Circular No. 05/2011/TT-BTNMT on technical methods for terrestrial gravity survey

CIRCULAR

Technical regulations on the method of terrestrial gravity survey

MINISTER OF NATURAL RESOURCES AND ENVIRONMENT

Pursuant to the Law on Minerals and the Law amending and supplementing certain articles of the Law on Minerals;
Pursuant to the Government Decree No. 25/2008/NĐ-CP dated March 4, 2008 stipulating the functions, tasks, powers, and organizational structure of the Ministry of Natural Resources and Environment;
Pursuant to the Government Decree No. 89/2010/NĐ-CP dated September 18, 2010 amending and supplementing Article 3 of the Government Decree No. 25/2008/NĐ-CP dated March 4, 2008 stipulating the functions, tasks, powers, and organizational structure of the Ministry of Natural Resources and Environment;
Considering the proposal of the Director of the Geological Survey Institute and the Head of the Legal Department,

c) Enterprises may be granted permission for no more than one block out of the total three dual-frequency blocks (FDD) B

PART I

GENERAL PROVISIONS

Article 1. Scope of Regulation

This Circular stipulates the procedures, contents, and requirements for the method of terrestrial gravity survey in geological basic investigation activities related to minerals, mineral exploration, engineering geology surveys, hydrogeological surveys, geological disaster investigations, and other relevant fields.

Article 2. Applicability

This Circular applies to units, organizations, and individuals participating in implementing tasks, projects, and programs related to geological basic investigation of minerals, mineral exploration, engineering geology surveys, hydrogeological surveys, geological disaster investigations, and other relevant fields (referred to as specialized projects) conducting the terrestrial gravity survey method.

Article 3. Explanation of Terms

1. The terrestrial gravity survey method is a method of measuring the absolute value, acceleration (first derivative), or acceleration gradient (second derivative) of the gravity field on the ground to conduct basic geological investigations of minerals and mineral exploration; engineering geology surveys, hydrogeological surveys, geological disaster investigations, and other relevant fields.

2. The unit of measurement in gravity survey is milligal (mGal) where 1 mGal = 0.0001 gal = 1.10^-5 m/s^2.

3. A gravity base network consists of multiple gravity points used to link and standardize gravity measurement data to the same level.

4. A closed gravity base polygon is a system comprising three or more gravity base points forming a closed polygon within the gravity base network system.

5. A hanging gravity base point is a gravity base point that only connects with one gravity base point of a closed gravity base polygon.

6. A hanging gravity base edge is an edge consisting of two or more consecutive gravity base points arranged along a route without forming a closed gravity base polygon within the gravity base network system.

7. Chain gravity measurement involves measuring the difference in gravity values twice or more to connect gravity values between hanging gravity base points.

8. A gravity base point marker is an object marked with the name and number of the gravity base point placed at the location of the gravity base point in the field.

Article 4. Fields and conditions for applying the terrestrial gravity survey method

1. The terrestrial gravity survey method is applied to study deep earth structures, regional tectonic zoning, geological and mineral surveys, engineering geology surveys, hydrogeological surveys, geological disasters, and other research fields.

2. Conditions for applying terrestrial gravity survey:

a) There must be a density difference between the research object and the surrounding rock environment. The research object must have sufficient size to cause reliable anomalies as specified in Appendix 1 attached to this Circular;

b) The terrain of the area being surveyed for gravity should not be too dissected;

c) The scale of the gravity map survey must be larger than one degree or equal to the scale of the geological map survey;

d) Terrestrial gravity survey must be prioritized to be conducted before or simultaneously with geological mapping and mineral investigation and evaluation work.

Article 5. Proportion and tasks of gravity survey, requirements for gravity survey

1. Proportion and tasks of gravity survey:

a) Gravity survey at a scale of 1:500,000 to 1:200,000 has the task of serving geological map compilation, mineral resource prediction, tectonic zoning of the Earth's crust;

b) Gravity survey at a scale of 1:100,000 to 1:25,000 has the task of determining geological structural zones, fault systems, intrusive bodies, volcanic formations, sedimentation, and delineating mineral prospects;

c) Gravity survey at a scale from 1:10,000 to 1:2,000 has the task of determining detailed geological structures, discovering mineral deposits, and participating in mineral resource reserve assessment; coordinating with other geological and geophysical methods to address engineering geological, hydrogeological, and geological disaster tasks;

d) Gravity survey at a scale from 1:1,000 to 1:200 has the task of evaluating and exploring mineral deposits; conducting engineering geological surveys, hydrogeological surveys, and geological disaster investigations; searching for and identifying buried objects and ancient structures;

