Controlled-source Electromagnetic System Research Articles

SWAN: A surface-towed modular controlled-source electromagnetic system for mapping submarine groundwater discharge and offshore groundwater resources

Offshore freshened groundwater (OFG) and submarine groundwater discharge (SGD) are important components of coastal hydrologic systems. A lack of understanding of offshore groundwater systems and their interactions with onshore systems along the majority of global coastlines still exists due to a general paucity of field data. Recently, controlled-source electromagnetic (CSEM) techniques have emerged as a promising noninvasive method for identifying and characterizing OFG and SGD. Unfortunately, only a few systems are available in academic and research institutions worldwide, and applications are limited to specific regions. These systems are often limited by relatively high deployment costs, slow data acquisition rates, logistical complexity, and lack of modification options. A relatively inexpensive and user-friendly CSEM system is needed to overcome these limitations. We present the initial theoretical and practical developments of SWAN — a low-cost, modular, surface-towed hybrid time-frequency domain CSEM system capable of detecting OFG and SGD to water depths of 100 m. A field test of the system was carried out in the central Adriatic Sea at water depths between several tens to approximately 160 m to illustrate its capabilities. Through its ability to facilitate continuous measurements in both the time and frequency domain, the system has demonstrated its effectiveness in acquiring high-quality data while operating at towing speeds ranging from 2.5 to 3 kn. The resulting data coverage enables the system to detect variations in subsurface resistivity to depths of approximately 150–200 m below seafloor. With its modular, user-friendly design, SWAN provides an accessible, cost-efficient means to investigate the hydrogeology of shallow offshore environments.

Advances in Marine Intelligent Electromagnetic Detection System, Technology, and Applications: A Review

This paper introduces the current state of the art in marine intelligent electromagnetic detection sensors, systems and platforms for the detection and health monitoring of offshore structures, especially for subsea cables and pipelines. The presented survey is by no means exhaustive, but introduce some notable results and relevant techniques in the detection and inspection of marine structure electromagnetic monitoring fields. Particularly, this paper presents a review of the main research works focusing on electromagnetic detection techniques and intelligent marine vehicles for subsea cables. The marine electromagnetic exploration techniques are elaborated as active and passive detections based on different electromagnetic field generation and sensing principles. The pulse induction-based detection method for subsea ferromagnetic lines detection, transient and controlled-source electromagnetic systems in marine geological fields under the category of active detection approach, are reviewed. According to the classical applications on subsea cable-like target inspection, the passive detection approach is detailed classified as crisscrossing detection and along-tracking detection, which are characterised by different trajectories of detection vehicles. Finally, the paper discusses the marine electromagnetic detection techniques in general and highlights challenges that need to be addressed in developing detection vehicles with lower electromagnetic noises and advanced autonomy on carrying out detection missions.

Drone-towed controlled-source electromagnetic (CSEM) system for near-surface geophysical prospecting: on instrument noise, temperature drift, transmission frequency, and survey set-up

Abstract. Drone-borne controlled-source electromagnetic (CSEM) systems combine the mobility of airborne systems with the high subsurface resolution in ground systems. As such, drone-borne systems are beneficial at sites with poor accessibility and in areas where high resolution is needed, e.g. for archaeological or subsurface pollution investigations. However, drone-borne CSEM systems are associated with challenges, which are not observed to the same degree in airborne or ground surveys. In this paper, we explore some of these challenges based on an example of a new drone-towed CSEM system. The system deploys a multi-frequency broadband electromagnetic sensor (GEM-2 uncrewed aerial vehicle, UAV), which is towed 6 m below a drone in a towing-bird configuration together with a NovAtel GNSS–IMU (global navigation satellite system–inertial measurement unit) unit, enabling centimetre-level position precision and orientation. The results of a number of controlled tests of the system are presented together with data from an initial survey at Falster (Denmark), including temperature drift, altitude vs. signal, survey mode signal dependency, and the effect of frequency choice on noise. The test results reveal the most critical issues for our system and issues that are likely encountered in similar drone-towed CSEM set-ups. We find that small altitude variations (± 0.5 m) along our flight paths drastically change the signal, and a local height vs. signal correlation is needed to correct near-surface drone-towed CSEM data. The highest measured impact was −46.2 ppm cm−1 for a transmission frequency of 91 kHz. We also observe a significant increase in the standard deviation of the noise level up to 500 % when going from one transmission frequency to five. We recommend not to use more than three transmission frequencies, and the lowest transmission frequencies should be as high as the application allows it. Finally, we find a strong temperature dependency (up to 32.2 ppm∘C-1), which is not accounted for in the instrumentation.

