ICS 75.180.10 E 11 Ref.No : 65583-2018
SY
The People' s Republic of China Standard of
Petroleum and Natural Gas Industry
SY/T 6957—2018
Replace SY/T 6957-2013, SY/T 6956-—2013
Technical specifications for marine gravity and
magnetic data acquisition
Issue date:2018 - 10 - 29
Implementation date:2019 - 03 - 01
Issued by National Energy Administration, P.R.C. SY/T 6957—2018
Contents
Foreword
:18
Scope Normative references
1
19
2
19
3 Technical design 4 Navigation and positioning 5 Marine gravity data acquisition 6 Marine magnetic data acquisition Anomaly accuracy calculation and leveling
19
·20
20
.· 22
7
23
8 Deliverables Annex A (Informative ) The template of marine gravity daily log Annex B (Informative ) The template of marine magnetic daily log Annex C (Informative)The template of marine gravity still reading Annex D (Normative)Gravity anomaly calculation Annex E (Normative) Magnetic anomaly calculation
24
26
27
28
29
2
17 SY/T 6957—2018
Foreword
Added the requirements for the maximum sailing speed (see 4.1 of this standard) ; -Modified the requirements for the stabilization time after the marine gravity meter charging (see 5.1.2.2 of this standard and 4.2.2 of SY/T 6957-—2013) ; -Modified the requirements for the recording time basis (see 5.4.1 and 6.3.1 of this standard and 5.4.1 of SY/T 6957——2013)
This standard is drafted in accordance with the rules given in GB/T 1.1--2009 Directives for standardization -- Part 1 : Structure and drafting of standards. This standard replaces SY/T 6957--2013 Technical specification for marine gravity survey and SY/T 6956--2013 Technical specification for marine magnetic survey. This standard is mainly based on SY/T 6957--2013, and integrated the contents of SY/T 6956—2013. Compared with SY/T 6957-2013, besides the the jurisdiction of the Petroleum Geophysical editorial revisions, this standard mainly includes Standardization Committee.
This standard was proposed by and under
This standard was drafted by GME & Geochemical Department of BGP Inc., China National
the following changes :
-Modified the standard name ; -Modified the normative reference documents Petroleum Corporation. (see Chapter 2 of this standard and Chapter 2 This standard was mainly drafted by Yang Zhanjun,
Ye Yuxing, Song Hailu, Sun Ximing, Li Keyi
of SY/T 6957--2013); -Added some requirements for technical design and Shang Liang.
This standard is published in both Chinese and
(see 3.2.1 of this standard) ; -Modified the requirements for sailing time after English.In the event of any discrepancy between the the end of survey line (see 4.1 of this standard texts, the Chinese version should prevail. and 5.3 of SY/T 6957—2013) ;
18 SY/T 6957—2018
Technical specifications for marine gravity and
magnetic data acquisition
1 Scope
a) Collecting the navigational information
including weather and sailing chart and understanding distributions of docks, islands, submerged rocks and fishing activity in the survey area.
This standard specifies the technical requirements for the technical design, navigation and positioning, data acquisition, anomaly accuracy calculation, leveling and deliverables of the marine b) Collecting rock geophysical properties, geology, gravity and magnetic survey.
drilling, geophysical and geochemical exploration information of survey area and surrounding area.
This standard is applicable to marine gravity and magnetic data acquisition and on-site data processing of the marine gravity survey with the c) Collecting the information of gravity base shipboard gravimeter and marine magnetic survey
stations and magnetic observatories around the survey area. 3.2 Contents 3.2.1 General contents
with the towed magnetometer.
2 Normative references
The following referenced documents are indispensable for the application of this document. The technical design includes the following
contents :
For dated references, only the edition cited applies. For undated references, the latest edition of the a) The scope of the survey area, geological tasks
survey line deployment and work load. b) The methods and requirements for instrument
referenced document (including any amendments) applies. SY/T 5819 Technical specifications for land gravity and magnetic surveys SY/T 6055 Technical specification for petroleum c) The involved personnel, instrument types
test and data acquisition, and their survey accuracy.
and quantity, operation plan and management measures.
gravity, magnetic, electric and geochemical exploration map SY/T 10019 Technical specifications for offshore d) The requirements for inspection and acceptance
as well as the final deliverables. 3.2.2 Survey line deployment The following principles of survey line deployment shall be complied with :
DGPS positioning and survey
3 Technical design 3.1 Preparation The following work shall be done before the a) The survey scale shall be determined according
to the geologic tasks, the common used line
technical design :
19 SY/T 6957—2018
inlines. 3.2.3 Anomaly accuracy
spacings are listed in Table 1. b) The inlines shall be perpendicular to the main strike
of geologic targets, and it should coincide with or be parallel to the existed seismic survey lines. The anomaly accuracy requirements are listed in
c) The tie lines should be perpendicular to the Table 1.