2. Requirements before conducting gravity survey:

a) Collecting and compiling density parameters and other physical parameters, determining excess density values between them;

b) Collecting topographic data, establishing requirements for the accuracy of coordinate and elevation determination of gravity points. Corresponding to the gravity mapping scale, larger-scale topographic maps or maps at the same scale as the gravity measurement must be available;

c) When mapping gravity at a scale of 1:10,000 and higher but without corresponding topographic maps, appropriate topographic work should be proposed based on the accuracy requirements of the gravity measurement project;

Article 6. Network and accuracy of measurement

1. Selecting measurement point network and accuracy:

a) The density of the gravity measurement network must ensure the reflection of relevant gravity field characteristics related to the research object. Anomalies measured over an area must be represented by at least five measurement points; when measured along a line, they must be represented by at least three measurement points;

b) A gravity anomaly map can only be established when the gravity measurement results meet the technical criteria specified in Appendix 1 issued together with this Circular and the topographic work requirements stipulated in Article 18 of this Circular. If these conditions are not met, only a survey sketch or a map at a lower scale than the specified scale can be established;

c) When selecting cross-sections of contour lines on summary diagrams, it must be based on the objectives and tasks of the project, the predicted intensity of gravity anomalies, and correspond to the scales specified in Appendix 1 issued together with this Circular. To better represent weak anomalies, smaller cross-sections of contour lines less than 1/3 to 1/2 of those specified in Appendix 1 issued together with this Circular may be used. Conversely, for local strong gravity anomalies (with high horizontal gradients), some contour lines may be omitted but not exceeding three to five times the specified cross-sections in Appendix 1 issued together with this Circular;

2. When mapping gravity in areas adjacent to already mapped areas, a number of old reference points must be linked to the new reference point network, balanced to unify the gravity measurement results across regions;

a) When the difference in reference values at each point does not exceed the project's allowable error, the gravity measurement data can be linked;

b) When the accuracy of measurements between two adjacent areas differs, the error of the anomaly map is chosen at a lower error level;

c) When the surveyed areas have different scales, if a combined map is created, the smallest scale among them should be selected;

3. The accuracy of determining Bouguer gravity anomalies is evaluated by the root mean square error. This error must not exceed 0.4 times the value of the contour interval when measuring over an area. When measuring along a line, this error must not exceed 0.2 times the smallest local anomaly value caused by the research object. On geological-geophysical cross-section lines, the measurement accuracy is higher by 1.5 times;

4. The main technical criteria for gravity mapping: contour interval, measurement network density, measurement error, anomaly determination error, coordinate determination error, and measurement point elevation are specified in Appendix 1 issued together with this Circular. When preparing a project, the specific errors for gravity measurement and topographic work must be defined based on the project's objectives and tasks. Depending on the geological characteristics (simple, moderate, complex), terrain, and measurement point network density, the network density can be increased within the upper limit;

Chapter II

PROJECT PREPARATION AND CONSTRUCTION. DATA COLLECTION, PROCESSING, COMBINATION, AND FIELD SURVEY

Section 2

PROJECT DESIGN

Article 10. Project Design Plan

1. The project design plan includes the explanatory note, drawings, and appendices.

2. The content of the explanatory note of the project shall be implemented according to Model No. 1 of Appendix 2 issued together with this Circular.

Article 11. Inspection, Acceptance, and Review of Projects

Projects must be inspected, accepted, and reviewed in accordance with current regulations and may only commence construction after being approved by the competent authority.

Article 12. Preparation for Organizing Construction

1. The head of the unit and the project manager have the responsibility to disseminate the project content to construction teams and assign plans regarding progress and funding.

2. Technical staff and workers participating in construction must study occupational safety, understand all professional and technical requirements before implementing each step of the project.

3. Only Investigators and Intermediate-level Environmental Resource Investigators assigned by the project manager or the construction unit's head may use machines for on-site construction work.

4. The person responsible for processing office documents and compiling the final report must have the qualifications of a Senior Investigator in environmental resources (geophysics specialization) or higher.

5. The time to compile the final report is from one-third to one-half of the actual on-site construction period, but not less than one month.

