Circular No. 56/2013/TT-BTNMT on technical magnetic sea measurements from ships

CIRCULAR

Technical Regulations for Marine Magnetic Surveying by Ship

MINISTER OF NATURAL RESOURCES AND ENVIRONMENT 

Pursuant to the Decree No. 25/2009/NĐ-CP dated March 6, 2009 of the Government on comprehensive management of marine resources and environmental protection of islands;

Pursuant to Decree No. 21/2013/NĐ-CP dated March 4, 2013, promulgated by the Government, stipulating the functions, tasks, powers, and organizational structure of the Ministry of Natural Resources and Environment;

At the proposal of the Director General of the Vietnam Administration of Seas and Islands, the Head of the Science and Technology Department, and the Head of the Legal Affairs Department;

The Minister of Natural Resources and Environment promulgates this Circular on technical regulations for marine magnetic surveying by ship.

PART I

GENERAL PROVISIONS

Article 1. Scope of Regulation

This Circular stipulates the contents, procedures, and technical requirements for marine magnetic surveying by ship to establish the T magnetic field map and the ∆Ta anomaly map.

Article 2. Applicability

This Circular applies to state management agencies, organizations, and individuals related to marine magnetic surveying by ship.

Article 3. Explanation of Terms

In this Regulation, the following terms shall be understood as follows:

1. Marine magnetic surveying by ship is a method of measuring the absolute value T of the magnetic field using marine magnetic equipment with a towed receiver following the ship.

2. Deviaxia measurement is the measurement of the magnetic field value at one point along different ship directions to determine the ship direction effect for linking magnetic field measurement data in the research area.

3. Deviaxia correction is the process of eliminating the influence of the ship direction effect on the magnetic field values on different measurement lines in the research area.

4. Magnetic variation is the change in the geomagnetic field over time, including centennial variation, diurnal variation, and geomagnetic storms.

5. Centennial variation is the change in the annual average value of the geomagnetic field over time.

6. Diurnal variation is the change in the geomagnetic field within a day-night cycle.

7. Geomagnetic storm is a sudden strong change in the geomagnetic field caused by charged particle streams emitted from solar eruptions (solar wind).

8. Magnetic variation correction is the process of eliminating temporal variations in measured geomagnetic field data.

9. Total magnetic field or full magnetic field (T) is the magnetic field measured at the observation point, including the normal geomagnetic field at the observation time and the magnetic field caused by the environment below and around the observation point.

10. Normal geomagnetic field (To) is considered as the field of a dipole with one pole near the geographic North Pole and the other near the geographic South Pole. The dipole axis forms an angle of approximately 11.3° with the Earth's rotational axis. The North Magnetic Pole has coordinates of 70°N, 96°E; the South Magnetic Pole has coordinates of 73°S, 156°E.

11. Magnetic anomaly ∆Ta is the component of the total magnetic field remaining after subtracting the normal geomagnetic field To.

12. Magnetic anomaly ∆T is the remaining part of the total magnetic field after subtracting a selected total magnetic field value but not the normal geomagnetic field To.

13. Tesla (T) is the unit of measurement for magnetic field strength in the International System of Units (SI).

14. IGRF is the simulated geomagnetic field of the Earth based on a standard mathematical model.

15. Internal linking is the linking of magnetic measurement data on lines of a survey area to a unified magnetic level.

16. External linking is the linking of magnetic measurement data from survey areas measured at different times to the same magnetic level at a unified time.

Article 4. General Requirements for Marine Magnetic Surveying by Ship

1. Requirements for the Survey Line Network

a) The survey line network must be appropriate to the scale of the magnetic field map to be established and the geological structure characteristics of the research area, in two forms: square grid and rectangular grid as specified in Appendix No. 5 attached to this Circular;

b) In the survey area, all types of survey lines, including regular lines, tie lines, linking lines, and check lines, must be fully arranged;

c) Marine magnetic survey lines must be navigated using satellite positioning technology (GPS system or others), with the deviation between the designed line and the actual surveyed line not exceeding 2mm according to the map scale.

