Implications for the fluid The RAMESSES Experiment III: Constraints on the model

Along axis variability

The bathymetric map of the AVR and surrounding area (Figure. 1) shows that although the main bathymetric features are elongated parallel to the strike of the AVR, neither the AVR itself, nor the surrounding topographic features, are strictly 2-dimensional. It was shown earlier that the broad shape of the central AVR, which has only a small effect on the off axis ELF data, cannot explain the on axis 0.35 Hz data, which require the more pointed topography seen at the northern end of the AVR. In addition, magnetotelluric data from instrument Kermit at the southern end of the AVR show no evidence for the low crustal resistivities which are required by the data from sites further north (Heinson , this issue).

It is therefore important to examine the possible effect on the response if the mid-crustal low resistivity anomaly were discontinuous under the AVR itself. This can be accomplished to some degree using a 2-dimensional approach, by assuming that the axis of the AVR runs perpendicular to the direction in which the resistivity is invariant. The source is also oriented in this direction. The effect of along axis variability in the structure beneath the AVR can then be studied by examining the behaviour of the source parallel component of the seafloor electric field.

Figure 8 shows the effect on the response of a gap in the low resistivity layer. Also shown are the responses of two 1-dimensional models, one with a continuous layer of low resistivity, and one without the low resistivity layer. The effect of the gap in the 0.2  layer is to interrupt the channelling of current, and therefore the degree of enhancement of the fields along the axis of the dipole is severely reduced. Since the low resistivity layer can no longer efficiently channel current, its effect for a receiver on the far side of the gap from the transmitter is to attenuate fields relative to the model without the 0.2  layer. It can also be seen from Figure. 8 that the reduction in enhancement begins before the gap itself is reached. Although approximate, these results suggest that in order to see an enhancement effect to a source receiver range of 15 km, the low resistivity anomaly must be continuous, at least along the northern half of the AVR.

Implications for the fluid The RAMESSES Experiment III: Constraints on the model

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