Article 7. Basis for Establishing Projects

1. Decision assigning tasks by the competent authority;

2. Contracts for gravity surveying with legal validity.

Article 8. Documents for Establishing Projects

To establish a gravity surveying project, it is necessary to collect all relevant geodetic, geophysical, geological, mineral, rock physical parameters, and other related documents already available in the area, including the following types of documents:

1. Geodetic documents:

a) Topographic maps of the working area at a larger scale or the same scale as the planned gravity map;

b) Topographic maps at a smaller scale than the gravity map for terrain correction calculations;

c) Data on coordinates, elevations, and descriptions of national geodetic markers related to the survey area;

d) Technical specifications of geodetic equipment used in the project.

2. Existing geophysical, geological, and mineral documents.

3. Rock and ore physical parameter documents.

4. Based on the synthesis and analysis of existing documents, establish appropriate methods and technical standards to address the project's tasks.

Article 9. Tasks for On-Site Survey to Establish Projects

1. Preliminary survey of topographical conditions and transportation.

2. Search and determine the positions on-site of elevation markers, coordinate markers; national gravity base points, adjacent regional gravity base points that have been constructed; take additional samples to measure density parameters when there is insufficient documentation as stipulated in Article 16 of this Circular.

3. If necessary, conduct 1 to 2 representative gravity lines in areas with different topographies to preliminarily assess the effectiveness of gravity exploration methods in addressing the project's objectives and tasks.

Chapter III

ON-SITE CONSTRUCTION

Section 1

GRAVITY WORKS

Article 13. Gravity Instruments

1. Before on-site construction, all gravity instruments must be monitored in static and dynamic states; calibrated, and the constant C determined.

2. Monitoring instruments in static and dynamic states; calibration, and determination of the constant C must be conducted before construction, six months after construction, after repairs, and when the instrument has been subjected to severe impact.

3. Instruments must be stored according to the manufacturer's instructions.

Article 14. Gravity Support Points and Gravity Support Networks

1. Gravity support points and gravity support networks are used for:

a) Eliminating accumulated errors at regular points;

b) Calculating the zero point displacement and checking the quality of regular measurement trips;

c) Bringing all data to a unified level.

2. Gravity support points must be located in easily accessible, quiet, and easily identifiable positions with fixed landmarks and markers ensuring their existence for at least five years. Each support point must have a description form and accompanying photographs.

3. The accuracy of measurements at gravity support points must be at least 1.5 times higher than the accuracy of measurements at regular points. Multiple readings from a single instrument or measurements using multiple instruments may be employed.

4. A gravity support network includes closed polygons, hanging sides, and must be established before regular point measurements. The density of the support point network must ensure that regular point measurement trips have linear zero point displacements of the instrument.

5. The gravity support network must be connected to national gravity support points or absolute method gravity points (g). During surveys, if the scale is 1:10,000 or larger, and the area is less than 70 km² or the measurement route is shorter than 20 km, the support network can be linked to a conventionally defined origin point within the region.

6. The gravity support network is carried out through independent measurement trips. In fragmented terrain conditions with difficult transportation, hanging gravity support sides may be arranged. Hanging gravity support sides must be connected to closed gravity support polygons. If a hanging gravity support side has two to three points, chain measurements must be conducted. The number of gravity increments measured on hanging gravity support sides must be at least 1.5 times greater than those measured on polygon support sides. Measurement errors on hanging gravity support sides must be equivalent to those on polygon support sides.

Article 15. Regular Gravity Point Networks

1. Regular gravity points are measurement points designed according to a network plan, evenly distributed across the area, along a route, or on a line.

2. Each measurement trip must start and end at a support point. The location of each measurement point must be marked with paint on a fixed landmark outside, plotted on the construction map, and described in a point description form as prescribed.

3. At scales of 1:10,000 and larger, gravity points are conducted along predetermined routes, marked with stakes and numbered. Depending on the accuracy requirements of the project, each regular point may be measured independently two or three times. If multiple instruments are used, the value measured by each instrument is an independent measurement value.

4. Density of measurement points: Regular points must be arranged according to the specified density. The general density of measurement points for different gravity map scales ranges from 0.33 to 1 point/cm² of the corresponding map scale. The specific density of the network for each map scale according to the approved project. 5 Measurement sequence: Regular points are sequentially measured over areas where the support point network has been completed. In special cases, some regular measurement trips may be conducted during support measurements. The number of measurement points and trips must be clearly stated in the project.

