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Soft ground

 

Soft ground map (Source: WesternGeco)

PRODUCT DESCRIPTION

Category

Component products

 

Terrain Information

  • N/A

 

Uses

  • Seismic Planning - Areas of poor coupling
  • Seismic Planning - Identification of adverse terrain for trafficability
  • Surface Geology Mapping - Lithological discrimination
  • Surface Geology Mapping - Terrain evaluation and geo-morphology characterization
  • Environmental Monitoring – Natural hazard risk analysis
  • Logistics Planning and Operations - Baseline mapping of terrain and infrastructure
  • Logistics Planning and Operations – Support to surveying crews

Geo-information requirements

  • Topographic information
  • Terrain information
  • Lithology, geology and structural properties of the surface

Description

A soft ground map is created on a project/basin scale. A soft ground map is a raster file that highlights areas that are not suitable for vehicles/operations or present a risk for vehicles/operations. Soft ground mapping derived from EO data provides the user a first look at the surface and its suitability for oil field operations. EO derived data are good at identifying areas of sabkha (wet and dry), peat/marsh/bog areas and potential high water table areas.

Soft ground mapping is a product of topography (soft ground typically occurs in topographic lows), terrain information such as vegetation cover and the surface lithology.

Optical-derived products:

The soft ground map is a supervised classification of multispectral optical satellite data combined with topographic data to produce a clear risk map that identifies locations of soft or wet ground. Soft ground tends to be found in coastal areas, but inland topographic lows are also possible risks.

Seasonal variations are important to appreciate and account for. Review of multi-temporal imagery can be beneficial to identify seasonal variations in soft ground.

Radar-derived products:

A radar dataset would not typically provide an input to this product, but may be applicable to geographies where there is a lack of cloud-free optical data e.g. tropics.

Elevation data

DEM data (optically or radar derived) can provide detail of soft ground at different resolutions. Elevation mapping helps identify topographic lows and areas where the water table maybe high.

Known restrictions / limitations

Cloud cover would be a limitation on the optical data, most commonly in tropical areas, however archive imagery should provide mitigation against this.

Seasonality will have an impact so it is important for the user to understand when the image was acquired and the preceding weather conditions.

In dense vegetation where bare earth models are needed (for example in Seismic Planning) EO derived products cannot provide a solution.

Soft ground mapping from DEM data is limited by the availability of DEM data. DEM data derived from stereo pairs can have a lead time of 3 weeks, but have a higher degree of accuracy than freely available lower resolution DEM’s. Commercial radar derived DEM data are available off-the-shelf, with affected in steep mountainous regions and densely vegetated regions.

Lifecycle stage and demand

Pre-licensing

Exploration

Development

Production

Decommissioning

 

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Exploration:

  • Soft ground identification is critical to map during and prior to undertaking a land seismic survey. Failure to appreciate soft ground for a seismic contractor would result in significant operational issues. Soft ground will have an impact on the data quality and the type of source that will be used.

Development:

  • Soft ground identification would be useful during field development for facility siting, pipeline and road routeing, airstrips, camps etc.

Decommissioning:

  • Soft ground identification for logistics planning and HSE assessments.

Geographic coverage and demand

Global coverage (with a few restrictions see below). Demand in remote regions is high with exposed non vegetated surfaces best suited.

CHALLENGES ADDRESSED

OTM:058 Identifying ground conditions susceptible to poor coupling

OTM:045 Identifying soft ground for seismic vehicles

OTM:043 Anticipating areas of high seismic impedance

OTM:046 Identifying variations in trafficability for seismic vehicle

OTM:054 Understanding the near-surface for anticipating seismic signal absorption properties

OTM:042 Identifying seasonal terrain changes e.g. for access

OTM:053 Understanding the near-surface for explosive charge placement

HC:1101 Identify areas with soft sediments to avoid strong attenuation

HC:1103 Identify soft and hard ground as areas of potentially poor source and receiver coupling

HC:1203 Identify areas with soft sediments to plan access and assess hazards

HC:1210 Identify soft ground to reduce environmental impacts

HC:2501 Characterization of surface/near-surface structural geological properties for infrastructure planning

PRODUCT SPECIFICATIONS

Input data sources

Optical: VHR1, VHR2, HR1, HR2, MR1

Supporting data:

  • Digital elevation models (DEM)
  • Topographic and geological mapping
  • Existing  GIS data such as infrastructure and assets
  • Local knowledge
  • Meteorological data/statistics

Spatial resolution and coverage

Spatial resolution: 1 m – 90 m pixel size

Minimum Mapping Unit (MMU)

Variable, depending on source data resolution MMU as small as 0.5 ha is possible.

Accuracy / constraints

Thematic accuracy: 80-90%

Spatial accuracy: The goal would be one pixel, but depends on reference data

Accuracies for selected off-the-shelf elevation products:

  • SRTM version 3: Absolute and relative vertical accuracy was anticipated to be less than 16 and 10 m, respectively
  • ASTER GDEM2 has a root-mean-square error (RMSE) in elevation between ±7 and ±15 m can be achieved with ASTER stereo image data of good quality.
  • WorldDEM (TanDEM-X) has a 2m relative and a 4m absolute vertical accuracy in a 12mx12m raster
  • WorldView Elevation Suite (for a 1m x 1m DEM) 30cm relative vertical accuracy and 50cm relative horizontal. However the accuracy is dependent on the quality of ground control points (GCP). Known locations need to be identified in the images.

Accuracy assessment approach & quality control measures

Stratified random points sampling approach utilizing in-situ measurements and any available published data or reports. Statistical confusion matrix with user’s, producer’s accuracy and kappa statistics for hydrological network and catchment area mapping.

Frequency / timeliness

Observation frequency: Archive data can be used although new acquisition may be a requirement in some situations. The frequency is constrained by client needs, satellite revisit and acquisition timeframes, but also processing requirements. Depending on the requirements of the customer the best suitable satellite sensor has to be chosen regarding spatial / spectral resolution as well as revisit frequency.

Timeliness of delivery: Delivery in time with project planning requirements. Archive data can be used to good effect if the prevailing conditions are known and the time of year is accounted for.

Availability

Freely available or commercially acquired depending on the selected sensor and methodology used.

Availability if DEM data:

1km, 90m (most common) and 30m DEM are freely available from SRTM version 3 and Aster GDEM2 for higher resolution this is a paid for data set. These data sets for example are Elevation 30 (SPOT DEM 30m),  WorldDEM (TanDEM-X 12m), Elevation 8 (SPOT DEM 8m), Elevation 4 & 1 (Pleiades DEM 4m & 1m), WorldView Elevation Suite (1m).

Delivery / output format

Data type:

  • Raster formats

File format:

  • Geotiff (standard - any other OGC standard file formats)
          

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Lead Author:WesternGeco
Peer Reviewer:Hatfield Consultants

Author(s):

Andrew Cutts

Document Title:

Soft Ground

# of Pages:

5

Circulation:

Internal – Project consortium and science partners

 

External – ESA

 

 

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