Table 1 Line spacing, line offset tolerance and anomaly accuracy requirements for marine gravity and magnetic survey at common used scales
Inline spacing Tie line spacing Line offset tolerance Gravity anomaly Magnetic anomaly
accuracy nT ± 5.0 ± 4.0 ± 3.0 ± 2.0
accuracy mGal ± 2.5 ± 2.0 1.5 ±1.0
Scale
(line offset/line spacing)
km 20 10 5 2
km 50 ~ 100 25 ~ 50 10 ~ 25
:1，000，000
1 :
1 : 500, 000 1 : 200, 000 1 :100,000
20%
<10
shall be recorded in daily log. The template refers to Table A.1 and Table B.1.
3.2.4 Navigation and positioning
The methods and technical specifications for navigation and positioning shall be implemented in 4.2 Requirements of positioning accordance with the provisions in SY/T 10019.
Positioning method and technology shall be implemented in accordance with the provisions in SY/T 10019. The speed error in the east-west direction shall be no more than ± 0.2 kn, the course error shall be no more than ±1°
3.3 Compilation The technical design structure shall be implemented in accordance with the provisions in SY/T 5819. The design map shall be compiled in accordance with the provisions in SY/T 6055.
5 Marine gravity data acquisition 5.1 Gravimeter preparation 5.1.1 Assembly, disassembly and checking The assembly, disassembly and checking of the
4 Navigation and positioning 4.1 Requirements of navigation During data acquisition, the navigation shall meet the following requirements : a) The vessel shall sail along the design survey line gravimeter shall follow the instructions in the user
at least 10min before the start of line and 5min manual. after the end of line.
5.1.2Static test
b) The sailing speed shall be no faster than 10kn. c) The vessel should sail at constant velocity along 5.1.2.1 The static test shall be done prior to the
the designed survey line. If the vessel deviates field operation. from the design line, the vessel shall be slowly 5.1.2.2The test should be done after 24h warming adjusted back to the design line, the adjustment The sampling time interval is 1s. The test duration shall be no more than 0.5° per second
requirements are listed in Table 2.
d) The information of course or speed changing 5.1.2.3 After earth tide correction, the static drift 20 SY/T 6957—2018
data shall be reduced and the curve shall be drawn. drift tolerance requirements are listed in Table 2. The drift curves should be linear approximately. The
Table 2 The static test duration and drift tolerance
Test duration
Working unit
Drift tolerance
h ≤ 72 >72
mGal 1.5 3.0
h 24
48
5.2 Gravity base establishment and tying 5.2.1 The gravity base shall be established on dock 5.4.1Requirements with obvious marks, a detailed description should
5.4 Marine gravity measurement
The requirements of gravity measurement include : a) The UTC time should be used as the gravity
be done. 5.2.2 The base should be tied to IGSN or National gravity base by using land gravimeters. The height difference from the base to sea level shall be noted down. The method and technical requirements for c) In case of bad sea conditions and the vertical
measurement time basis. b) The sampling interval shall be no more than 1s.
and horizontal swing angles of the platform exceeding 20°, the gravity measurement shall be paused.
gravity base tie shall be implemented in accordance with the provisions in SY/T 5819. 5.3 Still reading
d) In case a line is acquired in several segments ,
the repeated segment shall be more than 2km.
Still reading shall be done with requirements as e) The daily log shall be filled in detail, including
below : a) Still reading shall be done before and after each
the start time and end time of each survey line and abnormal situations. The template of daily log is shown in Table A.1.
sailing. b) The survey vessel shall be as close as possible to
the gravity base, and the distance should be no 5.4.2 Evaluation more than 200m.
The criteria of qualified marine gravity measurement include : a) The gravity drift of a voyage within 1 month shall
c) The still reading shall be done after the
gravimeter is stabilized, and the continuous recording time shall be no less than 30min.
be no more than 3mGal, the gravity drift rate for the voyages longer than 1 month shall be no more than 3mGal per month in average.
d) The elevation differences between the gravity
base, gravimeter sensor and the sea level shall be measured, and the reading accuracy shall be accurate to 0.1m.
b) The continuously lost data length shall be no
more than 10% of the survey segment, and cumulative lost data length shall be no more than 20% of the total survey lines.
e) The distance and azimuth from the gravimeter to
the base shall be measured, the distance shall be accurate to lm , and the azimuth shall be 1° , and a sketch shall be drawn.
c) The data length of continuously exceeded line
offset tolerance shall be no more than 10% of the survey segment.
f) The still reading table shall be filled on site,
with the template shown in Table C.1.
