18

Part  B  -  Minimum  Requirements  on  geological  survey

Table  5:  Requirements  of  seismic  investigations.

Geological  survey

Objectives

•  Detecting  the  type  and  location  of  geological  units

Scope

•  Use  of  grids  for  exploration  of  the  offshore  construction  site
Recommendation:  Spacing  of  the  seismic  grid  of  500  m  in  longitudinal  and
transverse  directions.  In  case  of  differences  in  similar  grid  positions  due  to
certain  features  of  the  construction  site,  a  maximum  spacing  of  1000  m
must  not  be  exceeded.

Timing

•  Once

Method

•  Boomers  or  alternative  systems  with  comparable  or  better  performance
and  sufficient  signal  penetration,  resolution  of  at  least  1  m  required  close
to  the  surface
•  Supplementary  sub-bottom  profilers  or  chirp  sonar  for  areas  close  to  the
surface  (e.  g.  along  the  planned  cable  route  corridors),  vertical  resolution  of
at  least  0.5  m
•  Cruising  speed:  max.  4  kn
•  Usage  up  to  sea  state  of  4,  when  systems  are  used  together  with  a  socalled
  “motion  sensor”  then  usage  up  to  swell  5  or  6  is  justifiable

Presentation  of
results

•  Section  with  interpretation  (i.e.  geological  longitudinal  sections  and
transects)
•  Map  with  the  geographical  location  of  geological  units  and  structure  elements ­
  (e.  g.  isolines  map)
•  Data  must  also  be  submitted  in  digital  format  and  with  sufficient  supporting ­
  documentation  (internal  format).

Before  carrying  out  geotechnical  surveys  (boreholes,  probes),  an  investigation  should  be  carried ­
  out  for  reasons  of  safety  in  accordance  with  DIN  4020  to  determine  whether  ordnance  or
cables  are  present  with  a  magnetometer  or  an  active  metal  detection  system.