6. Measurement time: Independent measurement trips must be conducted continuously without interruption. The duration of each measurement trip must be within the linear zero point displacement range of the instrument.

7. Quartz gravimeter measurement values are recorded in the logbook according to the model prescribed in Appendix 3 issued together with this Circular. Each point must record three readings and calculate the average value.

8. For automatic recording machines, technical operations must strictly follow the procedures specified in the machine's history for each type of machine. Each measurement trip must be recorded in the trip logbook. After each day of measurement, data from the gravimeter must be transferred to a computer for storage.

Section 2

DETERMINATION OF SOIL AND ROCK DENSITY

Article 16. Requirements for Determining Rock Density

1. The value of rock density parameters and ore is data serving as a basis for project development, calculating inter-layer corrections, topographic corrections, and solving quantitative gravity problems.

2. Each geological formation with different lithological components must have at least 30 samples of density parameter data. When this quantity is insufficient, additional sampling and density parameter measurement must be conducted.

3. Error in determining density parameters: In accordance with VILAS standards.

4. When establishing a gravity anomaly map, the inter-layer density selected uniformly is s = 2.67g/cm³ (if the main area of gravity exploration is geological formations from before the Neogene - Quaternary period and igneous rocks), and s = 2.30g/cm³ (if the main area of exploration is Neogene - Quaternary sedimentary deposits).

5. For gravity surveys at scales of 1:10,000 and larger, the average inter-layer density value must represent the characteristic density of geological formations distributed within the survey area.

Section 3

FIELD SURVEY WORK

Article 17. Tasks of Field Survey Work

1. Determine the coordinates and elevations of gravity measurement points.

2. Transfer elevations and coordinates from state survey markers to gravity points, determine surrounding elevations around gravity points when required by the project.

3. Establish gravity points in the field, set stakes, mark point numbers, plot points on construction maps (diagrams), and prepare point description sheets according to regulations.

Article 18. Requirements for Field Survey Work

1. Arrange and measure coordinates and elevations of base points and ordinary gravity points in the field strictly according to the project design.

2. If the elevation difference measured at gravity points exceeds one-third of the contour interval on the topographic map, supplementary surveying should be proposed based on the accuracy requirements of gravity measurements. Technical specifications and quantities of supplementary surveying are stipulated in the project.

3. Base points are constructed using reinforced concrete, engraved with all necessary information, ensuring durability for 5 to 15 years as specified in Model No. 7 of Appendix 3 issued together with this Circular. For gravity surveys at scales of 1:10,000 and larger, base points can be made of wooden stakes or fixed objects, ensuring durability for 5 to 10 years.

4. Ordinary points are made of wooden stakes, bamboo, or fixed objects, marked with point/line numbers, and must remain intact during the project construction period.

5. Geodetic work in geophysical operations must comply strictly with current geodetic regulations.

Section 4

FIELD OFFICE WORK

Article 19. Field Office Work

1. Field office work is carried out concurrently with field construction activities, including the following main tasks:

a) Adjusting and calculating gravity and geodetic measurement data;

b) Balancing the gravity and geodetic base networks;

c) Calculating elevations and coordinates of gravity points;

d) Calculating and establishing preliminary Bouguer anomaly diagrams;

e) Calculating errors and assessing the quality of gravity and geodetic measurement work. These tasks must be independently performed twice by two different individuals to ensure reliability and timely detection of errors for supplementary field checks.

2. The field office team must include at least the following members:

a) Project Leader: With a level of expertise from Senior Resource and Environment Investigator upwards;

b) Three Resource and Environment Investigators responsible for data calculation and drawing up charts;

c) Two Cartographic Geodesists responsible for data calculation and drawing up charts.

Section 5

INSPECTION AND FIELD ACCEPTANCE

Article 20. Organization of on-site inspection and acceptance

1. Time for on-site inspection and acceptance: Based on the annual plan, volume, progress of construction steps, and actual construction requirements to organize on-site inspection and acceptance.

2. The construction unit must prepare a written report on the volume and quality of completed works compared to the project and submit it to the direct management agency before organizing the inspection and acceptance.

3. The inspection team shall be established by a Decision issued by the competent authority head, responsible for conducting inspections according to the contents stipulated in Articles 22 and 23 of this Circular, and recording the results of on-site inspection and acceptance in accordance with current regulations.

