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GEOLOGY & GEOPHYSICS
versely, should the aim of the survey be, for example, imaging shallow gas in the overburden of a production field, EM streamer length
can be reduced to minimize operational risk during close passes.
The EM streamer is towed at a depth of up to 100 m (328 ft). This
can be reduced in shallow-water areas, as in the case of the 2013
Celtic Sea survey. This was acquired with the EM streamer at 50 m
(164 ft) and the broadband marine seismic system at 25 m (82 ft).
It should be noted that in contrast to streamer seismic, both 2D and
3D towed streamer EM projects are acquired with a single EM streamer.
Line spacing of less than 1. 5 km (0.9 mi) allows for anisotropic inversion
of both 2D resistivity sections (one per sail line) as well as a 3D resistivity volume. Projects with a line spacing of more than 1. 5 km (0.9 mi) are
considered 2D with resistivity sections generated from 2.5D inversion.
The EM streamer has 72 offset pairs, ranging in length from 200
m (656 ft) at the near offset to 1,100 m ( 3,608 ft) at the far offset. This
provides exceptionally dense resistivity data which is rich in both offsets and frequencies. This dataset enables optimization of data selection for inversion. These parameters can be evaluated during the feasibility study phase of a project, ensuring selection of the frequencies
and offsets which will be most useful to image the target in question.
It is this density of data which provides improved resolution in resistivity profiles and volumes and enables detailed reservoir level quality
improvement workflows to be applied.
Simultaneously towed streamer EM and GeoStreamer acquisition
provides additional benefits on top of the cost savings and improved
resolution of resistivity data discussed. HSEQ exposure is reduced,
as both datasets are acquired using a single vessel, less fuel is con-
sumed, and fewer vessel days are required compared to acquiring two
separate surveys. Operationally, the addition of EM to a 2D broad-
band marine seismic survey requires minimal extra personnel; the
equipment is deployed and operated by a standard seismic crew with
the addition of one or two EM specialist field engineers.
EM technology has traditionally been used as a de-risking tool,
in part due to the diffusive nature of the technique resulting in rela-
tively low resolution when compared to seismic. However, with the
advent of towed streamer EM and associated high-density data,
more focused, high-resolution applications have become viable. By
integrating seismic and resistivity data, the ability to estimate hydro-
carbon saturation can be applied in mitigating drilling hazards as
well as field development optimization. It has the additional potential
to be applied in a 4D sense, in appropriate environments.
Near-field exploration is of particular interest at present, as highlighted
in the article “Brownfields: making the most of mature fields,” which ap-
peared in the May 2016 issue of Offshore. If survey design is optimized
through a project specific feasibility study, the towed streamer EM data
acquired to investigate potential drilling hazards can also be used to inves-
tigate the near field potential. With the integration of seismic, resistivity,
and well log data reservoir characterization and QI workflows have been
developed, enabling the estimation of hydrocarbon volumes in place.
PGS’ towed streamer EM system resulted in a step change in
CSEM acquisition efficiency, in conjunction with dramatically improved sampling density. This advance has expanded the technology’s application from frontier exploration to more detailed reservoir
imaging and characterization. •
PGS wishes to acknowledge the assistance of the International Association of
Geophysical Contractors in the development of this article.