Jill Polak – Notes on Resistivity Survey

Stepstones Notes    Jill Polak     22/08/2009     

 TR/CIA resistivity kit (CBA South West)20 x 20 metre grids, with readings at 1 metre intervals

Top of grids is north and first traverses made from SW corners facing north. The base line was along the northern boundary of the field, at least 40 metres north of the edge of the grids.

The weather was dry and sunny, following a little rain during previous week. The field contained short stubble from a recently harvested wheat crop but had not been ploughed since harvesting.

 Site name & location:

SMR & SHER refs: 00414

National Grid Reference: 

Site Scheduled:

Landowner & occupier:  Alvis Bros

 Grids 1 -14 were downloaded under TR/CIA software. Grids 1 & 2 & 3 were already on the meter so were deleted after download.


Produced by Bob Smisson with Res2Dinv software and saved as bitmaps. Grid 14 is a pseudosection west to east crossing the eastern edge of the ring once near the centre of Grids 08 and 09.  Zero is at the west end.  Grid 10 is a pseudosection north to south, taken one metre to the east of the junction of Grids 08 & 09 and was intended to cross the ring at both ends. Unfortunately the line chosen was too far east and so it only touched the outside edge of the ring near the middle.  Zero is at the north end.

 The Grid 14 pseudosection crosses the ring at a right angle and is therefore more informative than Grid 10, which nearly misses the ring altogether. In both pseudosections the yellow/green=soft=low readings of the ring can be seen near the surface towards the middle. The third (bottom) version of each set of three is the significant one to study.  


Grids 04-07 and 11-13  are the usual sort of resistivity grids of measurements  in ohms up to 75 centimetres below the ground surface. They have been put together as raw data under TR/CIA software according to the grid plan below.

11 12 13
07 08 09
04 05 06

 The raw data ohms on this site have too wide a range to distinguish features (it appears uniformly black) but as almost all came within a narrow band from 15 to 50 ohms, these adjusted levels were used as a basis for the bitmap.

 All remote probe positions (15 metres from the nearest working positions) were adjusted to within 0.3 ohms of the initial reading of 25 ohms at a point on the west side of the junction of Grids 4 and 7. This was close enough to avoid any need to edge match the individual grids.

 The bitmap is uninverted so black=soft=low and white=hard=high readings.

 (Bob Smisson used Snuffler software to produce alternatives including a 3d version which was rather more informative but was not saved as a bitmap). 

 The ring is low resistance indicating soft ground, possibly a ditch. There is a hint of some contrasting high resistance, or hard ground, just inside the ring. There also seems to be an almost regular pattern of activity covering most of the inside of the ring. (Which might indicate postholes? jp).

 There appears to be an entrance gap on the south side, possibly ‘blocked’ in such a way as to reduce the view of the inside from a distance. (Which might explain why the initial three grids downloaded showed little sign of the ring. jp)  A line shows north to south to the east of the ring, which may be a recently trampled path. The old field boundary to the west is not within the 9 grid area.