2. The arrangement of tie lines and linking lines must meet the following requirements:

a) The location of tie lines and linking lines must have stable magnetic fields with little gradient change;

b) Tie lines must be arranged in a control polygon network covering the entire survey area. The number of tie lines must ensure that each vertical section of the survey area intersects at least one horizontal line. The distance between tie lines should be 10-15km, and in complex survey areas, a special tie line network with distances less than 10km may be applied;

c) Linking lines must intersect tie lines perpendicularly. If the number of intersection points is insufficient for secure linking, regularly measured lines can be used as linking lines;

d) The distance between linking lines should not exceed 100km; when conducting high-precision measurements, this distance should be less than 30km;

e) The length of each tie line and linking line during high-precision measurements should not exceed 200km, and during medium-precision measurements, it should not exceed 300km.

3. Requirements for Equipment and Means

a) High-precision continuous magnetic field T measuring instruments (accuracy of 0.1nT) unaffected by measurement direction, capable of stable operation in a temperature range from -45°C to 60°C, with synchronous GPS connection capability, numerical data output, and data storage on computer hard drives must be used;

b) Magnetic measuring instruments must be calibrated, tested, and evaluated for technical parameters before being put into use;

d) Ships used for marine magnetic surveying must have suitable structures, power, and load capacity to ensure safety during measurement and movement at sea.

3. Marine magnetic surveys can only be conducted under wind conditions below Beaufort force 5.

4. Technical personnel involved in marine magnetic surveying must have relevant expertise and training in occupational safety.

Chapter II

CONSTRUCTION OUTSIDE THE REAL ESTATE

Article 5. Office work before fieldwork and preparation for on-site construction

1. Office work before fieldwork includes:

a) Collecting information about geology and geophysics that has been carried out related to the survey area; the anchoring position of the ship, the location of the magnetic variation station, and the location of the field office;

b) Based on the approved marine magnetometer survey requirements and the information collected as stipulated in point a Clause 1 of this Article, the entity conducting the survey must develop a construction outline with the main contents specified in Appendix No. 1 attached to this Circular;

c) The construction outline for marine magnetometer surveys must be approved by the head of the project's leading unit.

2. Preparation work for on-site construction includes:

a) Inspecting and maintaining machines and equipment;

b) Calibrating marine magnetometer measuring devices and magnetic variation measuring devices at specialized calibration laboratories. In cases where marine magnetometer measuring devices are damaged during construction, they must be recalibrated before being put back into use. For marine magnetometer measuring devices that cannot be calibrated in domestic specialized calibration laboratories, the construction entity must monitor and check technical parameters to ensure compliance with the manufacturer's specifications;

c) Preparing specialized materials and accessories.

Article 6. Installation of machines and equipment on marine magnetometer survey ships

1. Installation of machines and equipment on ships

a) The installation room for machines and equipment must be equipped with air conditioning, have an appropriate size to ensure sufficient space for installing equipment and working for machine operators. The installation room for machines and equipment must be located far from the engine room and in a place with minimal vibration impact when waves occur;

b) Power cables, signal cable winches, and receivers must be arranged on the aft deck of the ship in an area ensuring convenient and safe operation;

c) Machines and equipment specified in points a and b of this Clause must be installed on bases and securely fastened to prevent movement during the ship's stationary or moving phases on the sea; power cables and signal cables must be laid separately; generators must be placed in well-ventilated areas and easily accessible for continuous monitoring during operation;

d) The magnetometer must be connected to the GPS device.

2. Checking the stability of machines and equipment after installation

a) Running the generator in no-load and load conditions;

b) Testing the GPS navigation positioning device when connected to the magnetometer in static mode under conditions where the ship's engine is not operating;

c) Testing the magnetometer in static mode under conditions where the ship's engine is not operating by moving the receiver more than 100 meters away from the ship, selecting an appropriate measurement range and cycle; conducting a test run for a minimum of 30 minutes. Test data is considered original documentation.

Article 7. Moving the ship to the marine magnetometer survey area

1. The magnetometer survey ship may only move to the survey area after the magnetometer machines and equipment have been installed and checked according to the provisions of Article 6 of this Circular.

