Marine Controlled-Source Electromagnetic Response: Campos Basin Experience
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 18003, "Marine Controlled- Source Electromagnetic: The Campos Basin Experience," by M.P.P. Buonora, Petrobras S.A., and R. Reddig and A. Zerilli, AGO Schlumberger, prepared for the 2006 Offshore Technology Conference, Houston, 1–4 May.
The marine controlled-source electromagnetic (MCSEM) response of known reservoirs yields anomalies that, although considered marginal, can be imaged with new processing and interpretation procedures. There is evidence of correlations between the anomalies and reservoirs. A proposed workflow enhances the ability to interpret weak anomalies accurately and gain benefit from the additional MCSEM data.

Introduction
The Campos basin MCSEM data were acquired on a rectangular grid with a line spacing of approximately 5 km. Three-component electrical-field and two-component magnetic-field data were recorded. All fields at each receiver location were processed and interpreted through a novel workflow. The main objective of the survey was to calibrate the MCSEM technology over known reservoirs and quantify anomalies associated with those reservoirs, with the expectation that a new prospective location(s) could be found. A further objective was to establish an industry-standard workflow.
In recent years, MCSEM technology has drawn the attention of major oil companies with its sensitivity in mapping resistive structures (such as hydrocarbon reservoirs) beneath the ocean bottom. Many surveys have been carried out on a variety of prospect scenarios in marine environments of north-west Europe, Africa, the Mediterranean, Southeast Asia, and offshore Brazil, and many have been verified with wells. MCSEM technology is in the early stage of application to real E&P problems, and a great deal of R&D is needed to improve its efficiency and reliability in acquisition hardware, accurate survey engineering, data processing, multidimensional modeling, and inversion. The success of MCSEM technology will depend on integration with seismic, geologic, and petrophysical data for the area under investigation.

Offshore Brazil
Petrobras has acquired approximately 1600 km of MCSEM data offshore Brazil, encompassing 36 towed lines as shown on Fig. 1. Data were acquired over three major sectors: southern sector covering a small portion of the Santos basin (339 km), central sector on portions of the Campos basin (1121 km), and northern sector on the Espirito Santos basin (153 km of towed lines). The Campos basin survey used two distinct tow patterns: star-like shape (green lines on Fig. 1), and 5-km rectangular grid (red lines on Fig. 1). This study focused on data acquired from the 5-km rectangular-grid tow pattern.
The reservoir is a rectangular thin sheet 3 km wide, 10 km long, and 50 m thick with resistivity of 10 Ω⋅m buried 1 km below the seafloor. The reservoir is illuminated by an electrical horizontal-dipole source (transmitter). Resistivities for the 1.5-km water layer and for the surrounding background were set to 0.3 and 0.8 Ω⋅m, respectively. The model parameters were derived from a nearby well that encountered the reservoir at the same depth.