Circular No. 04/2011/TT-BTNMT on technical procedures for seismic surveys in basic geological investigations for minerals and engineering geology

CIRCULAR

Technical Regulations on Seismic Survey in Basic Geological Investigation Related to Mineral Resources and Engineering GeologyRegarding Mineral Resources and Engineering Geology

________________

MINISTER OF NATURAL RESOURCES AND ENVIRONMENT

Pursuant to the Law on Minerals and the Law Amending and Supplementing Certain Provisions of the Law on Minerals;
Pursuant to Decree No. 25/2008/NĐ-CP dated March 4, 2008 of the Government stipulating the functions, tasks, powers, and organizational structure of the Ministry of Natural Resources and Environment;
Pursuant to Decree No. 89/2010/NĐ-CP dated September 18, 2010 of the Government amending and supplementing Article 3 of Decree No. 25/2008/NĐ-CP dated March 4, 2008 of the Government 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 Department 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

These Circulars prescribe the procedures, contents, and requirements for seismic survey work in basic geological investigation related to mineral resources and engineering geology.

Article 2. Applicability

These Circulars apply to units, organizations, and individuals participating in implementing tasks, projects, and programs of basic geological investigation related to mineral resources, mineral exploration, engineering geological surveys, hydrogeological surveys, geological disaster investigations, and other related fields (referred to as specialized projects) conducting seismic survey work.

Article 3. Explanation of Terms

1. Seismic survey method is the process of receiving elastic waves propagating in rock and water media from seismic wave sources to interpret characteristic parameters of seismic wave propagation.

2. The source of seismic waves can be a hand hammer, machine hammer, or explosives generating elastic waves propagating in rock, water, and air media.

3. Seismic receiver is equipment that converts ground vibrations caused by mechanical wave propagation into electrical signals transmitted to the recording station.

4. Seismic wave recording tape is the product of seismic survey work.

Article 4. Scope of Application

1. Basic geological investigation related to mineral resources, mineral exploration:

a) Mapping engineering geological maps at a scale of 1:50,000;

b) Evaluating mineral resource potential;

c) Determining fault zones, fractures, and karst caves.

2. Engineering geological surveys, hydrogeological surveys, geological disaster investigations, and other related fields:

a) Foundation surveys for construction projects;

b) Determining the depth of underground water levels;

c) Determining the elastic parameters of rocks in their natural state;

d) Determining the empirical correlation between the speed of elastic wave propagation and elastic modulus Ed, Poisson's ratio v, and with engineering geological criteria such as: deformation modulus, natural bulk density, porosity, moisture content.

Article 5. Conditions for Application

1. Seismic methods can be conducted on land, on river, lake, coastal surfaces, in drill holes, in mine shafts, and other excavation works.

2. Depending on map scales, objects under investigation, seismic survey work must ensure technical standards specified in Table 1.

Table 1. Technical Standards for Seismic Survey Work

3. The technical standards specified in Table 1 shall be applied under the following conditions:

a) Segments not within the scope of quality construction assessment;

b) Calculating errors using the root mean square method when the number of points exceeds 15 points; calculating errors using the arithmetic mean method when the number of points is 15 or fewer;

c) Taking the maximum value for deviation and error indicators when the terrain and geological conditions are complex (the surface has significant undulations; the wave propagation speed in the layer changes over a distance shorter than twice the distance between wave-receiving points);

d) The reflection wave method can only survey from depths of 5 meters or less.

PART II

PROJECT PREPARATION

Article 6. Preparation and establishment of projects

1. In cases where seismic work is a component of a specialized project and is carried out in combination with other survey methods, the preparation and establishment of the project shall be conducted according to the regulations on the establishment of proposals and geological resource investigation reports.

2. In cases where the seismic measurement project is independent, the conduct of surveys, determination of objects, proposal of measurement network lines, and selection of reception methods must be submitted for approval by the competent authority.

Article 7. Collection of materials related to tasks and working areas

1. Topographic maps at scales corresponding to the geological engineering drawing scale or larger.

2. Geological, hydrogeological, engineering geological, and geophysical materials.

3. Layout diagrams of relevant geological and geophysical survey works.

4. Surface layer conditions to evaluate wave emission and reception conditions, serving the selection of appropriate source types and wave reception point processing methods.

Article 8. Basis for determining the tasks of seismic methods

1. Based on the basic geological investigation tasks for mineral resources, mineral exploration tasks to survey engineering geology, hydrogeology, and geological disaster investigations in the survey area;

2. Based on the results of compiling related geological and geophysical materials collected;

3. Based on the applicability and construction conditions of seismic methods.

Article 9. Establishment, determination of network lines, and spacing of wave reception points

1. For surface seismic measurements, choose line layout arrangements as follows:

a) When the seismic work has the task of providing cross-sectional materials for geological engineering projects, arrange continuous lines that coincide (or at least extend half the reception point spacing on each side) with the lines for establishing geological engineering cross-sections;

b) When the seismic work has the task of providing materials for other methods, measurements are carried out on sections ranging from 200m to 500m, arranged in coordination with other works such as drilling, excavation, mechanical testing, and other geophysical methods;

c) When surveying by area or requiring concentrated materials along construction project lines to obtain comprehensive geological engineering cross-sections, such as in hydropower dam surveys, tunnel surveys, and construction foundation surveys, arrange according to a network including longitudinal and transverse lines to connect across the area.