Article 21. Responsibilities and contents of construction unit's inspection work

1. The construction unit is responsible for self-inspecting 90-95% of the project's inspection volume regularly.

2. Contents to be inspected:

a) Position and coordinates of support points and common points already constructed compared to the project design;

b) Quality of support edge measurements, closure error of closed support polygons, quality of the entire support network;

c) Quality and volume of common point measurements, check measurements;

d) Volume and quality of geodetic measurement items, quality of determining coordinates and elevations of gravity points;

đ) Points of abnormality and doubts requiring inspection;

e) Abnormal gravity points;

g) Accuracy of describing points and positions of gravity measurement points from the field onto the construction topographic map.

Article 22. Tasks of the Inspection Team

1. Inspect compliance with the provisions of this Circular and technical requirements of the project.

2. Inspect the quality of surveying materials and the volume of technical works already implemented.

3. Preliminarily evaluate the geological effectiveness and economic efficiency of technical works; propose solutions for changes, adjustments, and supplements (if necessary).

Article 23. Requirements for on-site inspection work

1. Requirements for the support network:

a) Check all support edges by independent closed measurements;

b) All polygons must be measured closed and meet the closure errors specified in Article 26 of this Circular;

c) For hanging and chain-linked support edges, the network closure errors after balancing must comply with the standards set out in Article 26 of this Circular.

2. Requirements for the common point network:

a) The volume of check measurements should be 10 to 15% of the total number of points specified in the project;

b) Each measurement trip must have at least one independent check point; if no check measurement is arranged, then two to three points must be remeasured. Check points from remeasurement trips cannot be used to calculate errors;

c) Check points must be distributed relatively evenly across the area (or along the route) and across measurement trips;

d) At each gravity anomaly, at least two check points must be measured;

đ) All abnormal points must be checked for both gravity and geodetic measurements;

e) When the first check measurement result exceeds the project's allowable error, a second check measurement must be conducted to determine which trip was incorrect and to remeasure it.

3. For geodetic materials:

a) Check and assess coordinate, elevation, and polygon closure errors of gravity support networks; coordinates and elevations of common gravity points;

b) Check and assess the density of actual measurement points compared to the project.

Chapter IV

CONSOLIDATION DEPARTMENT

Section 1

DATA CORRECTION

Article 24. Office Summary Section

The Office Summary Section consists of:

1. Project Director: Senior Investigator for Natural Resources - Environment or higher;

2. Three investigators for Natural Resources - Environment;

3. One map surveyor;

4. College-level investigator for Natural Resources - Environment;

5. Intermediate-level investigators for Natural Resources - Environment.

Article 25. Tasks of the Office Summary Section

1. Verify 100% of the calculated volumes of the field office. If the verification error exceeds the specified limit, an additional 20% of the point volume must be verified to find the cause and corrective measures.

2. Calculate gravitational corrections, abnormal gravity values, establish support point lists, and normal gravity points.

3. When measuring gravity at a scale of 1:10,000 and larger, if each measurement trip takes more than 2 hours, lunar and solar attraction must be corrected according to specialized charts established annually.

4. Establish Bouguer and Fai abnormal gravity maps or abnormal gravity chart maps as required by the project.

5. Process, analyze, and interpret geological data to address project objectives and tasks; prepare summary reports, print, and submit for storage.

Section 2

QUALITY ASSESSMENT OF DOCUMENTS

Article 26. Steps for assessing the quality of the gravity support point network

1. Preliminarily assess the accuracy of determining the difference Dg of a support edge (eT) using the formula: (1) Where: dDgi - The difference between the ith Dg value and the average Dg value;

Chapter m. Number of Dg values of the support edge.

2. If the support point system forms closed polygons, preliminary quality assessment is based on the closure error of each polygon using the formula: (2) Where: Wcp - Allowable closure error for each polygon; eT - Average square root of the determined support measurement error; K - Number of edges of a polygon.

3. Assess the overall quality of the support network system based on the formula: (3) Where:

Chapter di. Difference in Dgi before and after balancing; . S - Total number of edges in the polygon system;
r - Total number of support points, excluding the origin point.

4. Assess the accuracy of the support network based on the results of the support network balancing and the measurement error of some support edges during verification trips.

Article 27. Quality Assessment of Normal Point Measurement

1. Assess the quality of normal point measurement based on the calculation of the measurement verification error. The average square error of a single measurement is calculated using the formula: (4) Where:

Chapter di. Deviation of the ith measurement result from the average value;

Chapter m. Total number of measurements at verification points; . n - Total number of verification points.

