OR/14/030 What the dataset shows

Entwisle, D C, White, J C, Busby, J P, Lawley R S and Cooke, I L. 2014. Electrical Resistivity Model of Great Britain: User Guide. British geological Survey. (OR/14/030).

Values of resisitivity

The geology of Great Britain within the PMM is mapped with 1,464,139 polygons. Each polygon is attributed with a LEX-RCS code. This rigorous lithostratigraphic attribution scheme is derived from two attributes: a LEX (or Lexicon) description giving the name of the specific geological unit; and a RCS (Rock Classification Scheme) code giving its lithology. The compiled data contain 9514 unique LEX-RCS codes. Each LEX-RCS code is then parameterised with its expected range of saturation, porosity, clay content and pore fluid resistivity values.

5000 values of electrical resistivity are derived for each LEX_RCS code by randomly selecting the values of the input variables (from within their defined ranges) and running the resistivity calculation each time. This produces a modelled statistical distribution of resistivity for every geological classification. From these the 50th percentile (median) of the resistivity distribution is extracted along with the 20th and 80th percentile values and minimum and maximum values. These percentiles best summarise the expected range of resistivity values in the upper 3 to 5 m for each LEX-RCS code.

In most instances the RCS defines a single lithology but there are a substantial number of cases where multilithic units are described. In these instances, lithologies with different matrix resistivity are considered separately and two sets of resistivity data are derived for the unit. The two ‘lithologies’, with different resistivity characteristics, are hereafter referred to as lith1 and lith2, where lith1 is the primary lithology as identified from the BGS Lexicon description

The resistivity values are verified by comparison with electrical resistivity sounding data, held in the National Resistivity Soundings Database (Barker et al. 1996) and apparent resistivity estimates from airborne electromagnetic surveys (Beamish and White, 2012)

Field descriptors

The data fields included in this dataset are described below:

Lithology (GEN_PMLITH)

This field describes the lithology(ies) identified in the parent material GIS.

Lexicon Rock Classification Scheme (LEX_RCS)

This field is the standard DiGMapGB-50 code that describes the lithostratigraphy of the geological units found in Great Britain. It provides the starting point for the parent material characterisation. It comprises a ‘stratigraphic’ code (LEX) and ‘lithology’ code (RCS).

Value of lith1 20th percentile resistivity (A20)

The 20th percentile value of resistivity calculated from the 5000 different realisations for lith1 of the LEX-RCS code.

Value of lith1 50th percentile resistivity (A50)

The 50th percentile (median) value of resistivity calculated from the 5000 different realisations for lith1 of the LEX-RCS code.

Value of lith1 80th percentile resistivity (A80)

The 80th percentile of resistivity value calculated from the 5000 different realisations for lith1 of the LEX-RCS code.

Range of lith1 resistivity (ARANGE)

The range of expected resistivity values for lith1 of the LEX-RCS code.

Value of lith2 20th percentile resistivity (B20)

The 20th percentile of resistivity value calculated from the 5000 different realisations for lith2 for the LEX-RCS code.

Value of lith2 50th percentile resistivity (B50)

The 50th percentile (median) of resistivity value calculated from the 5000 different realisations for lith2 of the LEX-RCS code.

Value of lith2 80th percentile resistivity (B80)

The 80th percentile of resistivity value calculated from the 5000 different realisations for lith2 for the LEX-RCS code.

Range of lith2 resistivity (BRANGE)

The range of expected resistivity values for lith2 for the LEX-RCS code.

Nominal Scale (NOM_SCALE)

This field describes the notional x-y spatial scale of the data. Most geological map data in the dataset is captured and presented at a scale of 1:50k. The field identifies a combination of scales used to create the map from the bedrock and superficial map sources. The available scales are show as follows:

Note: where the geology consists of a single lithology, lith1 and lith2 resistivity values are the same.

Table 1 Explanation of the nominal scales of geological map data.

Field Value	Meaning
50	No superficial data is present for this sheet and bedrock data is available at 1:50 000 scale
250	No superficial data is present for this sheet and bedrock data is available at 1:250 000 scale
625_50	Superficial data is present for this sheet at a scale of 1:625 000 and Bedrock data is available at a scale of 1:50 000
50_50	Superficial data is present for this sheet at a scale of 1:50 000 and Bedrock data is available at a scale of 1:50000
35_50	Superficial data is present for this sheet at a scale of 1:35 000 and Bedrock data is available at a scale of 1:50 000
35_250	Superficial data is present for this sheet at a scale of 1:35 000 and Bedrock data is available at a scale of 1:250 000

Unique polygon identifier (UID)

This field gives a polygon identifier for DiGMapPlus products. This field uniquely identifies each object in the map. It is used for auditing and identification purposes.

Version of DiGMapPlus Resistivity Model

This is the code used by BGS to identify a type of version of DiGMapPlus product. In the case of this dataset the value of this field is set to 'DiGMapPlus_Resistivity_V1'.