2. For underground seismic methods, wave projection, vertical cross-sections, and borehole measurements, arrange according to the specific conditions of drilling and excavation works.

3. Choose the spacing of reception points according to Table 1 in Article 5 of this Circular.

4. Choose denser spacing of reception points when performing special tasks:

a) Measuring at sections with bedrock depth less than the current reception point spacing;

b) Determining wave propagation velocities in overburden layers to serve the calculation of mechanical parameters for construction projects, because with normal reception point spacing, the time-distance curve segment corresponding to that layer does not have at least three reception points to calculate wave propagation velocity.

Article 10. Refraction Wave Method

1. The refraction wave method is used when the research depth does not exceed 50 meters.

2. Select the refraction wave observation system by choosing the number of sources and the length of the reception section L.

b) The commonly used observation system is a five-point source system consisting of three near-source points and two far-source points. These source points create intersecting and chasing wave diagrams.

- Far-source points are placed at a distance greater than the surface wave exposure distance from the reception section so that the first arrival wave on the reception section is completely exposed from the surface.

- When the line is long and has multiple reception sections, the source points are arranged at fixed positions so that some near-source points of one section are far-source points of the next section.

4. The length of the reception section L is selected based on two cases:

a) When the survey area has hard rock at depths less than 1.3 times the required survey depth, select the length of the reception section L greater than 1.3 times the distance from the source point to the surface wave exposure point XN on the first arrival wave and round up to the nearest multiple of the receiver cable bundle length (Figure 3). The distance XN is chosen from common geological environments, with a wave propagation velocity in the surface layer VG » 2 VTB in the overburden layer, which is: XN » 2.5h, where h is the depth of the surface layer;

b) When the survey area has hard rock at depths greater than 40 meters, test measurements must be conducted to select an appropriate reception section length L;

c) Based on the estimate in point a of this clause, observe with a reception section length L ranging from 2 to 3 adjacent measurement sections.

d) Analyze the data, determine the environmental model and research depth achieved;

When establishing a project, it is necessary to collect data on water flow and terrain to select appropriate construction methods. At the same time, consider the possibility of stringing cables across the river to measure simultaneously on both banks to reduce errors in determining the arrival time of waves. If necessary, conduct a site survey beforehand. From this, draw up a layout diagram for measurement, cable placement, receiver installation points, and technical instructions for construction.

Article 11. Reflection Wave Method

1. Select the observation system for deep common depth point recording within the frequency band from 150 to 2000 Hz.

a) The distance between the wave receivers should be selected from 1 to 4 meters. Do not use a receiver distance greater than 1/20 of the maximum depth to be surveyed to ensure the quality of the deep common depth point.

b) The number of stacked waves should be chosen from 6 to 24.

c) Use a wave source such as a hammer strike, air gun, electric detonator, or small explosive charge.

2. Conduct trial measurements to determine the observation parameters according to the following sequence:

a) Measure with a receiver spacing of approximately 1 meter and a stacking level of 12 or more.

b) Process the test results with different stacking levels and receiver distances.

c) Compare the results and choose the parameter corresponding to the minimum stacking level where the processed deep common depth point clearly shows the reflection boundaries at the required maximum depth.

d) Selection of parameters must take into account that the receiver spacing should not be less than the correlation radius of the main random noise on the unstacked trace, but should not make the stack too large, to the extent that the outermost reflected wave has an angle of reflection greater than the critical angle (Figure 4); increasing the stacking level will increase the signal-to-noise ratio for the total trace, highlighting deeper reflection waves, but will increase the cost of the survey.

Article 13. Seismic Wave Illumination Method

1. Seismic wave illumination measurements between drill holes, mining structures, on the surface, or arranged on both sides of a mountain slope.

2. The distance between wave receivers is selected according to Table 1 of Article 5 of this Circular.

3. The number and arrangement of sources are determined based on the survey requirements and the geometric relationship between the object and the receiving array, ensuring that the wave rays cover the area under study.

Article 14. Underground Mine Seismic Measurement

1. Underground mine seismic measurement involves receiving and correlating PSP cross-wave from the original rock face together with the corresponding vertical wave to calculate the elastic modulus Ed and Poisson's ratio ν of the rock mass, and to determine the stress reduction zone caused by tunnel excavation (Figure 6).

2. Observations are carried out using the refraction wave method:

a) Survey network at a scale of 1:200 to 1:1000.

b) Receiver spacing of 1 to 2 meters.

c) The observation system consists of 4 to 5 sources, including 2 distant sources.

d) The wave source is generated by a hammer strike or electric detonator.

Article 15. Selection of Seismic Wave Reflection Sources

a) The type of source, main characteristics, depth of refraction wave study, depth of reflection wave study and illumination of corresponding types of sources are specified in Table 2;

b) The frequency of emitted and received waves achieving resolution and accuracy of boundaries depends on the type of source used, environmental features at the emission and reception points (Appendix 1.3).

