravity in each of their countries, but with the help of gravimeters, gave Absolute Gravity values to the National Gravimetric Network and the Secondary Networks along the Leveling and Geodesic Triangulation points. Fortunately, these organizations have the "Main Characteristics" of these gravimetric stations in digital form, therefore, this information constituted the beginning of the TERRESTRIAL Gravimetric Database.
Surveys of the Operators. The oil companies that have carried out terrestrial gravimetric exploration in this part of South, Central and North America have deposited this information in the indicated institutions. Only information from this source that is in digital form was incorporated into the Terrestrial Database. The Department of Earth Sciences and the Geophysics Section of Laboratory "D" of the Simón Bolívar University carried out the evaluation and certification of this information. Topographic correction was performed with the Oasis Montaj “Gravity Terrain Correction” Module and using the topographic relief shown in Figure 1-C. For each observation shown in Figure 1-A, the near topographic correction was made up to a radius of 2Km and the far 125Km.
Figure 1-A shows the gravimetric land cover that has been evaluated and certified in this part of South, Central and North America. In addition to the PRIMARY and SECONDARY GRAVIMETRIC NETWORKS of each country, a total of 116 terrestrial surveys carried out between 1952 and 2004 were evaluated. Each one of these surveys was linked to some point of the indicated networks, they were analyzed individually to elaborate the best possible mesh. that it did not show high-frequency noise and individual stations were eliminated whose noise was very possibly due to some editing error when copying its Main Characteristics from its analog recording. The files “NOAA GRAVITY STATION DATA”, corresponding to Colombia, Bolivia, “South America” 1997 and 1993, US NGS of 1999, Alaska and Mexico, were analyzed individually, innumerable repeated stations were found, and noise anomalies were eliminated from high frequency produced by individual stations.
The GRAVIMETRIC BASE OF TERRESTRIAL DATA consists of an ASCII list ordered by LONGITUDE that in addition to the LONGITUDE, LATITUDE, HEIGHT, OBSERVED GRAVITY, and the
TOPOGRAPHIC CORRECTION includes a FLAG that would allow the STATIONS to be classified.
Flag Type of Station
1 Ground Station with height above the MSL
2 Ground Station with height below MSL
3 Marine Station
5 Submarine Station
Each station is a LINE of this ASCII file which includes LONGITUDE, LATITUDE, HEIGHT, OBSERVED GRAVITY, FLAG and TOPOGRAPHY CORRECTION.
To manipulate this ASCII file, the SECBOUGTierra.FOR program was implemented, which allows not only to extract the gravimetric observations by WINDOWS of LONGITUDE and LATITUDE, but also calculates for each terrestrial, marine and submarine station the Free Air and Bouguer Anomalies for a Bouguer density and returns an ASCII file for the selected window with the following information for each gravimetric observation:
LONGITUDE, LATITUDE, STATION, HEIGHT, OBSERVED GRAVITY, FLAG, FREE AIR ANOMALY, SIMPLE BOUGUER ANOMALY, TOPOGRAPHIC CORRECTION and TOTAL BOUGUER ANOMALY
This ASCII file, once introduced in OASIS MONTAJ, will allow the creation of the base to determine the flat UTM coordinates and subsequently design within Geosoft: the base map of the window and the color maps of the Free Air and Bouguer Total Anomalies. The
ANNEX at the end of this report includes a list of the source of the SECBOUGTierra.FOR program.
Figure 2-A shows the TERRESTRIAL BOUGUER ANOMALY, using a Bouguer Density of 2.67g/cc. The TERRESTRIAL GRAVIMETRIC DATABASE currently consists of 789539 observations.