2. Before the magnetometer survey ship moves, it must check the removal of connecting wires from survey equipment to shore power sources; the safety of winches, signal cable pullers, power cables for equipment, and the magnetometer system.

3. Survey measurements can only be conducted after checking and ensuring that all magnetometer machines and equipment on the ship and magnetic variation stations, GPS positioning stations operate normally.

Article 8. Selection of Cable Length for Receiver Deployment

1. The selection of cable length for receiver deployment must be conducted in areas without magnetic anomalies or with minimal magnetic field changes.

2. The cable length selection shall be carried out by gradually increasing the cable length until the measurement data remains unchanged, measuring sequentially in the directions of 0 and 180 degrees corresponding to each cable length deployed.^° ° tương ứng với mỗi chiều dài cáp thả.

3. Based on the measurement results as stipulated in Clause 2 of this Article, select a cable length sufficient to ensure that the influence of the ship on the measurement data is less than one-third of the allowable error for creating magnetic field maps.

Article 9. Measurement of Deviation (Dviaxia)

1. Deviation measurement (Dviaxia) must be conducted before line measurements in each survey season. If the magnetometer needs to be replaced or repaired during measurement, the Dviaxia measurement must be repeated.

2. The Dviaxia measurement time must be as short as possible and should be carried out at the time of the day when magnetic variation is minimal.

3. The location for Dviaxia measurement must be arranged within the survey area, where the seabed topography is relatively flat and the magnetic field gradient is small.

4. The Dviaxia measurement must be performed twice (outbound and return) at a position with a defined coordinate according to the directions of 0 and 180 degrees.^°, 45^° , 90^°, 135^°, 180^°, 225^°, 270^{b) ADDING CLAUSES 10 AND 11 TO ARTICLE 2 AS FOLLOWS:}, 315^°.

5. The Dviaxia measurement data must be corrected for magnetic variation, plotted on a Dviaxia curve to determine a consistent correction value when establishing magnetic field maps.

Article 10. Measurement of Magnetic Variation

1. Magnetic variation measurement must be conducted simultaneously with the marine magnetic measurement process.

2. Magnetic variation measurement stations must be located in areas without magnetic anomalies and must meet the following requirements:

a) The distance between magnetic variation measurement stations must be less than 100km along the meridian direction;

b) The distance from the magnetic variation station to the magnetic measurement area must be less than 200km along the parallel direction;

c) Must be far from traffic routes, power transmission lines, and sources of electromagnetic interference to avoid affecting the measurement results;

d) Within a radius of 2.5m, there must be no more than a 5nT difference in the magnetic field.

3. In each survey season, there must be a minimum of three days of continuous 24-hour monitoring to determine the variation pattern of the area.

4. For automatic recording magnetic variation meters, the measurement cycle is 1-2 minutes. For non-automatic recording magnetic variation meters, the measurement cycle is 5 minutes, and each measurement must record two values and the measurement time in the logbook.

Article 11. Regular Line Magnetic Measurements

1. Regular line magnetic measurements must be conducted using continuous measurement methods, with the measurement cycle depending on the type of magnetometer used in the survey. Marine magnetic measurements should not be conducted during geomagnetic storm periods.

2. Extension of the magnetic measurement line shall be carried out in the following cases:

a) When magnetic anomalies are detected, follow-up measurements must be made until the entire anomalous area is covered;

b) When conditions permit the survey vessel to approach closer to shore than originally designed.

3. When using two or more magnetometers to measure the magnetic field in the same survey area during the same season, it is necessary to evaluate the synchronization of the instruments using one of the following methods:

a) Simultaneous measurement of all instruments on a section of the line;

b) Re-measurement of a section previously measured by another instrument.

4. Magnetometer operators must record measurement logs according to the template specified in Appendix 6 attached to this Circular.

5. During the measurement process, regular safety surveillance and monitoring of equipment towed behind the vessel must be organized. Any incidents must be promptly reported to responsible personnel for handling.

6. Marine magnetic measurement data must be recorded on CDs or external hard drives to be transferred to the field office processing unit after each measurement trip.