Table 2. Main Types of Sources

Article 16. Content of the Seismic Project

1. An independent seismic survey project must include the following contents:

a) Objectives and tasks based on requirements from the decision-making authority or other legal basis, the seismic project determines specific technical tasks and work procedures, and the time schedule for implementation;

b) Work area location, geographical, economic, cultural characteristics, and impact assessment on construction processes;

c) History of geological, engineering geological, and geophysical studies conducted in the work area. Evaluation of the extent to which existing materials are utilized in seismic data analysis;

d) Methods and workload, summarizing the issues:

- Geological-geophysical basis for selecting seismic work types, technical tasks, and workload of such works;

- Observation methods, network, and technical measures to be applied;

- Geodetic requirements for different types of work;

- Allowable errors of working methods.

e) Construction organization: general plan of construction including organization, manpower, steps of execution. If the project is prolonged, a construction schedule chart should be provided;

f) Office work: detailing processing and calculation methods. Classical methods only need brief mention;

g) Project products: reports, seismic-geological cross-sections, tables of mechanical parameters of strata, diagrams based on tasks specified in point a of this clause;

h) Budget estimate: calculated based on current norms, unit prices, and auxiliary costs;

i) List of reference documents;

j) Appendices and drawings accompanying the project:

- General layout diagram of the work area;

- Geological and engineering geological diagrams at survey scale (1:50,000, 1:25,000 or larger);

- Layout diagram of seismic lines and related geological and geophysical surveys;

- When conducting illumination, drilling holes, or vertical sections with multiple wave emission directions, a planned observation diagram should be drawn.

2. When seismic work is a component of a geological engineering survey project, the above contents should be briefly stated, consistent with the overall project content.

3. Members of the project team include:

a) Project Leader: Senior Surveyor of Geophysics Grade 4 or higher;

b) Two senior surveyors specializing in geophysics;

c) One senior surveyor specializing in geology or engineering geology;

d) One surveyor specializing in geodesy;

e) Technical staff.

Article 17. Conditions for Implementing a Project

A project may only be implemented upon obtaining a decision approving it from the competent authority or upon signing a seismic service contract with legal validity.

Article 18. Components of a Seismic Survey Team

1. The machine measurement unit includes:

a) One machine operator, holding the position of chief investigator, specializing in geophysics at level 3;

b) One recorder, holding the position of chief investigator, specializing in geophysics at level 2;

c) Four wireline supervisors, holding the position of investigator, specializing in geophysics at level 3;

d) Six to twelve wireline workers.

2. The signal source unit includes:

a) One signal source commander;

b) Blasting crew members or hammer operators, with a minimum of two groups; each group consisting of two to four people.

3. The surveying unit includes:

a) One machine operator holding the position of chief investigator, specializing in surveying at level 3;

b) One recorder holding the position of chief investigator, specializing in surveying at level 2;

c) Two technical assistants.

4. The field office unit includes:

a) One general supervisor, holding the position of chief investigator at level 4 or higher specializing in geophysics;

b) Four chief investigators specializing in geophysics for analyzing data;

c) Two investigators;

d) Two technical assistants.

Article 19. Occupational Safety Work

1. Prior to construction, occupational safety training must be organized according to current regulations on basic geological surveys related to minerals.

2. Professional production participants must be trained in occupational safety regarding work in areas with explosive materials and first aid measures in case of accidents.

3. When using explosive sources, implementation must follow Circular No. 23/2009/TT-BCT dated August 11, 2009, issued by the Ministry of Industry and Trade detailing certain provisions of Decree No. 39/2009/NĐ-CP dated April 23, 2009, of the Government on industrial explosives, and Circular No. 35/2010/TT-BCA dated October 11, 2010, issued by the Ministry of Public Security on issuing permits for transporting industrial explosives and dangerous goods.

4. Establish and disseminate workplace safety rules and coordination measures between blasting units and specialized teams.

PART III

FIELD CONSTRUCTION

Article 20. Requirements for Field Construction

1. Before commencing field construction, geological measuring equipment must be calibrated and tested at a facility authorized to operate testing services and having a calibration procedure consistent with the equipment being measured.

2. For newly produced or imported seismic measuring equipment without established calibration procedures, calibration measures should be carried out according to the contents specified in Sections 1, 2, 3, 4, 5, and Section 6 of Appendix II attached to this Circular.

Article 21. Selection of Wave Recording Parameters

1. Select the digital signal sampling interval Δt based on measurement resolution and noise reduction from mirror images.

2. The time measurement resolution required to determine seismic boundaries and calculated according to the formula: Δt ≈ δh/V.

a) Δt is the sampling interval, measured in seconds;

b) δh is the error in determining the boundary that needs to be achieved, chosen as 1 to 2% of the predicted depth h, measured in meters;

c) V is the average wave propagation speed in the overburden layer, measured in meters per second, which can be set at approximately 500 to 800 m/s for dry overburden and 1200 to 1500 m/s for water-saturated overburden.

3. To reduce noise from mirror images, the digital frequency must be more than four times the highest frequency f (Hz) in the signal band to be received. In this case, the sampling interval must satisfy the expression: Δt < 1/(4fmax)._max 38 |||_fmax).

