Three methods for mitigating airwaves in shallow water marine controlled-source electromagnetic data

Abstract

In the past several years, marine controlled-source electromagnetic (MCSEM) techniques have been applied successfully in deep water (depth > 1 km) for oil and gas exploration. The application of this technology in shallow water is challenged, however, because of “airwaves” that mask the signal from the target reservoir at depth. Based upon the understanding that an airwave is a lateral wave, which can be analytically expressed in a dual-half-space resistivity model, we propose three airwave-mitigation approaches to reduce the effects of these airwaves on MCSEM data. In the EM “x-bucking” approach, the effect of the airwaves can be “bucked” out from two measurements by using the analytic expression of the airwave. The frequency derivative (dE/dFreq) approach takes advantages of the unique characteristics of the airwaves in frequency domain, enhancing the reservoir signals while suppressing the airwave. The magnetotelluric (MT) stripping method uses the plane-wave feature of the airwaves and subtraction of the lateral wave electric component, which is obtained from measured marine MT impedance and controlled-source electromagnetics (CSEM) data, to generate a new data set in which the effects of the airwaves are removed substantially. By comparing the detectability, which is defined as the ratio of inline Ex fields between a reservoir model and a corresponding baseline model, for a reservoir target in deep water versus shallow water with a moderate 2D bathymetry, we show that the effects of the airwaves in shallow water can be reduced in the data, leading to greater reservoir detectability. In addition, these approaches have been applied successfully to a real shallow water MCSEM data set in which the detectability to the deeper resistive basement is enhanced.

Research on the Propagation of Extremely Low Frequency Electromagnetic Wave in Shallow Sea Area

“…Extremely low frequency (ELF) electromagnetic (EM) wave radiation and propagation underwater has attracted great attention for a long time [1], such as underwater target detection [2,3], submarine navigation and communications [4][5][6], and marine controlled source electromagnetic (MCSEM) [7,8]. In shallow sea area, EM methods are reconsidered for communications between autonomous underwater vehicles (AUV) while acoustic communication is limited by ambient interference, unlike in deep water.…”

Marine controlled-source electromagnetic interferometry

“…Various approaches like modelling and adjacent subtraction (Nordskag & Amundsen, 2007), weighted differences between pairs of receivers or sources (Chen & Alumbaugh, 2011;Løseth et al, 2010), taking the frequency derivative (Chen & Alumbaugh, 2011;Maaø & Nguyen, 2010) or a difference between fields at two frequencies that are further apart (Wirianto et al, 2011), or magnetotelluric impedance stripping (Chen & Alumbaugh, 2011) exist. Assuming that only the upward decaying field at the receiver level bears information about the subsurface and that the field which interacted with the air-water interface is downward decaying at the receiver level, Amundsen et al (2006) proposed to decompose the diffusive fields into upward and downward decaying fields in order to remove the effects related to the air-water interface.…”

“…For frequency-domain CSEM, different approaches exist to minimize the effects of the airwave from the data. These approaches include modelling and adjacent subtraction (Nordskag & Amundsen, 2007), weighted differences between pairs of receivers or sources (Løseth et al, 2010;Chen & Alumbaugh, 2011), taking the frequency derivative (Maaø & Nguyen, 2010;Chen & Alumbaugh, 2011) or a difference between fields at two frequencies that are further apart (Wirianto et al, 2011), magnetotelluric impedance stripping (Chen & Alumbaugh, 2011), wavefield decomposition (Amundsen et al, 2006), and interferometry (Wapenaar et al, 2008b). Weiss (2007) shows that the CSEM time-domain response from a subsurface resistor in shallow water may be easier to distinguish from the airwave arrival than in the frequency domain because of the possible separation of the two events in time.…”

Effects of the airwave in time-domain marine controlled-source electromagnetics

“…For frequency-domain CSEM, different approaches exist to minimize the effects of the airwave from the data. These approaches include modeling and adjacent subtraction (Nordskag and Amundsen, 2007), weighted differences between pairs of receivers or sources (Løseth et al, 2010;Chen and Alumbaugh, 2011), taking the frequency derivative (Maaø and Nguyen, 2010;Chen and Alumbaugh, 2011) or a difference between fields at two frequencies that are further apart (Wirianto et al, 2011), magnetotelluric impedance stripping (Chen and Alumbaugh, 2011), wavefield decomposition (Amundsen et al, 2006), and interferometry (Wapenaar et al, 2008). Weiss (2007) shows that the CSEM time-domain response from a subsurface resistor in shallow water may be easier to distinguish from the airwave arrival than in the frequency domain because of the possible separation of the two events in time.…”