Acquisition of Magnetotelluric Data for Studies of Pipe-to-Soil Potentials on Pipelines: Preliminary Results From the 2003 Ottawa River Valley Survey
Abstract
A long-term study has been initiated as part of the POLARIS project to investigate the effect of Earth resistivity on pipe-to-soil potentials (PSPs) on pipelines. The pipeline chosen for investigation is a natural gas pipeline extending from the Alberta/Saskatchewan border to Quebec and Vermont. A previous study of this pipeline revealed large PSPs variations near the contact of the Precambrian Shield and Paleozoic sedimentary rocks of the Ottawa Embayment in eastern Ontario. The POLARIS Ottawa River Valley Survey was done in October 2003 to investigate the PSPs. Objectives included: (1) magnetotelluric (MT) soundings to define the sub-surface resistivity structure; (2) simultaneous recording of MT and PSP time-series; and (3) field-testing MT instruments acquired for the POLARIS project. Remote-referenced MT soundings were made at four sites spaced 25 km apart along a NNW-SSE line parallel to the pipeline and perpendicular to the geological contact. Recordings were made using audiofrequency and broad-band equipment allowing the MT impedance to be defined between 10,000 Hz and 0.001 Hz and providing continuous electric and magnetic field time series with 15 Hz sampling frequency for time segments of up to 12 hours duration. During the MT survey, the pipeline was instrumented with 5 dataloggers measuring PSPs at a sampling rate of 1 Hz. This survey may be the first one in which simultaneous and co-located MT and PSP recordings have been made. The MT responses sampled the Earth resistivity at depths ranging from tens of metres to tens of kilometres. The results reveal that the most conductive responses occur at site ORV003, at the margin of the Paleozoic sedimentary rocks. However, the enhanced conductivity at this site is not caused by the Paleozoic rocks. Modelling of the data from ORV003 indicates a contribution to the response from a 30 m thick, conductive (2 ohm.m) surface layer of glaciomarine and marine silt and clay sediments and also a contribution from conductive rocks of the Precambrian basement at depths exceeding 5 km. The next step in quantifying PSP fluctuations consists of combining electric field data with a model of the pipeline based on a distributed-source transmission line model. The PSP modeling results will be further refined using the 2-D resistivity model defined by the MT data.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2004
- Bibcode:
- 2004AGUSM.S31A..05S
- Keywords:
-
- 0600 ELECTROMAGNETICS;
- 0900 EXPLORATION GEOPHYSICS;
- 0925 Magnetic and electrical methods;
- 0994 Instruments and techniques;
- 1515 Geomagnetic induction