4. If there is no basis for selecting the sampling interval, test measurements should be conducted with smaller intervals according to the bandwidth characteristics specified in Table 3. Examine the recorded bands with time ratios ranging from 1:2 to 1:10, select the ratio where the wave pulses are still clearly visible, and calculate the appropriate sampling interval.

5. Choose the length of the signal recording time needed to capture the latest useful signal.

a) When measuring refracted waves and vertical cross-sections, typically choose according to the horizontal exchange wave PSP formed from the ground surface, calculated for the farthest source-receiver distance as stipulated in Section 2 of Appendix III attached to this Circular;

b) When measuring borehole seismics and reflected waves, choose twice the transmission time of the P-wave along the ray path;

c) When measuring reflected waves, choose according to the P-wave reflected from the deepest boundary to be surveyed;

d) In cases where the machine specifies the recording length as the number of digital samples of the signal channel, calculate this length by dividing the required recording time by the sampling interval. Table 3. Bandwidth Characteristics of Waves and Selection of Sampling Interval Method Main BandwidthSampling Interval

Surface Refraction, Water Surface

- When measuring deep point reflections, select a high-pass filter with a lower limit from 150 to 200 Hz. b) When using filters and needing to select filter parameters, perform:- Test measurements on several sections of the machine, with a wide range or filtering according to the lower limit of the bandwidth specified in Table 3, Clause 4 of this Article; - Use the digital filter (Digital Filter) function of the machine with different filters to select the clearest signal display, then choose the appropriate analog filter parameters.negative.

7. Select similar analog filter:

a) Select the similar analog filter (Analog Filter) of the machine via the parameter setting menu (Setting):

- This filter cuts the signal spectrum before recording, so it will be completely lost in the data;

 - When measuring other methods (refraction waves, reflection waves), record the signal over a wide range:

- When measuring deep point reflection waves, choose a high-pass filter with a lower limit from 150 to 200 Hz.

b) When using filters and needing to select filter parameters, perform the following:

 - Conduct test measurements on several sections of the machine, over a wide range or with a low-frequency limit according to the band specified in Table 3 Clause 4 of this Article;

- Use the digital filtering function (Digital Filter) of the machine with different filters to select the clearest signal display band, from which suitable analog filter parameters can be chosen.

Article 22. Broadcasting

1. Explosive sources, blasting caps:

a) Arrange the initiation marks in a loop wire configuration or an electric pulse blasting configuration. If the electric pulse configuration is used, check the delay time of the cap series;

b) At close source points, use small caps or small explosive charges:

- When blasting on land, place the charge in a hole and cover it with soil, but the depth should not exceed 1/15 of the distance to the signal receiving point;

- In areas with very hard rock, place the charge on the surface of the rock;

- When measuring in water, use a float to keep the charge at the predetermined depth and use a rope from the boat to the cable bundle to maintain the charge's position accurately.

d) Ensure a safe distance from the explosive charge to people and equipment, as stipulated in Article 19 of this Circular.

2. Dam hammer, sledgehammer, machine hammer sources:

a) Arrange the initiation marks using a hammer switch or a signal receiver placed at the source point. When using a signal receiver, ensure that the receiver does not detach from its placement point after each strike, maintaining a stable start signal level;

b) On soft ground, use iron bars, plastic or hard wooden boards, placed horizontally or at the designed angle of the measurement method for the hammer or sledgehammer impact point;

c) When measuring refracted waves on land, strike the hammer at an angle of 20° to 45° relative to the bar, directed towards the receiving section, aligning the impact force with the incoming wave ray, enhancing the sliding wave intensity at the boundary and increasing the likelihood of forming horizontal PSP exchange waves (Diagram 7).

3. Vibration sources: use a signal receiver placed at the source point to capture standard signals and initiate recording measurements. The vibration emission direction follows the equipment capability and project measurement design.

4. Electric pulse, air gun sources:

Article 23. Construction of Refracted Wave Measurement Methods on Land and in Underground Mines

1. Position the signal receiver points on the line using a tape measure or by connecting the receiver ends on the measurement cable.

2. Lay out the cable and receivers along the measurement line

a) For the first receiver segment on the line, lay out the receivers such that one receiving point coincides with a certain landmark on the line. Subsequent receiver segments overlap at least one receiving point with the previous segment;

b) To place the receivers, drill holes slightly wider than the receiver tail perpendicular to the ground rock surface. Firmly insert the receiver into the ground rock. If the ground rock is too hard, use clay to fill around the receiver.

3. Determine the positions of the source points within the cable bundle (close source points) based on the positions of the receiving points:

a) To avoid damaging the receivers, the source points can be shifted parallel to the line, but not more than 20% of the distance between receiving points;

b) When using explosive sources at close source points at the beginning of the receiving bundle and the wave strength is insufficient to reach the farthest receiving point, choose the following handling methods:

- Explode multiple times with small charges and sum the waves;

- Increase the charge size, remove nearby receivers near the source point, record tapes showing waves from the distant source point.

4. Determine the positions of the source points outside the cable bundle (distant source points) using a tape measure or based on the location of the previously constructed source points.

