Determining reserves of aggregates by non-invasive electrical tomography (DRAGNET) : MIST project MA/6/1/008 (2008)

Three-dimensional electrical resistivity tomography (3D ERT) is a geophysical imaging
technique that has developed rapidly in recent years. The great strength of the technique is that it
is a relatively low cost non-invasive method of providing high-resolution spatial information that
reveals the structure of the subsurface. Despite its obvious suitability, 3D ERT has not yet been
applied by the minerals industry to the investigation of sand and gravel deposits. The
DRAGNET project was therefore established to begin the process of researching and developing
3D ERT for sand and gravel reserve assessment. More specifically, we have demonstrated the
technique at UK extraction sites, we have sought to establish procedures to integrate 3D ERT
with conventional investigation methods, we have developed best practice guidance for the
future use of the technique in this area, and we have begun to consider the economics of the 3D
ERT for minerals surveys.
The DRAGNET demonstration studies were carried out at Marfield Quarry, North Yorkshire,
and Bull’s Lodge Quarry, Essex. These sites were selected due to their challenging geologies.
The Marfield Quarry site comprised extremely coarse gravels, which have proved to be nearly
impossible to drill, whilst the Bull’s Lodge test area had overburden to mineral thickness ratios
of as much as 2:1, with in excess of 10 m of till overburden. Integrated models were generated
for both sites to demonstrate our methodology for 3D data fusion, visualisation and interpretation
using combined 3D ERT and conventional data sets.
The 3D ERT model of the Marfield Quarry site was used to identify the distribution of
overburden, depth to bedrock and quality variations within the gravels. Operationally relevant
information derived from the Marfield 3D ERT model was presented in a form that could be
directly used by minerals industry geologists for reserve calculations.
The Bull’s Lodge 3D ERT model successfully defined the overburden, but was not able to
resolve the base of the mineral. The failure of ERT in this case was due to the unfavourable
overburden to mineral ration, and the very low resistivity of the till, and highlights the limitations
of the technique. Synthetic modelling studies carried out as part of this project were used to
determine the range of overburden to mineral rations over which 3D ERT would be successful,
and to trial a new survey strategy that has the potential to resolve thin mineral layers buried
beneath thick overburden. The results from the synthetic modelling and the case studies
represent an invaluable resource with which to inform the planning of future minerals surveys,
and have provided a foundation on which to begin developing best practice guidance.
The key elements of our best practice guidance for 3D ERT mineral reserve assessment surveys
are as follows: (1) Overview of ERT; (2) Limitations of ERT; (3) Survey planning and design
(including array type, vertical and lateral resolution); (4) Data collection; (5) Data processing
and interpretation (including inversion and visualisation); (6) Data quality assessment; (7) Case
studies.
Based on our experience during the project, and other published sources, we assess the likely
cost benefits of applying 3D ERT to sand and gravel investigations. Due to the limited scope and
resources available to us this assessment is necessarily qualitative. We consider the relative costs
of ERT (particularly compared to drilling), its effectiveness for sand and gravel deposit
investigations, and the added value that it can provide to site operators. It is probable that 3D
ERT will be most suited both to the investigation of complex deposits with significant lateral
heterogeneity, and those sites that are difficult to drill. In such situations 3D ERT can be used to
reveal the structure of the deposit between intrusive sample points, it can provide targets for
drilling, and can potentially reduce the number of intrusive sample points required.