Magnetic Gradiometry: Equipment
Instrumentation
The Discovery Programme has a Bartington Instruments Grad601-2 Dual Gradiometer. This is consists of two vertical component fluxgate Grad-01-1000 sensors with a 1m separation between the sensing elements to give a strong response to deep anomalies. The Grad601-2 records two lines of data during each traverse and reduces the survey time and distance walked to one half compared to using a single gradiometer. It can operate as either a survey tool, where data are logged while covering the site in parallel or zigzag paths, or in a scanning mode where it is used as a search tool with an audible output for locating and tracing pipes, cables etc. without data logging.
When used as a survey tool, data are saved in grids of 10 x 10, 20 x 20, 30 x 30 or 40 x 40m. The magnetic gradient is measured along a series of lines spaced at 0.25, 0.5 or 1m depending on the resolution required.
The data logger has a simple six-key control panel with menu-selected operations and a 20 character, two-line liquid crystal display. External push buttons are provided for use during survey operations. The gradiometer operates from a 12V rechargeable battery giving more than 24 hours operation. The memory will hold the equivalent of 30 grids of 30 x 30m with a 1m line separation and a resolution of 4 readings per metre.
The maximum depth for detection will depend on the size, shape and magnetic properties of the object compared to the background material. However, the instrument is designed specifically for archaeological surveys and typically detects features up to 2m.
Data Processing Software
Data is exported from the logger using Grad601 Data Download software from Bartington Instruments and then data is processed in GEOPLOT 3.0 software from Geoscan Research.This software enables processing and presentation of geophysical data collected from a variety of instruments including resistance meters, and gradiometers. It is used for the geophysical processing of both the sensors currently operated by the Discovery Programme.
Processing functions include : high pass, low pass, median, Wallis & periodic filters, spectrum and variance analysis, despiking, interpolation, edge matching, zero mean traverse correction, destagger correction, several numeric functions and a powerful cut and combine function for combing data sets mathematically. A record of every edit and process is maintained with each data file. Graphics may be produced as shade plots (grey scale or colour), trace plots (stacked profiles or 3D), dot-density or pattern plots and may be printed out at any scale or saved as bitmaps for use in other software packages