5. Record in the measurement logbook the characteristics of the line segment: survey markers, terrain features, changes in the measurement line direction, geological exposure marks, surface condition changes, relative to the receiver positions, to locate the receiving segment and explain the data later.

Article 24. Construction of Refraction Wave Measurement on Water Surface (Rivers, Lakes)

1. Prior to construction on water surface, preparatory work must be carried out, including:

a) Determining the method to fix the buoy receiver array on the water surface;

b) Manufacturing the floating platform for the receiver array on water;

Figure 9. Refraction wave measurement on rivers and lakes.

a) When there is a cable anchoring point;

b) When there is no cable anchoring point;

c) A triangular floating platform made of bamboo tubes or plastic pipes to place the wave receiver machine.

d) Determining the actual coordinates of the receiving section using GPS or a theodolite;

d) The receiving cable should be at a uniform depth between 0.2 to 1 meter.

2. Transmitting waves:

a) When recording the source signal from a nearby point, the wave transmitter must be correctly positioned with an error not exceeding 10% of the distance to the receiving point;

b) Positioning distant sources by estimation within a range of -5% to +20% vertically and 15% horizontally.

 Figure 10. Construction of common-depth profiling measurement when recording 24 channels, using a 48-point receiver array, transmitting waves from one point at channel 1 position.

3. Reverse seismic measurement requires consistent marking of the wave transmission time, thus only loop-starting methods can be used and high-quality cables must be employed to transmit start signals.

4. Recording in the field logbook the topographic features, layout diagrams of the receiver array, and the actual depth of the receiver array relative to the water surface.

5. When using a theodolite intersection method, the survey log must clearly record the time and minute of measurement for easy comparison with seismic data.

Article 25. Construction of Common-Depth Reflection Wave Measurement Method

1. Conducting measurements according to the number of channels of the machine:

a) Implementing the deployment of the receiver array according to the designed receiving points;

b) Recording measurements sequentially at each receiving point;

c) When moving to subsequent sections, overlap 1 to 2 receiving points.

2. Conducting profiling:

a) Using a multiple receiver array with the number of receiving channels being a multiple N = 2, 3, or 4 times the number of recording channels, connected to a sliding switch selector.

b) Selecting the multiple N based on equipment capability, for example, when using 24 recording channels, having 48, 72, or 96 receiving points, and the switch selector has 24 output pairs, the number of input pairs depending on the number of receiving points;

c) Using a level 6 addition, recording 12 channels with a 24-channel recorder like Mark-6 and a multiple receiver array N = 2, then use the roll-up mode (Roll up Mode) of the machine.

3. Conducting common-depth profiling on water surface: carried out when the ship runs along the route in a pseudo-profiling manner. During construction, select the appropriate speed and firing rate to achieve a profiling distribution of receiving sections along the route.

4. Positioning the measurement point by GPS: Record the logbook including time and passing landmarks to compare the measurement point location with the map.

Article 26. Construction of Vertical Section Measurement, Borehole Seismic

1. When measuring vertical sections and forward borehole seismic, lower the receiver array into the borehole to the deepest required depth, measure sections while pulling the cable up. Implement wave transmission at the designed source points.

2. During measurement, if the source points need to be moved, choose the nearest point in the same direction with similar reception and transmission conditions as before, and arrange overlapping observation of at least one receiving point.

3. For reverse borehole seismic measurement, place the wave receiver at the designed positions, lower the blasting tube into the borehole to the deepest required depth, measure the source points while pulling the cable up.

4. Record in the field logbook the condition of the borehole, draw a diagram of the actual layout of the source-receiver points. After completing an observation, record the specified data in the logbook as stipulated in Point 3 of Appendix I issued together with this Circular.

Article 27. Seismic Wave Survey Construction

1. Implement the placement of receiving points and source points as designed in the project;

2. During the survey, if the source point is placed on the ground and for some reason must be moved, the new source point must ensure control over the target area requiring wave projection and must arrange at least one overlapping receiving point.

Article 28. Geodetic Work to Determine Coordinates and Topography of Seismic Line

1. When conducting seismic surveys on land (refracted waves and reflected waves), geodetic work shall be carried out according to current geodetic regulations, with the task of arranging the line according to the design on-site and collecting topographic cross-sections at the survey ratio of the seismic survey.

2. When measuring refracted waves on water surfaces, determine the coordinates of both ends of the measurement section using GPS or triangulation from markers on the shore, with errors specified in Table 1 Clause 2 Article 5 of this Circular.

3. When measuring reflected waves at deep common points on water surfaces, use GPS positioning combined with recording topographic and physical features on the shore in a logbook.

4. When conducting seismic wave projection and vertical cross-section measurements, geodetic work shall determine the height and distance between these points and the mouth of the drilling excavation works.

Article 29. Field Office Work

1. After each day of measurement, copy the data from the measuring instrument to another storage medium.

2. Check, verify, and systematize field record books, delineate the line segment on the construction map.

3. Check the quality of the recorded tapes by reviewing on a computer or printing on paper. Pay attention to detecting startup recording wave addition errors, channel non-functioning errors, receiver polarity reversal errors, and direct construction to find measures to correct errors if present and re-measure sections that do not meet quality standards.

