The use of rotational invariants for the interpretation of marine CSEM data with a case study from the North Alex mud volcano, West Nile Delta

Abstract

S U M M A R YSubmarine mud volcanos at the seafloor are surface expressions of fluid flow systems within the seafloor. Since the electrical resistivity of the seafloor is mainly determined by the amount and characteristics of fluids contained within the sediment's pore space, electromagnetic methods offer a promising approach to gain insight into a mud volcano's internal resistivity structure. To investigate this structure, we conducted a controlled source electromagnetic experiment, which was novel in the sense that the source was deployed and operated with a remotely operated vehicle, which allowed for a flexible placement of the transmitter dipole with two polarization directions at each transmitter location. For the interpretation of the experiment, we have adapted the concept of rotational invariants from land-based electromagnetics to the marine case by considering the source normalized tensor of horizontal electric field components. We analyse the sensitivity of these rotational invariants in terms of 1-D models and measurement geometries and associated measurement errors, which resemble the experiment at the mud volcano. The analysis shows that any combination of rotational invariants has an improved parameter resolution as compared to the sensitivity of the pure radial or azimuthal component alone. For the data set, which was acquired at the 'North Alex' mud volcano, we interpret rotational invariants in terms of 1-D inversions on a common midpoint grid. The resulting resistivity models show a general increase of resistivities with depth. The most prominent feature in the stitched 1-D sections is a lens-shaped interface, which can similarly be found in a section from seismic reflection data. Beneath this interface bulk resistivities frequently fall in a range between 2.0 and 2.5 m towards the maximum penetration depths. We interpret the lens-shaped interface as the surface of a collapse structure, which was formed at the end of a phase of activity of an older mud volcano generation and subsequently refilled with new mud volcano sediments during a later stage of activity. Increased resistivities at depth cannot be explained by compaction alone, but instead require a combination of compaction and increased cementation of the older sediments, possibly in connection to trapped, cooled down mud volcano fluids, which have a depleted chlorinity. At shallow depths (≤50 m) bulk resistivities generally decrease and for locations around the mud volcano's centre 1-D models show bulk resistivities in a range between 0.5 and 0.7 m, which we interpret in terms of gas saturation levels by means of Archie's Law. After a detailed analysis of the material parameters contained in Archie's Law we derive saturation levels between 0 and 25 per cent, which is in accordance with observations of active degassing and a reflector with negative polarity in the seismics section just beneath the seafloor, which is indicative of free gas.

Rapid resistivity imaging for marine controlled-source electromagnetic surveys with two transmitter polarizations: An application to the North Alex mud volcano, West Nile Delta

“…In Hölz et al (2015), we more rigorously examine a variety of rotational invariants, all of which can be used to interpret data from an electromagnetic survey with two TX polarizations that are not necessarily perpendicular. Indeed, under the assumptions of two perpendicular TX antennas with equal source moment, our invariant given in equation 5 is identical to the first invariant I1 described in this companion paper.…”

“…The focus of this paper is on the eastern mud vol-cano, North Alex, which is located in close proximity to several gas production fields, and therefore it is a feature of interest to the oil and gas industry. Figure 1b shows a 2D seismic image across the center of North Alex (modified from Hölz et al, 2015). A near-surface gas cap is clearly visible (at a depth of no more than approximately 10 m beneath the seafloor, although in some cases almost reaching the surface) but the seismic image beneath this horizon is highly incoherent due to the lack of underlying reflectors by the active reworking of the sediments, as well as from scattering and attenuation by the overlying low-velocity, gas-saturated zone.…”

“…A modified version of this technique was adapted and modified by the marine electromagnetics group at GEOMAR for the WND project. In a companion paper by Hölz et al (2015), we describe the experiment and instrumentation in detail. Here, we will summarize some of the most important aspects of the experiment and describe a method to rapidly map seafloor resistivity using this type of system.…”

“…Figure 2 shows a sketch of the GEOMAR controlled-source electromagnetic (CSEM) system. A remotely operated vehicle (ROV) places an electric bipole TX stationary on the seafloor in an arbitrary orientation; the 9.1-m-long bipole TX injects a time-varying current of 23.4-A Hölz et al, 2015).…”

“…This paper is mainly concerned with step (2), which has required the development and examination of some new theory. In our companion paper by Hölz et al (2015), we describe and examine step (3) in detail also using the WND data set. In the present paper, we will first describe the fundamental theory of a rapid imaging technique.…”