The Airborne Gravimetric Database is made up of information from the following sources: During the last 35 years, the oil companies that carry out exploratory activities in this part of South America have carried out airborne AEROGRAVIMETRIC, AEROMAGNETIC and GRADIOMETRIC surveys. Fortunately, all this information is found in the indicated institutions in digital form. The companies that have worked are CARSON Aerogravity, FUGRO Airborne System and SANDER Geophysics. In previous years, it is worth noting the acquisitions of only Aeromagnetics from AERO SERVICE Corporation and GEOTERREX Ltd. and Aerogravimetrics from LCT and EDCON Aerosurveys. In the case of Aerogravimetry, the presentation of the data varies from one company to another. CARSON digitally delivers “LAT, LONG, X, Y, Free Air and Bouguer Anomaly Anomalies and flight parameters”, FUGRO delivers “LAT, LONG, X, Y. For the FALCON Gradiometric System, All Gravity Tensor Components, and parameters of flight”, however. for each of the FUGRO - FALCON surveys, from the Gdd Component it is necessary to obtain gd which is the vertical component of the Acceleration of Gravity already converted into the high frequency component of the Bouguer, finally the existing Bouguer must be added to add the low frequency component. SANDER delivers “LAT, LONG, X, Y, flight parameters and Free Air and Bouguer Anomalies”. Although the output formats of the Main Characteristics of the aerial surveys of each company are different, by simple digital manipulation it is easy to read them and convert them to a single format for the definitive elaboration of the aerial gravimetric database. For each of the observations of CARSON, FUGRO, SANDER and LCT, a line was drawn that included: LONGITUDE, LATITUDE, BOUGUER ANOMALY. The SECBOUGAire.FOR program allows the extraction of gravimetric observations by WINDOWS of LONGITUDE and LATITUDE, and returns an ASCII file for the selected window with the following:
LONGITUDE, LATITUDE, TOTAL BOUGUER ANOMALY
This ASCII file, once introduced in OASIS MONTAJ, allows the creation of the base to determine the flat UTM coordinates and subsequently design within Geosoft: the base map of the window and the color maps of the Free Air and Bouguer Total Anomalies. The ANNEX at the end of this report includes a list of the source of the SECBOUGAire.FOR program. Figure 1-B shows the aerial gravimetric coverage that has been evaluated and certified in this part of South and Central America. It was possible to have access to 92 Surveys that were referred to the gravimetric networks in their respective countries. Individually, each of these surveys proceeded to build Bouguer meshes from 500mX500m and the distance between points of the mesh was increased, until no high-frequency noise associated with the flight lines was observed. Figure 2-B shows the Final Bouguer Anomaly Map with the Aerogravimetry analyzed in this work.
The Land-Air Database. Once the final grids of the Bouguer Anomaly were obtained, both from the terrestrial and aerial data, using the "gridkint" module of Oasis Montaj, a final grid was obtained for the Total Bouguer EARTHAIR shown in Figure 2-C . There are land areas where there is no land or air coverage. To complete the total land coverage of the area selected in this work, information from both the “South America Gravity Project, SAPG”, and Bouguer, obtained from the SRTM Digital Terrain and the Sandwell Free Air Anomaly, were used. Figure 2-D shows the Bouguer that was obtained in the terrestrial areas where there is neither terrestrial nor aerial coverage of the terrestrial areas in South and Central America. Figure 2-E shows the union of these two surveys including the area occupied by the Caribbean Sea and a small part of the Pacific Ocean
The Marine Gravimetric Database. The area shown in Figure 1-C includes a large portion of the Pacific and Atlantic Oceans and the entire Caribbean Sea. To determine the Bouguer corresponding to the marine areas, the corresponding meshes were extracted from the Geosoft Database, not only from the (SRTM World Elevation 3 arc-secons.GRD), but also from the BATIMETRY (SRTM 30 Plus V7.GRD ) and the Free Air Anomaly (AAL) (Free Air Gravity (mgal) Sandwell v23.1.GRD). To obtain the Satellite Bouguer, a database was created that includes, for each point of a 1KmX1Km grid, LONGITUDE, LATITUDE, BATIMETRY and AAL. Using the algorithm shown below, the ESCSAndwell.for program calculates the Bouguer Anomaly (AB) for each point in this database.
The output file of this program for each point of the grid is obtained LONGITUDE, LATITUDE, BATIMETRY, AAL and SATELLITE BOUGUER ANOMALY (AB). Figure 2-F shows the Satellite Bouguer of the indicated marine areas.
THE TOTAL DATABASE and THE TOTAL BOUGUER ANOMALY MAP of South, North Central AMERICA and SURROUNDING AREAS. The FINAL GRID OF UNION OF ALL THE BOUGUER ANOMALIES constitutes the TOTAL GRAVIMETRIC DATABASE. The final file corresponding to a 1KmX1Km grid has for each grid node LONGITUDE, LATITUDE, TOTAL BOUGUEER ANOMALY. The SECBougUSB.FOR program extracts the corresponding values of the AB Total in LONGITUD – LATUTUD windows. Table 2 summarizes the databases that were created and the respective program that allows the extraction of information.
Extracting using SECBougUSB.FOR all the values that exist in Final500BougMarzo22.DAT, Figure 3 shows the BOUGUER ANOMALY MAP OF SOUTH AND CENTRAL AMERICA
Conclusions and recommendations.