4. Input the coordinates of the source point and receiving point for each tape file: when there are no geodetic coordinates, enter the X coordinate based on the distance from the receiving point to the zero mark of the line. When full XYZ coordinates are available, enter these coordinates.

5. Preliminarily phase the waves of interest: initial waves and lateral waves if observed.

6. Create time interval charts, check mutual time.

7. Identify missing tapes and tapes of poor quality that do not meet usage requirements, deciding to remeasure.

Article 30. Quality Assessment of Construction

1. Assess the quality of construction based on the quality of the recorded tapes as stipulated in Article 31 of this Circular and the quality assurance index for each measurement section.

2. The number of required recorded tapes for a measurement section is the number of data files needed corresponding to the observation system and the direction of the source pulse already designed for those source points; excess tapes are only for reference; eliminate tapes without clear transmission and reception positions.

3. Assess the quality of seismic tape documentation based on the following criteria:

a) The actual location of the measurement section, clearly recorded in the field book;

b) The number of channels not working out of the total number of channels arranged for measurement;

c) The recording start time, which must be earlier than the appearance time of the wave;

d) The wave picture obtained on the tape, which must clearly show the first wave peak;

d) Random noise level, affecting the determination of the wave and the first wave break point or not.

4. Adjusting factors when assessing quality:

a) Unable to complete the measurement section according to the design due to terrain and physical obstacles, such as when placing the source point is obstructed by civilian infrastructure, embankments, rock walls, dangerous cliffs;

b) Failure to obtain waves due to strong wave absorption characteristics of the soil and rock, which cannot be overcome with existing equipment, such as in areas of destruction, fault zones, karst caves, extremely strong weathering zones.

Article 31. Quality Evaluation of Recording Tapes

1. Good quality recording tape:

a) The number of non-operational channels does not exceed 5% of the total number of measured channels and must not be the channels of the first and last machines, nor adjacent to each other;

b) The waveform picture on the tape is clear: The start of the wave and the initial break point of the wave are distinct;

c) The amplitude of noise before the appearance of the initial wave is less than one-third of the amplitude of the initial wave;

d) The first wave appears clearly in 85% of the measured channels.

2. Fair quality recording tape:

a) The number of non-operational channels does not exceed 10% of the total number of measured channels and must not be adjacent to each other;

b) The first wave appears clearly in 70% of the measured channels.

3. Average quality recording tape:

a) The number of non-operational channels does not exceed 15% of the total number of measured channels and must not be adjacent to each other;

b) The first wave appears clearly in 50% of the measured channels.

4. Poor quality recording tape:

a) The number of non-operational channels does not exceed 20% of the total number of measured channels, or there are two adjacent non-operational channels;

b) The first wave appears clearly in 30% of the measured channels.

5. Recording tapes with poor quality must be discarded:

a) Recording tapes with more than two adjacent non-operational channels, or those that do not meet the criteria for poor quality mentioned above;

b) Measurement sections where more than 45% of the recording tapes have poor quality must be remeasured.

CHAPTER IV

OFFICE WORK

Article 32. Requirements for Office Work

Office work involving the processing of office documents shall be carried out simultaneously and immediately after the completion of fieldwork.

Article 33. Systematization and Completion of Field Documents

1. Conduct verification, correction, systematization of field record books, determine the positions of line segments on construction maps.

2. Reorganize printed recording tapes, compare them with field record books and data files.

3. Determine the coordinates of the source point and the receiving point, and enter them into the recording tape data files.

4. Remove defective or redundant data files from the main data directory.

5. When marking waves on paper recording tapes and encountering multiple waves with significant amplitude differences, print at least two tapes, including a "strong" tape with a high ratio of the first wave clearly visible and a "weak" tape with a low ratio of subsequent waves clearly visible (Figure 11).

Article 34. Phase Marking and Seismic Waves

Phase marking and seismic waves should begin immediately after construction and rechecked during the final review of materials.

Article 35. Establishing Time Interval Graphs

1. Plot phase lines and establish time interval graphs for observed waves, with a spacing of 5 ÷ 10mm on a single-point receiver drawing:

a) Use different colors for different source points and waves;

b) Establish time interval graphs closely related to wave lines on the recording tape, which often requires multiple attempts, especially when searching for lateral waves;

c) When working on millimeter paper, create preliminary time interval graphs for each segment of reception, marking all plotted phase lines. Adjust the wavehead using a compass on the drawing. Then adjust or redraw to obtain the initial time interval graph of the wavehead for the line;

d) When working on a computer, switching between the recording tape and the graph, checking and calculating adjustments will be more convenient. Therefore, only print the linked wave product, which is the initial time interval graph of the wavehead for the line.

2. For the refraction wave method:

a) Check the parallelism of the chasing time interval graphs for continuously observed waves corresponding to clear refraction boundaries, particularly surface waves;

b) From the chasing graphs, perform shifts (phantom) to establish the expanded time interval graph t_A and Advanced Water-saving Irrigation_B for two source points near the ends of the reception segment (Figure 12);

Shifting, calculating θ and t₀ can be done using a compass on millimeter paper or through office software calculation tables. 

