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Part D - Minimum requirements on site investigations
Part D: Minimum Requirements on site investigations for cable routes
for the inter-array network and the power cables
The objective of geophysical and geotechnical survey of cable route corridors is to gain
sufficient information regarding the characteristics and the geology of the seabed such that
cables can be installed in the proper manner. The findings of such investigations are used on
the one hand to optimise a planned cable route corridor whilst taking unknown situations into
account whilst also establishing a suitable installation method. The findings must therefore
make it possible to determine the quality of the seabed regarding cable layering and to calculate
the expected installation depth for the planned installation method (so-called “Burial
Assessment”) within each part of the cable route corridor.
Measurement of the cable route corridor must cover the entire cable corridor. This applies to
both the horizontal coverage as well as the measurement of vertical sections. It is usually expedient
to separate the measurement into two independent exploration phases which are organised
into one measurement cruise.
The first phase contains the geophysical measurements so that a linear insight and overview
of the measured area is gained. During this first measurement cycle, bathymetric measurements
shall be carried out with a multi-beam echosounder (MBES), recording of the surface of
the sea bed shall be carried out with SSS as well as subbottom profiling (e. g. boomers, chirp
sonar pinger, parametric echosounder) for determining geological units within the profile. It is
strongly recommended to combine the SSS measurement with metal detection measurements.
Detailed requirements of the respective measuring principle can be seen in Table 10.
The scope of sediment sampling or geotechnical investigations is established on the basis of
the assessed and interpreted geophysical measurement data. In case of cable route corridor
measurements previously carried out in the EEZ within the North Sea, the median sampling or
probing intervals were typically up to 4 km in geologically homogeneous cable corridor sections,
but in more geologically complex cable corridor sections it was typically 0.5 km. It may
be necessary to go under this interval to clarify complicated geological conditions.
After assessing and interpreting the geophysical test results, locations are selected for the
geotechnical and geological sampling based on the knowledge gained, and such locations
should enable a comprehensive knowledge of the geological conditions to be gained along
the cable route corridor with regard to the installation of cables. This covers the sampling of
all geophysical units which are identified and mapped on the basis of the investigation findings
with SSS or subbottom profiling measurements. Locations which appear to make geological
interpretations from geophysical measurements more complicated should also be
sampled. If the geophysical test results show conspicuous and/or unusual seismic reflectors
then it is strongly recommended that these are investigated with geotechnical methods. This
is particularly so when e. g. peat, clay lenses, till etc. are expected. For sampling or geotechnical
examination of the sediments, vibrocoring equipment, piston and gravity corers, grippers
as well as cone penetration testing (CRT) can be used. Further details regarding minimum
requirements can be found in Table 10.
The results of cable corridor measurements are illustrated in an independent, comprehensive
report. Alongside a short project description it is important to name the persons involved in
the project, describe the systems used for gathering data as well as illustrate the measuring
procedures. Data processing as well as subsequent assessment and interpretation must also
be explained. Geophysical and geotechnical findings shall be presented separately before a