2. In cases of double measurement (two measurements) or simultaneous measurement with two instruments, use the following formula: (5) Where: gi- Difference between the two measurements at the ith point; n - Total number of verification points.

Section 3

CALCULATING TOPOGRAPHIC CORRECTION FACTORS

Article 28. Calculation of topographic correction factors for gravity measurement points

1. When the topographic influence value Dgđh is greater than 0.7ed, topographic correction must be calculated (ed- is the measurement error at normal points).

2. The selection of calculation methods, palette radius, and map scale must ensure that the average square error of the topographic correction calculation does not exceed 0.7ed.

3. The quality of the topographic correction calculation is assessed by the independent verification error of the topographic correction. The number of verification points should account for 5 to 15% of the total topographic correction volume and be evenly distributed across the area. The error calculation formula uses either formula (4) or (5).

4. Types of topographic maps and geodetic data used for topographic correction calculations depend on the accuracy of the abnormal gravity map established and are specifically defined in the project.

5. When measuring gravity at a scale of 1:25,000 and larger, if there is a high-quality digital topographic map, computer-based topographic correction programs may be used.

6. Coastal topographic correction. When measuring gravity at a scale of 1:10,000 and larger along coastal areas, lakes, and ponds where measurement trips take more than 2 hours, coastal topographic influence correction must be calculated. Abnormal gravity values are calculated with intermediate layer density excluding water density.

Section 4

ESTABLISHING BOUGUER AND FAI ABNORMAL GRAVITY MAPS

Article 29. Calculation formula for the anomalous gravity of Bughe and Fai

1. The anomalous force of Bughe is calculated according to either formula (6) or formula (7): DgB = gd - g0 + (0,3086 - 0,0419

s) H + Dgs (6) DgB = Dgd - Dg0 + (0,3086 - 0,0419

s) H + Dgs (7)

2. The anomalous gravity of Fai is calculated according to either formula (8) or formula (9): DgF = gd - g0 + 0,3086H (8) or DgF = Dgd - Dg0 + 0,3086H (9) Formula (6) and (8) are used for scales of 1:25,000 and smaller with: gd(mGl) being the measured gravity value at the reference point; g0(mGl) being the normal gravity field value; H (

m) being the height value of the measurement point relative to sea level; s (g/cm3) being the average density value of the intermediate layer; Dgs being the total terrain correction and other corrections.

Formula (7) and (9) are used for scales larger than 1:10,000 with: Dgd being the gravity increment at the reference point compared to a specified origin point in the region; Dg0 being the normal gravity increment at the reference point compared to the specified origin point.

Article 30. The value of the normal gravity field γ0 is calculated according to the Helmert formula and converted to the new Potsdam system

g0 = 978.016 (1 + 0,005302 sin²j - 0,000007 sin² 2

j) (10) j is the latitude of the measurement point. When measuring gravity at scales larger than 1:10,000 on a small area, the normal gravity increment can be calculated using the following formulas: Dg0 = 1,51 sin²j .Dj (11) or Dg0 = 0,82 sin²j .Dd (12) With j being the latitude of the measurement point. Dj is the latitude increment of the measurement point compared to the regional origin point expressed in minutes; Dd is the distance increment along the latitude from the measurement point to the origin point expressed in kilometers; g0, Dg0 are expressed in milligals. Depending on the research purpose, other formulas for calculating the normal gravity field as prescribed in Appendix 7 accompanying this Circular may be used.

Article 31. Evaluation of the quality of gravity maps

1. All gravity maps are evaluated based on the mean square error in determining the anomalous gravity: (13) eT - the mean square error of the control network as stipulated in Article 26 of this Circular. For maps at scales of 1:200,000 and smaller, eT must also include the national control network error. ed - the mean square error at the reference point as stipulated in Article 27 of this Circular; eH - the error caused by measuring height; exy - the error caused by determining coordinates; eđh - the error caused by calculating terrain corrections. The value of ea must meet the requirements approved in the project.

2. When a gravity map includes supplementary measurements and denser reference point networks, to evaluate the measurement quality and reliability of the anomalous gravity, the interpolation error must be calculated using the following formula: ( 14) Dgdi - the value at the supplementary measurement point i used for detailed study of anomalies; Dgnsi - the value at the measurement point i obtained by interpolation from the contour map when these points are not used for drawing; n - the number of supplementary and denser points used to assess the error. A map is considered to have good quality when: (15) It meets the requirement when: Poor quality when: E ≥ D. D is the contour interval of the map.