Article 36. Calculating and Summarizing Average Speeds and Layer Speeds for V_t, and V_s

1. In the borehole measurement and wave projection method, calculate the wave transmission speed V_t, and V_s (if applicable) for each reception point, in sequence:

b) Correct for the angle of the wave ray with the borehole axis φ to obtain the actual speed V, measured in m/s (Figure 13):

c) The calculated results are plotted on a graph, from which layer division is performed based on speed, and the layer speed is determined as the average value obtained within that layer.

2. For the refraction wave method, to accurately calculate boundaries, it is necessary to calculate the speeds of the waves and compile them into a velocity profile:

c) If the wave propagating in the overlying layer is a penetrating wave, the first time interval graph curve, then perform formal layering to calculate the layer speed V_{nh cơ} for sub-layers: Consider each segment of the time interval graph between adjacent reception points as part of a wave formed from the corresponding underlying formal layer and calculate gradually until reaching the stable-speed layer, which is the base (Figure 16). Penetrating waves typically appear in areas of exposed bedrock and increasing weathering levels with depth;

d) For waves formed from the base layer, or from strong enough refraction boundaries to form intersecting expanded graphs, calculate the boundary speed V_G for each linear segment of the difference θ graph (Figure 12).

Where:

x Distance to the source point, measured in meters;

θ Time value read on the difference graph, measured in seconds;

V_G Transmission speed of the boundary, measured in m/s.

d) Calculate the average speed V_AVG to a boundary: based on the intersection of wave graphs, when the graph of the lower layer begins to appear in the first arriving wave (Figure 14). The calculated result should be compared with the effective speed V_AVG obtained after initially determining the thickness of the overlying layer;

d) Layer speed V_{nh cơ} and boundary speed VG are used for calculating mechanical parameters, while the average speed V_AVG and V_G is used for calculating the thickness of the overlying layer h at the boundary;

e) To minimize abrupt jumps in boundary calculations from one segment to another, smoothing the average speed V_AVG profile along the line before calculating the cross-section may be necessary.

Article 37. Determining Seismic Boundary Limits

1. With seismic well data: Determine the boundary using characteristic points on the time interval graph and actual velocity changes, combined with considering the dynamic characteristics of the wave.

2. With vertical section measurement data and seismic reflection data: Determine by characteristic points on the time interval graph, velocity, and geometric ray path diagram. Recheck the results by recalculating the time interval graph based on the obtained environmental model. This process may need to be repeated multiple times to achieve accurate results.

3. With refraction seismic data: In each step of constructing a section, use the following methods:

a) The t0 method to determine the perpendicular depth h of the boundary ; where V_tb is the average velocity of the overlying layer ;

b) Using the t0 method for overlying layers and base surface. Other variants of the t₀method can also be used, such as calculating layer thickness with correction factors;

c) The time domain method, implemented on graphs, when necessary to observe the time domain of waves and to check the calculation results using the t₀;

method; d) The reciprocal ray method (GRM,Generalized Reciprocal Method

), implemented on computers using software like WiewSeis or equivalent software, will more accurately determine the spatial position of the base boundary.

Article 38. Determining Fractured Zones and Karst Features Based on Dynamic and Kinematic Characteristics1. Kinematic characteristics: On seismic sections, calculated depths fluctuate, boundary velocities decrease, and the time interval graph shows strong fluctuations.2. Dynamic characteristics: On wave recordings, as they move away from the source point, there is strong wave absorption causing signal amplitude and apparent frequency reduction. Observing wave traces on forward and reverse source point tapes will identify the dynamic characteristic change point X

. Mark these sections, then adjust the ray angle on the constructed section and determine the strong absorption zone on the base (Figure 16).3. For water-filled karst, chaotic wave patterns are often observed, Figure 16. Determining the location of fractured zones in bedrock based on strong dynamic characteristic change points of waves._A X_Ba) Position X

according to the forward source point tape;

b) Position X

according to the reverse source point tape;_A c) Adjust the boundary position on the section according to the ray angle.

Depending on the type and size of the cave, the results should be verified and compared with those from other methods._B Article 39. Evaluating Errors

1. When observing networks, determine the error in the depth of strong boundaries (base or underlying layer) at intersection points of lines. Relative depth errors at intersection points must not exceed 5%.

2. When geological drilling projects are available, the error in determining boundaries based on seismic and drilling data must not exceed 7%.

Article 40. Establishing Seismic-Geological Sections

1. Seismic-geological sections are established based on data obtained from seismic observations and cross-checked with results from other observations.

2. For geological mapping, engineering geology, and geological disaster surveys at a scale of 1:50,000, establish seismic-geological sections with a horizontal scale of 1:5,000 and a vertical scale of 1:1,000 to 1:500.

3. For mineral resource evaluation, geological disaster assessment, and construction site investigation, establish seismic-geological sections with a horizontal scale of 1:2,000 to 1:1,000 and a vertical scale of 1:1,000 to 1:200.

4. On the section, represent determined boundaries, fractured zones, positions of some source points, typical parameter values (Vp, Vs, Ed, n), and geological symbols for layers and rock masses.

5. Plot drill holes, wells, trenches, and geological exploration pits along selected seismic lines and display their time interval diagrams on the section.