Article 12. Measurement along reference lines, connecting lines, and check lines

1. Measurement along reference lines and connecting lines

a) Magnetic field measurements along reference lines and connecting lines must be conducted within the shortest time with the smallest magnetic field variations during the day;

b) Reference lines must be measured using the same type of equipment and under the same measurement conditions as for regular lines, with the measurement error on reference lines being smaller than the allowable error for the magnetic map to be established;

c) Magnetic field measurements along reference lines and connecting lines must be carried out continuously twice, in both directions, with the magnetic field value taken as the average of the two measurements;

d) The measurement along the network of reference lines can be conducted before or after completing the measurement of all regular lines.

2. Measurement along check lines shall only be performed after completing the magnetic field measurements on all regular lines according to the designed network. Check lines must be arranged to intersect all regular lines. The number of check lines is determined based on the area and shape of the survey region, ensuring that each regular line has at least one intersection point with a check line.

3. The accuracy of magnetic field measurements is determined by the mean square error m1 of the magnetic field values measured at the intersection points between regular lines and check lines.

where:

m1: magnetic field measurement error;

di: difference value at the i-th point between two measurements;

n: number of intersection points between check lines and regular lines

4. The accuracy of magnetic field measurements is classified as follows:

a) Low accuracy when m1 > 15nT;

b) Medium accuracy when m1 = 5 ÷ 15nT;

c) High accuracy when m1 < 5nT.

Article 13. Content of office fieldwork tasks

1. Receiving magnetic field data from marine surveys and magnetic variation data.

2. Checking the quality of materials for each survey trip, including the synchronization between magnetic field measurement data and GPS data, the accuracy of the actual measurement line positions compared to the design, and the quality of collected magnetic field materials. In cases where the material quality does not meet requirements, re-measurement must be requested.

3. Constructing curves of magnetic variation.

4. Preliminary calibration of deviaxia and daily magnetic variation.

5. Establishing a preliminary magnetic field map T with the results of initial calibration.

6. Conducting preliminary analysis to identify magnetic anomalies, delineating areas or sections of lines with changes in the magnetic field that may be related to geological objects to provide timely information to construction departments and other project specialties for joint research.

Article 14. Removal of Equipment

1. At the end of the survey season, all equipment must be dismantled immediately after the vessel docks and moors at the port.

2. Inventory, clean, package, load, and transport the equipment to shore and deliver it to the unit's headquarters.

Chapter III

FIELD REPORT OFFICE

Section 1

POST-FIELD OFFICE

Article 15. Tasks of the post-field office

1. Completing and compiling original materials collected, including marine magnetic field data, magnetic variation data, magnetic field measurement logs, checking and completing the synchronization between magnetic field measurement data and GPS data.

2. Calculating correction values for deviaxia, daily magnetic variation, and instrument synchronization (if applicable).

3. Connecting within the survey area to calculate the magnetic field value T and anomaly ΔTa corresponding to the timing of the survey season.

4. Establishing the magnetic field map T and magnetic anomaly map ΔTa for the survey season.

5. Preparing a report on the results of field construction work.

Article 16. Adjustment value for daily variation correction, Deviaxia correction

1. The adjustment value for daily variation is determined according to the formula:

where:

dTbt: daily variation adjustment value;

Tđbt: magnetic field value measured at the geomagnetic station at the same time on the sea;

Ttbn: average magnetic field value during the measurement period of the field season at the geomagnetic station.

2. In cases where measurements are taken in distant sea areas where daily variations cannot be measured, data from the nearest geomagnetic observatory shall be used to correct daily variations. The daily variation adjustment value is calculated based on the data from the geomagnetic observatory using the following formula:

where:

dTbt: adjustment value to correct the magnetic field at a specific time;

Tbt geomagnetic observatory: value measured at the geomagnetic observatory at the same time as the measurement on the sea;

Ttbn geomagnetic observatory: annual average value obtained from the observatory's data. If there is no Ttbn value from the observatory for correction, the previous year's Ttbn value may be used but must adjust for the century variation according to the year of measurement.

3. The Deviaxia adjustment value is determined for each measurement direction along the survey line and is equal to the algebraic sum of the magnetic field values at that direction on the Deviaxia curve.

