FM256 Application Examples - Page 2                                                        Back to Page 1

Double Speed survey at Jarlshof Environs, Shetland 2003

A rapid fluxgate gradiometer survey was carried out on the perimeter of the prehistoric, norse and medieval settlement of Jarlshof, Shetland. A dual FM256 system, operated in double speed mode, was used to collect data in 20m grids. Data was sampled at a walking rate of 0.7s/m, 4 readings/m, traverse interval 1m. Processing interpolates this to sample and traverse intervals of 4 readings/m. Two extensive midden deposit areas of magnitude 5-20 nT are clearly visible, together with possible ridge and furrow, and track-ways.

Survey reproduced with permission of the Old Scatness Broch and Jarlshof Environs Project and Shetland Amenity Trust

0

40 m

Plot range -5 nT to +5 nT

High resolution survey at Jarlshof Monument, Shetland 2003

Comparative high resolution resistance and gradiometer surveys were made in two locations just inside the prehistoric, norse and medieval settlement of Jarlshof, Shetland. A dual FM256 fluxgate gradiometer system operating in Quad Density mode was used to collect data with a sample interval of 0.125m and traverse interval of 0.25m. An experimental 0.75m wheeled Square array was also used to collect data at a sample interval of 0.25m and traverse interval of 0.5m.

The survey areas were only 13m x 8m and 10m x 10m but reveal considerable fine detail because of the high sampling density. The smaller survey area was situated immediately adjacent to a metal fence and this shows as extensive red and blue banding in the lower gradiometer plot. However, since the FM256 is a 0.5m sensor gradiometer, it is able to reject the effects of the fence more effectively than a 1m gradiometer, allowing details near the fence to be seen. For example, a significant pair of 0.5m diameter anomalies, just to left of centre, are visible that correspond very clearly on both the gradiometer and resistance high resolution plots.

Survey reproduced with permission of the Old Scatness Broch and Jarlshof Environs Project and Shetland Amenity Trust

Resistance Data

Gradiometer Data

0

5 m

Plot range -2 nT to +2 nT

Plot range -2 SD to +2 SD

Double Speed and Single surveys at Clevigarth Broch, Shetland 2003

A large area dual FM256 fluxgate gradiometer survey was made at Clevigarth Broch and its environs, covering 3h. Data was sampled on 20m grids at 0.7s/m, 4 readings/m, traverse interval 1m, with data subsequently interpolated to 0.25m. The steep slopes of the broch made use of a dual system impossible but the flexibility of the dual system meant that a single gradiometer from the pair could be used to survey the broch itself at 1m intervals (see movie clip below). Noise in the quietest area was measured as 0.09nT and based on this, statistical detection (right hand two plots) was used to identify significant positive and negative features, ie those features with a magnitude greater than 3 standard deviations (0.27 nT).

Survey reproduced with permission of the Old Scatness Broch and Jarlshof Environs Project and Shetland Amenity Trust

Significant Positive Anomalies                     Significant Positives and Negatives

Plot range -1 nT to +1 nT

Statistical detection with threshold = 3.0 standard deviations of quietest area, ie plot < -0.27 nT and > +0.27 nT

0

40 m

Page 1       Back to top of page 2