3. When serving mineral resource evaluation, geological disaster assessment, and construction project surveys, establish seismic-geological cross-sections with horizontal scales of 1:2,000 to 1:1,000 and vertical scales of 1:1,000 to 1:200.

4. On the cross-section, represent the boundaries, identified fault zones, positions of some source points, typical parameter values (Vp, Vs, Ed, n), and geological symbols for rock layers and blocks.

5. Plot drill holes, wells, trenches, and geological excavation pits along the selected seismic profile onto the cross-section, displaying time interval charts.

Article 41. Establishing the Relationship between Seismic Parameters and Engineering Geological Indicators

1. When serving the mapping of engineering geological maps and large construction site surveys that implement a combination of various types of engineering geological surveys, the horizontal wave velocity Vs should be sought and calculated. If the number of pairs of vertical wave velocity Vp and horizontal wave velocity Vs obtained is sufficiently large, over 25 pairs, calculate the Poisson's ratio v and elastic modulus Ed according to the formula:

where:

Vp, Vs : Vertical and horizontal wave velocities, measured in m/s;

 v : Poisson's ratio;

Ed : Elastic modulus, measured in PA;

σ : Natural bulk density (density), measured in Kg/m3;

g : Acceleration due to gravity, measured in m/s2.

2. Establish the relationship between Ed and n with Vp. The relationship Ed (Vp) has the form:

Lg(Ed) = αLg(Vp) - β

In cases where there is insufficient data to establish the correlation, use the Hawkin formula: Lg(Ed) = 2,3416 Lg(Vp) - 4,9

3. Use the results obtained to calculate Ed at positions where only vertical waves can be observed and only Vp is obtained.

Article 42. Products of Seismic Survey for Engineering Geological Mapping

When conducting seismic survey to serve the creation of engineering geological maps at a scale of 1:50,000, establish diagrams of zoning areas at the same scale as the engineering geological map, including parameters such as elastic modulus, deformation coefficient, natural bulk density, porosity, moisture content for the area being mapped.

Article 43. Preparing a Summary Report

1. The summary report on seismic survey work includes an explanatory report, appendices, and accompanying drawings.

2. The explanatory report includes the following main contents:

a) Introduction: Summarize the legal and economic-technical basis of the project; the implementation situation of the workload, changes in content compared to the project; applied technical methods, quality of work and main results achieved; implementing units and key participants;

b) Chapter 1: Overview of the Work Area presenting the following contents: location of the work area (administrative location, coordinates, accompanied by a small-scale guide map at A4 size); topographical features, rivers, streams, climate, population, economy, transportation; brief history of geological, engineering geological, and geophysical research related to the work, the extent of using existing materials to solve assigned tasks. In cases where the seismic survey is part of a specialized project, only relevant information about the conditions for implementing the task needs to be provided;

c) Chapter 2: Fieldwork Methods and Techniques presenting the following contents: methods and techniques used in fieldwork; quality of field data evaluated according to the acceptance certificate, or current regulations; key points in analyzing, processing, interpreting seismic data, and integrating with other geological, engineering geological, and geophysical data;

d) Chapter 3: Results of the Work presenting the following contents: results of the work, presented along survey lines, tunnels, drill holes; results of correlating seismic data on plan view; results correlated with other geological, engineering geological, and geophysical data; assessment of the degree of solving assigned technical tasks;

đ) Chapter 4: Economic Section;

e) Conclusion summarizing the main results achieved; unresolved issues and directions for resolution.

3. Drawings and appendices accompanying the report:

a) Diagram of seismic survey line layout established on the base map and other geological-geophysical works at the same scale;

b) Time interval charts on survey lines, tunnels, drill holes;

c) Seismic-geological cross-sections of survey lines, tunnels, drill holes;

d) When seismic survey lines form a network, establish maps dividing rock masses according to physical properties, fracture zones, and fault zones;

đ) When seismic work has the task of determining elastic and mechanical parameters of rocks and establishing experimental correlations with engineering geological parameters, establish comparison tables, correlation charts, and engineering geological parameter maps based on geophysical data for layers with sufficient data and reliable information.

4. Components of the report preparation team include:

a) Chief investigator of the geophysics specialty level 4 or higher;

b) Two geophysics specialty investigators analyzing data;

c) Three calculation technicians;

d) One chief investigator of the engineering geology specialty.

Article 44. Approval and handover of results

1. The final report shall be reviewed and approved according to the current regulations on managing projects under the Ministry of Natural Resources and Environment.

2. After being approved by the competent authority, the report must be submitted for geological storage in accordance with the current provisions.

Chapter V

IMPLEMENTING PROVISIONS

Article 45. Effective Date

1. This Circular takes effect from March 15, 2011.

Repeal the technical procedure for seismic measurement issued by Decision No. 661/QĐ/ĐCKS-KHTC dated December 23, 2004 of the Director of the Vietnam Geological and Mineral Resources Department.

2. The Director of the Vietnam Geological and Mineral Resources Department, units under the Ministry of Natural Resources and Environment, organizations and individuals implementing specialized projects using seismic measurement methods shall be responsible for implementing this Circular./.