This work summarizes and allows digital manipulation of all the GRAVIMETRICAL information that has been collected from the land and airborne acquisition works carried out mainly by government organizations and oil companies that have carried out exploratory activities in Venezuela, Colombia, Ecuador and Peru. Additionally, the terrestrial gravimetric data that the Geophysics Section of Laboratory "D" of the Simón Bolívar University, of the Geophysical Institute of El Perú (IGP-INGEMET), of the Agustín Codazzi Geographical Institute of Colombia (IGAC), of the National Institute of Geography Statistics and Informatics of Mexico (INDESI) and partial data from the Military Geographic Institute of Argentina (IGM), have been able to evaluate and certify. ALL THE JOINTS OF terrestrial, aerial and satellite GRIDS were made with the Oasis Montaj “GRIDKNIT” Module
The TOTAL Gravimetric Database is a 1KmX1Km grid, which was built taking into account that in those areas where the terrestrial coverage is better than the air, the BOUGUER TERRESTRIAL DATA were used; and vice versa where aerial data have better coverage than terrestrial data, these were used. Extraction of all the Bouguer values allowed the elaboration of the BOUGUERANOMALY MAP of South, Central and North America that is presented in Figure 3.
At the end of 2018, Perupetro opened a tender to standardize its GRAV/MAG information. When presenting the volume of information to be processed, it was verified that SANDER Geophysics and FUGRO Airborne System have carried out airborne surveys mainly in the Marañón Basin. Similarly, SANDER Geophysics carried out an extensive survey on the Ecuadorian Coast and a large part of the Oriente Basin. In Colombia, Fugro Airborne System, carried out a Falcon-Gradiometric survey for Ecopetrol in the El Caguán area and another for BHP in the Llanos Orientales of Colombia. Also in Argentina, the IGM has a database of more than 16,000 gravimetric stations in Geodetic Leveling Points, as well as the surveys in Mexico of more than 12,000 INGESI stations. Once this information is available, the databases have to be fed back to include this new existing data.
THE AEROMAGNETIC DATABASE (IMT) of South, Central AMERICA AND SURROUNDING AREAS
Introduction. The Aerogravimetric surveys are always accompanied by the Aeromagnetic measurement; therefore, it is only necessary to extract from the information provided by LCT, CARSON, FUGRO and SANDER the position data, the Aeromagnetic measurement (Total Magnetic Intensity, IMT) already corrected for temporal effects, leveling and the IGRF (“International Geomagnetic ReferenceField”). The final IMT is simply subtracting the corresponding IGRF value from the IMT observation.
The magnetic database (BaseMAGColVenEcuPeru.DAT), is an ASCII list ordered by LONGITUDE that includes the MAIN CHARACTERISTICS of each magnetic observation: LONGITUDE, LATITUDE, IMT.
The SECMAG.FOR program allows you to extract the magnetic observations by LONGITUDE and LATITUDE WINDOWS and outputs an ASCII file for the selected window with the following information for each magnetic observation: LONGITUDE, LATITUDE, IMT. This ASCII file, once introduced in OASIS MONTAJ, allows the creation of the base to determine the flat UTM coordinates and subsequently design within Geosoft the base map of the window and the color maps of the Total Magnetic Intensity and the Total Magnetic Intensity Reduced to Pole to the Equator. The ANNEX at the end of this report includes a list of the source of the SECMAG.FOR program.
By applying a LONGITUDE-LATITUDE window to the Magnetic Database that includes the entire NW of South America and surrounding areas, the SECMAG.FOR program will extract all existing Aeromagnetic observations that, when introduced in OASIS MONTAJ, will allow the elaboration of the TOTAL MAGNETIC INTENSITY (IMT) map. With the use of the "Magmap" Module of Oasis Montaj, the IMTRP grid and the respective map of the TOTAL MAGNETIC INTENSITY REDUCED AT THE POLE would be generated.
The Geophysics Section of Laboratory "D" of the USB digitized the aerial survey of MENEVEN S.A. made by Geoterrex Ltd. of Canada). The attached Figure is the AIR IMT that could be processed in this work
Conclusions and recommendations.
This work allows digital manipulation of all the MAGNETIC information that has been collected from the airborne acquisition work carried out mainly for the National Hydrocarbons Agency (ANH) of Colombia, Petróleos de Venezuela (PDVSA) and oil companies that have carried out exploratory activities in Venezuela, Colombia, Ecuador and Peru. Similarly, the SANDER Geophysics and Fugro Airborne System surveys for operating companies in Colombia constituted a source of information for the MAGNETIC DATABASES presented.
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