Time synchronization and data transfer method for towed electromagnetic receiver.

Deep marine controlled-source electromagnetic (CSEM) prospecting has attracted extensive interest because it enables high efficiency and high horizontal-resolution prospecting of gas hydrate and oil. However, the elimination of time errors between the transmitter and the receiver and realization of long-distance high-speed real-time data transmission (submarine towed body status information and raw electromagnetic field data stream) are worthwhile challenges that require continuous effort. We developed a novel towed CSEM system using double-vessels that have high time synchronization accuracy and real-time data transmission. The near-seafloor-towed CSEM receiver contains a deck user terminal, master node, slave nodes, tail buoy, and neutrally buoyant towed cable. The deck user terminal generated and transmitted a pulse per second to the master node through a fiber converter and optical fiber. The RS-485 transceiver then turned the pulse signal into a differential signal and transmitted it to each slave node for error-free synchronization. The time information was also transmitted from the deck user terminal to various nodes through ethernet switches, optical fibers, and serial to ethernet converters. The deck user terminal can conveniently communicate with each node cascaded by the ethernet switch through ethernet and fiber optic communication technology. During an offshore experiment involving oil and gas exploration in the South China Sea, the towed CSEM receiver continuously acquired all electromagnetic components and status information, which achieved a preliminary prospecting result. The maximum transfer rate of real-time data can reach 10 Mbps with 300 m distance between each slave node, and the time synchronization error between transmitter and receiver is less than ±3 µs.

Surface towed electromagnetic system for mapping of subsea Arctic permafrost

Sea level has risen globally since the late Pleistocene, resulting in permafrost-bearing coastal zones in the Arctic being submerged and subjected to temperature induced degradation. Knowing the extent of permafrost and how it changes over time is important for climate change predictions and for planning engineering activities in the Arctic environment. We developed a controlled source electromagnetic (CSEM) method to obtain information on the depth, thickness, and lateral extent of marine permafrost. To operate in shallow water we used a surface towed electric dipole–dipole CSEM system suitable for deployment from small boats. This system was used to map permafrost on the Arctic shelf offshore Prudhoe Bay, Alaska. Our results show significant lateral variability in the presence of permafrost, with the thickest layers associated with a large river outflow where freshwater influx seems to have a preserving effect on relict subsea permafrost.

The marine controlled source electromagnetic response of a steel borehole casing: applications for the NEPTUNE Canada gas hydrate observatory

ABSTRACTGas hydrates are a potential energy resource, a possible factor in climate change and an exploration geohazard. The University of Toronto has deployed a permanent seafloor time‐domain controlled source electromagnetic (CSEM) system offshore Vancouver Island, within the framework of the NEPTUNE Canada underwater cabled observatory. Hydrates are known to be present in the area and due to their electrically resistive nature can be monitored by 5 permanent electric field receivers. However, two cased boreholes may be drilled near the CSEM site in the near future. To understand any potential distortions of the electric fields due to the metal, we model the marine electromagnetic response of a conductive steel borehole casing. First, we consider the commonly used canonical model consisting of a 100 Ωm, 100 m thick resistive hydrocarbon layer embedded at a depth of 1000 m in a 1 Ωm conductive host medium, with the addition of a typical steel production casing extending from the seafloor to the resistive zone. Results show that in both the frequency and time domains the distortion produced by the casing occurs at smaller transmitter‐receiver offsets than the offsets required to detect the resistive layer. Second, we consider the experimentally determined model of the offshore Vancouver Island hydrate zone, consisting of a 5.5 Ωm, 36 m thick hydrate layer overlying a 0.7 Ωm sedimentary half‐space, with the addition of two borehole casings extending 300 m into the seafloor. In this case, results show that the distortion produced by casings located within a 100 m safety zone of the CSEM system will be measured at 4 of the 5 receivers. We conclude that the boreholes must be positioned at least 200 m away from the CSEM array so as to minimize the effects of the casings.