Article 32. Establishment of contour maps, graphic maps, and anomaly gravity graphs

1. The main content of the anomalous gravity contour map includes:

a) Position, name of the point, anomalous gravity value;

b) Contour lines and values of the anomalous gravity contour lines. Anomalous gravity contour lines are drawn by linear interpolation between the anomalous values using specialized software (Surfer, VerticalMapper, ER Mapper) or by hand.

2. The force of the contour lines is defined as follows:

a) Contour lines with values equal to 0 and subsequent numbers that are multiples of 5 (the 5th, 10th, 15th, 20th... lines), the force is 0.25mm;

b) Other contour lines have a force of 0.15mm;

c) The values of the anomalous gravity on the contour lines are marked with corresponding forces and directed towards the center of the anomaly;

d) In areas where the density of measurement points is sparse and the actual distance between measurement points exceeds three times the designed distance between points, the anomalous gravity contour lines will be represented as broken lines;

đ) Points with sudden changes in anomalous gravity values are not used when drawing contour lines and are marked with special symbols.

3. The anomalous gravity map is established on a simplified topographic map of the same scale.

4. When establishing contour maps, graphic diagrams of anomalous gravity, the charts are drawn using specialized software (Surfer, Grapher...) or by hand. The force of the chart, vertical scale, and symbols on the chart are selected to clearly reflect the anomalies. The horizontal scale of the chart is the measurement scale. If the anomalous gravity contour map is colored, there must be a color legend.

Section 5

GEOLOGICAL INTERPRETATION OF GRAVITY DATA

Article 33. Requirements for interpreting geological data from gravity survey materials

Based on the objectives and tasks of the gravity survey project, appropriate processing and analysis methods shall be selected. The main requirements for interpreting geological data from gravity survey materials are as follows:

1. Classify objects causing gravity anomalies according to their different densities.

2. Determine the spatial location (in terms of area and depth), distribution scale, size, shape, depth, orientation, and dip angle of objects causing anomalies.

3. Interpret geological data from gravity survey materials according to the project.

Article 34. Establishing Result Maps of Geological Interpretation

1. The base map (diagram) of gravity anomalies, Fai anomalies, and results of geological interpretation must be established on a simplified topographic map at the same scale, ensuring that physical-geological objects can be related to basic topographical and geographical elements.

2. When establishing geological-physical cross-sections, they must be built on topographic cross-sections at the same scale. Results of geological interpretation of gravity survey materials combined with other physical materials must be fully and detailedly presented up to the reliable research depth.

3. Result drawings must comply with current regulations on map and geological diagram preparation.

Chapter 6

PREPARATION OF THE FINAL REPORT

Article 35. Final Report

1. The final report includes the explanatory note, main documents, and accompanying appendices.

2. The format and structure of the explanatory note must follow current regulations on geological report preparation and depend on the nature, scale, objectives, and tasks of the project.

3. Main documents of the report include:

a) Construction site maps or diagrams;

b) Maps (diagrams, charts) of Bouguer and Fai gravity anomalies;

c) Geological interpretation maps (diagrams) based on gravity survey materials;

d) Geological-physical cross-sections;

e) Diagrams of gravity network leveling balance;

f) Registers of gravity measurement points.

4. Appendices accompanying the report:

a) Legal texts; Decisions approving, reviewing the project; inspection and acceptance documents for each stage;

b) Intermediate processing and analysis documents; quantitative calculation results, modeling.

5. The contents of the main chapters of the explanatory note shall be implemented according to Model No. 2 of Appendix 2 issued together with this Circular.

6. The final report must be accepted, approved by competent authorities, and submitted to the Geological Archive in accordance with current regulations.

Chapter V

IMPLEMENTING PROVISIONS

Article 36. Effective Date

1. This Circular takes effect from March 15, 2011. The technical regulation on gravity exploration methods issued by Decision No. 661/QĐ/ĐCKS-KHTC dated December 23, 2004, of the Director of the Vietnam Geological Survey and Mineral Resources Management Department is hereby repealed.

2. The Director of the Vietnam Geological Survey and Mineral Resources Management Department, subordinate units under the Ministry of Natural Resources and Environment, organizations, and individuals implementing projects using gravity exploration methods are responsible for enforcing this Circular.