Article 17. Determining the total magnetic field value T

1. The total magnetic field value T is calculated according to the formula:

where:

T: magnetic field value T after correcting daily variation and Deviaxia;

Tđo: magnetic field value measured on the survey line;

dTbt: daily variation adjustment value;

dTđe: Deviaxia correction value.

2. In cases where the results of repeated measurements between different marine magnetometers used have a difference greater than 1/3 of the allowable error of the magnetic field map, synchronization calibration of the instruments must be performed. At that time, the total magnetic field value T is calculated according to the formula:

where:

T: magnetic field value T after correcting daily variation, Deviaxia, and instrument synchronization;

Tđo: magnetic field value measured on the survey line;

dTbt: daily variation correction value; dTđe: Deviaxia correction value;

dTđbm: instrument synchronization correction value.

Article 18. Internal connection and calculation of the total magnetic field value T of the survey area

1. Internal connection is carried out by using a reference network or using several vertical and horizontal lines forming a closed polygonal network on the survey line network.

2. The content of the methods for balancing the reference network and calculating the total magnetic field value T of the survey area is specified in Appendix 4 attached to this Regulation.

Article 19. Calculation of the magnetic anomaly value ΔTa

1. The magnetic anomaly value ΔTa is calculated according to the formula:

ΔTa = T - To (III.5)

where:

T: total magnetic field strength value after internal connection;

To: normal magnetic field value IGRF corresponding to the construction season time.

2. In cases where magnetic measurements are taken at a scale larger than 1:50,000, with a small survey area and little change in the normal magnetic field To within the survey area, a single adjusted T value point within the area can be selected as the origin and the anomaly ΔT = T - Torigin can be calculated, or the first-order trend of the total magnetic field T can be calculated and then ΔT = T - trendT can be calculated.

Article 20. Evaluation of magnetic map errors

where:

e: map error;

di: difference value at the ith point between two measurements of the check line and the regular line after correction and internal connection;

n: number of intersection points participating in the error calculation with n ≥ 20. In cases where the survey area is small and there are fewer than 20 intersection points between the regular line and the check line, additional intersection points between vertical and horizontal lines can be included in the error calculation.

Article 21. Preparation of Magnetic Field Maps for Survey Areas

1. The magnetic field map of the survey area includes:

a) The T magnetic field map;

b) The ∆Ta magnetic anomaly map.

2. The T magnetic field map and the ∆Ta magnetic anomaly map must be established on the basis of the simplified seabed topography map or the simplified seabed depth map, using the VN-2000 coordinate system.

3. The T magnetic field map must be established in the form of an isogonic map. Isogonic lines are drawn in solid black, with the magnetic field value clearly marked. The isogonic section is taken at two to three times the map's error.

4. The ∆Ta magnetic anomaly map must be established in the form of a colored isogonic map. The isogonic section is taken at two to three times the map's error. The colored isogonic map is represented in one of the following forms:

a) Solid black isogonic lines, with a pink or red background indicating positive magnetic anomalies with increasing intensity according to the strength of the field; a blue background indicating negative magnetic anomalies, with increasing intensity according to the negative value;

b) Solid isogonic lines with changing colors indicating changes in the magnetic field. The principle for selecting the color of the isogonic lines follows the selection of the background color as specified in point a of this Clause.

5. In cases where the magnetic survey project requires the establishment of a ∆Ta magnetic anomaly graph map, it shall be carried out as follows:

a) The ∆Ta magnetic anomaly graph map must display the magnetic field graphs of each line in their correct positions on the map. Positive magnetic fields are represented by red, and negative magnetic fields by blue;

b) The horizontal scale of the ∆Ta magnetic anomaly graph map is taken from the horizontal scale of the T magnetic field map and the ∆Ta magnetic anomaly map;

c) The vertical scale is chosen so that 1mm on the map corresponds to two to three times the mean square error in determining the ∆Ta magnetic anomaly. In areas with strong magnetic anomaly intensity, the vertical scale may be increased and the area must be framed and marked.

Article 22. Preparation of Construction Result Reports

After completing the survey season, a construction result report must be prepared. The main contents of the construction result report are stipulated in Appendix 2 attached to these Regulations.

