8.3　Geophysical Detecting Methods and Techniques of Detecting the Underground Pipeline

8.3.1　The underground pipeline present investigation diagram and underground pipeline present investigation deliverables shall be applied for detecting concealed underground pipelines，which is based on site investigation and site confirmation for distribution conditions of underground pipelines laying and pipelines exposure.The detecting shall be done by geophysical detecting method according to the assignment requirement，detecting target and geophysical conditions，and the following principles shall be complied with:

1　From surface to inside，from known to unknown.

2　From shallow to deep，from simple to complex.

3　The instrument，technology and method that are effective shall be applied.

4　Different detecting methods shall be used in complex conditions for interactive verification.

8.3.2　The geophysical detecting method for detecting the concealed underground pipeline shall be provided with the following conditions:

1　There shall be distinct difference in physical attributes between the underground pipeline and the surrounding soil detected.

2　The abnormal field generated by the underground pipeline detected shall have adequate intensity，and shall be distinctly identified from the interference background.

3　All methods shall comply with the relevant specifications in Section 8.5 of this Code.

8.3.3　The selection of pipeline detecting instruments shall be compatible with the method，technology and the type/material of pipeline，and the detecting shall choose proper geophysical detecting method and pipeline detecting instruments according to the difference in assignment requirement，detecting target and geophysical conditions.

8.3.4　The pipeline detecting instrument shall be provided with the following properties:

1　May receive distinct abnormal signal from the underground pipeline detected.

2　Have strong anti-interference capacity，may distinguish between the signal generated by pipeline and the interference signal.

3　Shall meet the detecting accuracy requirements in 3.0.22 of this Code，and have strong resolving capacity for adjacent pipelines.

4　Have adequately large transmitting power or magnetic moment，which may meet the requirement of detecting depth.

5　Have multiple transmitting frequency for selection to meet the requirement of different detecting conditions.

6　May observe multiple abnormal parameters.

7　The non-electromagnetic inductive pipeline detecting instrument shall meet the requirements of relevant geophysical detecting technical standard.

8.3.5　For inflammable and explosive pipelines，the detecting instrument shall have fireproof，explosionproof，and antistatic attributes，or necessary measures shall be taken.

8.3.6　Before detecting of underground pipelines，method experiment shall be done in the detecting area or adjacent known pipelines to decide the effectiveness，accuracy and relevant parameters of the method，technology and instrument/equipment used.For underground pipelines of different types，materials，pipe diameters and burial depths，and for areas of different geophysical conditions，the method experiment shall be done respectively and shall comply with the following requirements:

1　When doing conformance test for all the committed instruments on the known pipeline，the instrument may only be put into operation after meeting the accuracy requirement.

2　The minimum and optimal receiving and transmitting distance for each instrument shall be decided.

3　The optimal frequency of use on pipelines of different types，materials and burial depths for each and every instrument shall be decided.

4　The corrected parameter for each instrument in detecting pipelines of different types，materials and burial depths shall be determined.

8.3.7　For detecting of metal pipelines，the detecting method shall be selected based upon the type，material，size，burial depth and exposure condition of pipeline，and the geophysical condition on site:

1　For steel pipe，direct method，pliers clamp method and electromagnetic induction method should be used per specific conditions.

2　For steel pipe of large diameter（relative to burial depth），direct method or electromagnetic induction method should be used，or geological radar method，magnetic method，single channel shallow seismic method may be used.

3　For steel pipe with considerable burial depth（relative to pipe diameter），direct method，or electromagnetic induction method with high power and low frequency should be used.

4　For thermal piping or HT（High Temperature）petroleum transmission piping，active source electromagnetic induction or IR（Infrared Radiation）method should be used.

5　For metal piping with joint of high-resistance body，higher frequency electromagnetic induction method or pliers clamp method should be used，or the geological radar method may be used for detecting.In case of low ferromagnetic interference in the detecting area，magnetic field intensity method or magnetic gradient method may be used.

8.3.8　The detecting of non-metal piping should apply tracer electromagnetic method，geological radar method，single channel shallow seismic method，DC resistivity method，which may be selected according to the following principles:

1　For non-metal piping with inlet/outlet，tracer electromagnetic method should be used.

2　For reinforced concrete piping，magnetic dipole induction method may be used but the transmitting power or magnetic moment need to be increased and the receiving/transmitting distance needs to be reduced，or electro-charging method or magnetic gradient method may be used.

3　For non-metal piping with large diameter，and other underground facilities including culvert，covered trench，geological radar method and single channel shallow seismic method should be used.When ground connection condition is provided，DC method including high density resistivity method may be used.

4　The non-metal piping including plastic piping with metal line should be detected by direct connection（metal line）method.

8.3.9　For detecting of cable or optical cable，the detecting method may be selected according to the different types and exposure conditions of cable or optical cable:

1　The electric power cable should be detected initially by passive source power frequency method for initial location，and then by active source electromagnetic induction method for accuracy positioning and depth location.In case of cable exposure，pliers clamp method should be used.

2　For lighting cable，control/telecom cable or optical cable，active source electromagnetic induction method and pliers clamp method should be used.

3　Cable identifier or detecting instrument with cable identification function may be used to identify and trace cable.

8.3.10　During detecting of pipeline in dead zone，active source induction method and passive source method shall be used first for search，the searching methods include parallel searching method and circular searching method，and after finding abnormality，the active source method should be used for tracing，accuracy positioning and depth location.

8.3.11　In case of positioning by pipeline detecting instrument，the maximum value method or the minimum value method may be used.The two methods should be integratedly applied to decide the plane position of pipeline after comparison and analysis.

8.3.12　The methods for depth location by pipeline detecting instrument mostly include feature point method（ΔHx percentile method，Hx feature point method），direct reading method and 45° method.In detecting process，different methods should be integratedly applied，and method experiment shall be conducted first based upon different circumstances to select the proper depth location method.The depth location shall meet the following requirements:

1　The position of depth location point should be placed on the straight section without branch or bend in the middle.

2　The piping extension length at both sides of the depth location point shall be no less than 3 times～4 times of the burial depth of the pipeline center.

3　Before depth determination，the plane position of the depth location point shall be decided precisely on site.

4　For depth location by direct reading method，the receiver should be kept vertical，and direct reading result shall undergo depth correction according to the depth location correction coefficient determined by method experiment.

8.3.13　In order to distinguish two or more than two parallel piping or cables，direct method or pliers clamp method should be used，and distinguished by respectively and directly imposing signal on each pipeline.When electromagnetic induction method is used，the pipeline should be distinguished by changing the position and status of transmitting device and the frequency and magnetic moment transmitted，and by analyzing features of signal including abnormal intensity and width.

8.3.14　When multiple pipelines run in parallel，section detecting should be done，and positioning and depth location should be done respectively by inversion.

8.3.15　For detecting of underground pipelines by direct method or charging method，electrical contact shall be maintained，the grounding electrode shall be set up appropriately，and there shall be good grounding condition for the ground connection point.

8.3.16　In case of electromagnetic induction method being used for underground pipeline detecting，the transmitter and pipeline shall be kept at the optimal coupling condition，and optimal receiving/transmitting distance shall be maintained between the receiver and transmitter.When there is interference existing in the nearby area，method experiment shall be done to determine the method of excluding or minimizing interference.

8.3.17　In complex case，comprehensive geophysical detecting method may be used for detecting.For questionable point with intensive interference where the geophysical detecting methods are invalid，the excavation，feeler inspection or drill-rod detection methods shall be used for detecting，but the safety of pipeline must be ensured.