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

Abstract

To image the internal resistivity structure of the North Alex mud volcano offshore Egypt, the marine electromagnetics group at the Helmholtz Centre for Ocean Research Kiel (GEOMAR) developed and conducted a novel transient marine controlled-source electromagnetic experiment. The system, which was specifically developed to image the mud volcano, is also generally suitable for surveys of other small seafloor targets, such as gas-hydrate reservoirs, fluid-flow features, and submarine massive-sulfide deposits. An electric bipole antenna is set down by a remotely operated vehicle on the seafloor sequentially in two perpendicular polarizations at each transmission station. Two orthogonal horizontal electric field components are recorded on the seabed by an array of independently deployed nodal receivers (RXs). With two transmitter polarizations, the unique acquisition geometry of the system provides a very rich data set. However, for this geometric setup, conventional marine electromagnetic interpretation schemes (such as normalized magnitude variation with offset plots) have been difficult to implement. We have developed a simple imaging technique, which can be used for a first-step mapping of seafloor apparent resistivity with the GEOMAR system. Images can be produced in just a few minutes on a regular laptop computer, and the robustness of the approach was demonstrated using two synthetic data sets from simple seafloor models. The method was then applied to the real data acquired at the North Alex mud volcano in 2008. Results found increased apparent sediment resistivities of up to 4 Ωm near the center of the mud volcano occurring at source-RX offsets greater than 500 m, which mapped to apparent depths of greater than 150 m. This may be caused by large quantities of free gas or freshwater in the sediment pore space.

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

“…salinity and temperature) as well as the geotechnical parameters are sufficiently well known, and gas saturation levels are estimated to falls in a range between 0 and 25 per cent for observed bulk resistivities between 0.5 and 0.7 m. Consequently, high temperatures in the central part of the mud volcano lower bulk resistivities and, thus, mask the presence of potentially significant amounts of gas in the resistivity section. Nevertheless, the estimated saturation levels are a quantitative measure, which could not be deduced from apparent resistivities in Swidinsky et al (2015). Furthermore, they complement the information from reflection seismics, which is indicative for the existence of gas but not suitable to determine an upper bound on the saturation level.…”

“…The calculation of apparent resistivities from one of the rotational invariants, which is useful for the fast imaging of data (step 2), is published in a companion paper (Swidinsky et al 2015). The dimensionality analysis (step 4) and the 3-D interpretation (step 5) are both work in progress and will be included in future publications.…”

“…In this paper, we will show that rotational invariance can be established for the marine case, since it is only necessary to normalize the measured fields by a quantity, which reflects the source geometry. In the aforementioned companion paper we show how these invariants can be used to derive an apparent resistivity, which can be used for the quick imaging of CSEM data of experiments with two transmitter polarizations (Swidinsky et al 2015).…”

“…We have used this finding to derive an apparent resistivity transformation. In a companion paper by Swidinsky et al (2015) we use this invariant to define an apparent resistivity transformation, which can be used for a fast imaging of CSEM data, which was measured with two separate TX polarizations. For I 4 and even more so for I 2 (Fig.…”

Geoelectric hazard assessment: the differences of geoelectric responses during magnetic storms within common physiographic zones

“…In addition to direct monitoring of the geoelectric field, one can estimate the geoelectric field though characterizing and mapping the behavior of the Earth's ground-level geomagnetic field across broad geographic regions (e.g., Love et al 2016) and characterizing an impedance (e.g., Chave 2012) at a given location, which is dependent on the 3-dimensional conductivity distribution of the Earth's interior (e.g., Bedrosian and Feucht 2014;Meqbel et al 2014;Alekseev et al 2015). Subsurface conductivity can also be characterized with active sources, as described in works such as Spies and Frischknecht (1991) on land or Swidinsky et al (2015) in the marine environment; such conductivity models can be used to supplement impedances typically constructed using passive magnetotelluric methods.…”