Section 2

OFFICE FOR PREPARATION OF THE FINAL REPORT

Article 23. Tasks of the Office for Preparation of the Final Report

1. To compile and systematize all original materials implemented throughout the entire project.

2. To adjust and link the magnetic measurement data conducted over different seasons to a unified time point.

3. To establish the T magnetic field maps and the ∆Ta magnetic anomaly maps for the entire survey area of the project.

4. To process, analyze, and interpret marine geological magnetic data.

5. To write the final report.

Article 24. Adjustment for Century Variations

1. Adjust century variations to calculate the T magnetic field and ∆Ta anomalies of the survey years converted to the year of the magnetic field map creation, using century variation data from IGRF.

2. The T magnetic field and ∆Ta anomaly values for the year of map creation are calculated using the following formulas:

T_{(year of map creation)} = T_{(survey year)} - dT_km/h             (III.7)

∆Ta_{(year of map creation)} = ∆T a_{(survey year)} - dT_tk      (III.8)

where:

T_{(year of map creation)}:value of the T magnetic field in the year of map creation;

∆T a_{(year of map creation)}:value of the ∆Ta magnetic anomaly in the year of map creation;

T_{(survey year)}:value of the T magnetic field in the survey year;

∆T a_{(survey year):} value of the ∆Ta magnetic anomaly in the survey year;

dT_km/h:quantity of century variation required to adjust the measured T magnetic field value in the survey year to the year of map creation.

Article 25. Full Project External Linkage

1. Full project external linkage shall only be conducted after century variation adjustments have been made, as stipulated in Article 18 of this Circular.

2. In cases where survey lines for external linkage pass through all survey areas and adjacent linkage zones, two to three straight lines may be used for external linkage.

3. Where there are no survey lines for linkage, overlapping sections of regular survey lines or support lines must be used for external linkage.

Article 26. Map Creation

1. The content and methods for creating T magnetic field maps and ΔTa anomaly maps shall be carried out according to the provisions set forth in Article 21 of this Circular.

2. The map creation error shall be calculated using formula (III.6).

Article 27. Processing, Analysis, and Geological Interpretation of Magnetic Survey Data

1. All magnetic survey data must be processed, analyzed, and geologically interpreted with the aim of:

a) Clarifying the characteristics of the magnetic field and magnetic anomalies;

b) Determining geological structures, faults, magmatic bodies, and related mineral deposits.

2. The results of processing, analyzing, and interpreting magnetic survey data shall be presented in the form of geological structure maps or diagrams based on magnetic data. Such geological structure maps or diagrams based on magnetic data must include the following main contents:

a) Fault systems and key fault elements (depth of development, dip direction, slip direction);

b) Structural units based on magnetic data: belts, structural blocks, sub-blocks, uplift and subsidence zones of the basement;

c) Magmatic formations.

Article 28. Final Report

1. The final report on marine magnetic surveys includes explanatory notes and accompanying drawings. The final report must fully reflect the implemented contents and achieved results as specified in Appendix 3 attached to this Circular.

2. The final report on magnetic surveys must be digitized, classified by information layers, displayed, and managed using appropriate GIS tools compatible with the project's basic resource investigation maps.

Article 29. Original Documentation of Marine Magnetic Surveys

Original documentation of marine magnetic surveys includes:

1. Disks containing magnetic field measurement results.

2. Disks containing coordinates of survey lines.

3. Documentation of magnetic variation measurements.

4. Survey line maps.

5. Calibration and testing documentation for equipment.

6. Diagrams of actual survey line documentation.

7. Physical survey notebooks and other field record books.

Article 30. Products of Marine Magnetic Surveys

1. Final reports.

2. Original documentation.

Chapter IV

IMPLEMENTING PROVISIONS

Article 31. Effective Date

This Circular takes effect from March 1, 2014.

Article 32. Implementation Organization

1. The Vietnam National Marine Administration is responsible for inspecting and annually reporting to the Minister on the implementation of this Circular.

2. During the implementation of this Circular, if difficulties or issues arise, relevant agencies, organizations, and individuals must promptly report them to the Ministry of Natural Resources and Environment for study and supplementation as necessary.