Data reduction The RAMESSES Experiment III: The CSEM method

The experiment

The source, DASI (Deep Towed Active Source Instrument), consists of a deep towed vehicle which in turn tows a neutrally buoyant streamer that forms a 100 m long dipole (for details see Sinha etal, 1990). The DASI has a source moment of approximately 10 ,000 Am, and is towed at heights of 50-80 m above the sea bottom, monitored acoustically by a 3.5 kHz echo-sounder mounted on the deep tow vehicle. The source is kept as close as possible to the seafloor to ensure good coupling of electromagnetic energy into the crust, without coming into contact with the rugged ridge topography and risking damage. A high voltage shipboard power supply unit generates a 256 Hz signal which is passed via an armoured coaxial cable to the DASI package, where it is transformed to a high current output, typically 300 A peak to peak. Four ancillary electrodes spaced along the source dipole are connected to a data logger mounted on the deep tow vehicle (the DASI logger) and are used to monitor the transmitted fields. The location of the source is determined by acoustic ranging between the ship, the deep tow and the seafloor instruments, augmented by an array of navigation transponders.

Three types of receiver were deployed. ELF (ELectric Field) and LEMUR (Low Frequency ElectroMagnetic Underwater Receiver) instruments measure two components of the horizontal electric field at the seafloor, using HED receivers (Young & Cox, 1981; Sinha etal, 1990; Constable & Cox, 1996). Each 10 m receiver dipole consists of a pair of silver/silver chloride electrodes supported at the ends of orthogonal plastic arms. These instruments can detect signals to a source-receiver separation of approximately 15 km. LEM (Long-wire ElectroMagnetic) instruments have a single long antenna (300 m in this case) deployed on the seafloor behind the instrument package (Webb etal, 1985; Constable & Cox, 1996). Although only able to detect one component of the seafloor electric field, the long receiver dipole length makes these instruments more sensitive and less susceptible to internal instrumental noise than the short arm ELFs and LEMURs, extending the source-receiver range to around 90 km. The ELF instruments were modified from earlier experiments by replacing tape drives with high capacity disk drives, allowing the stacking algorithm used previously to be replaced with the collection of continuous time series at 64 Hz sampling. All the LEMs and several of the ELFs were equipped with MT amplifiers for use in the MT experiment described by Heinson (this issue), as well as CSEM amplifiers. Instrument positions were determined to within 50 m by acoustic surveying.

The experimental geometry is illustrated in Figure. 1. Instruments were deployed in lines along and across the AVR axis, coincident with the seismic wide angle profiles (Navin , this issue). The short arm ELFs Quail, Noddy and Kermit were deployed close to the axis, with the long wire LEMs, Macques, Rhonda, Lolita and Pele at longer range to the east of the axis. Modelling by Unsworth (1991) suggests that trans-ridge transmission into an array of seafloor instruments on the opposite side of the axis from the source is the optimum geometry for detecting sub-axial crustal resistivity structure. The geometry of the experiment was designed with this in mind, to maximise the sensitivity of the data to both 1- and 2-dimensional structure.

The first source tow was down the crest of the AVR and was cut short due to source failure after three hours of transmission. However, the source passed directly over ELF Quail, providing valuable short range data capable of constraining the shallow resistivity structure on axis. Frequencies of 0.35 Hz and 11 Hz were transmitted in a pattern of low and high frequency bursts. The second tow, 4 km west of the AVR axis, was completed successfully with seven hours of 0.75 Hz transmission both along and through the axial region. DASI tows are carried out at a speed of 1 - 2 knots (approximately 1 m/s) to minimise the phase shift at the receivers during synchronous detection of the signal, reduce wear on the deep tow and maintain small tow cable angles. It thus takes a considerable amount of time to cover significant distances with the source, and instrument failure coupled with unfavourable weather conditions limited total transmission time to ten hours.


Data reduction The RAMESSES Experiment III: The CSEM method

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