d) The data meeting all the above requirements are
21 SY/T 6957—2018
meter system shall be mounted in an instrument room.
qualified data. 5.5Bathymetry measurement Bathymetry measurement shall be done during marine gravity survey. 5.6 Tidal height measurement The tidal height measurement shall be done during near shore marine gravity survey. 5.7 Gravity anomaly calculation
b) The magnetometer cables shall be fixed on the
deck. c) The tow fish cable shall be no less than 2.5 times
of the vessel length. d) The sensor shall be far away from magnetic
interference sources. 6.1.2Noise level test The noise level test shall be done prior to magnetic
The gravity anomaly shall be calculated according survey :
a) The test site shall be selected in a place where
to the method described in Annex D.
magnetic field is gentle and stable and no cultural interferences nearby.
6 Marine magnetic data acquisition 6.1 Magnetometer preparation 6.1.1The assembly of magnetometer The assembly requirements include a) Monitoring and recording devices of magneto-
b) The test shall be done, with a reading interval
of 10s and an observation duration no less than 20min in a stable magnetic diurnal variation period.
c) The noise level shall be calculated with at least
100 readings according to Equation (1) :
(1)
x.
180°，45°，270°，135°）respectively within a distance of 20 m between the tow fish and the test point.
Where : S- -the value of noise level, in nT ;
-the magnetic value of the ith reading, in nT ; x -the seven-point moving average value of the c) The ship azimuth magnetic curve shall be plotted
xr
with the diurnal variation corrected data. d) If the direction difference between the designed
ith reading, in nT ;
N-The number of readings for calculation d) The noise level error shall be no more than
line and any of the eight azimuths exceeds 10°, the test azimuth should be adjusted to the designed survey line direction.
±2nT. 6.1.3 Ship azimuth magnetic test The ship azimuth magnetic test shall be done prior to survey : a) The test point shall be selected in an area where
e) If two or more survey vessels are used in one
project, the ship azimuth test shall be done respectively
6.2 Magnetic diurnal measurement
magnetic field is gentle and stable and near the survey area, and the test shall be done in a 6.2.1 The establishment of magnetic base magnetic quiet day.
b) The survey vessel sails through the test point The requirements of magnetic base establishment
include :
along eight azimuths (0°, 225°, 90°, 315°, 22 SY/T 6957—2018
a) The magnetic base shall be established in a
template of daily log is shown in Table B.1.
place near the survey area where the magnetic field is gentle and stable and far away from 6.3.2Evaluation electromagnetic interferences.
The criteria of qualified marine magnetic measure-
b) The resolution of the magnetometer for the ment include : magnetic base observation shall be no lower than a) The continuously lost data length shall be that of the marine magnetometer.
less than 10% of the survey segment, and cumulative lost data length shall be less than 20% of the total survey lines.
c) The time of magnetic observation at magnetic
base shall cover the time of the marine magnetic survey
b) The data length of continuously exceeded line
If there is a geomagnetic observatory near the survey area, the geomagnetic observatory data
offset tolerance shall be no more than 10% of the survey segment.
should be used for magnetic diurnal correction, c) The data meeting both the above requirements and the magnetic base should not be established
are qualified data. 6.4 Magnetic anomaly calculation The magnetic anomaly shall be calculated according to the method described in Annex E.
anymore. 6.2.2 Magnetic observation and absolute magnetic value calculation of magnetic base The magnetic observation and absolute value
calculation shall be implemented in accordance with Anomaly accuracy calculation and leveling the provisions in SY/T 5819.
7.1 Anomaly accuracy calculation 7.1.1 The anomaly accuracy is calculated by
6.2.3 Magnetic diurnal observation
The magnetic diurnal observation shall be using the root mean square error of the intersection implemented in accordance with the provisions in differences between inline and tie line according to SY/T 5819.
Equation (2) :
6.3 Marine magnetic measurement 6.3.1 Requirements The requirements of marine magnetic measurement include : a) The tow fish shall be released into the water in a
28
(2)
...
8:
2n
where :
-Anomaly accuracy, in mGal or nT ;
E
uniform speed, and the length of tow fish cable 0r -The ith intersection difference between inline
and tie line, in mGal or nT ; -The number of intersections.
shall be kept fixed during the marine magnetic survey.
b) The UTC time should be used as the magnetic 7.1.2 The number of intersections with difference
no more than three times of designed anomaly accuracy shall be no less than 97% of the total
measurement time basis. c) The sampling interval shall be no more than ls. d) In case a line is acquired in several segments, intersections. The intersections with difference more
than three times of designed anomaly accuracy shall
the segment longer than 2km shall be repeated. e) The daily log shall be filled in detail. The not be used for the anomaly accuracy calculation.
23