Through use of a model-based solution, the SPAR 300 computes an estimated 3-D coordinate (horizontal and vertical position which is tied to the project survey control) on targeted underground utilities and qualifies each discrete observation with a derived “accuracy” metric that is based on the apparent circularity of the observed magnetic field. A computed RMS (root-mean-square) between the fitted model solution and the observed magnetic field provides the reported SPAR 300 “accuracies” which are categorized as “VA, VB, VC, VD and VE”. Note: these are simply arbitrary measures of data confidence based on empirical methods and are not to be confused with Subsurface Utility Engineering (SUE) Standard (CI/ASCE 38-02) quality levels A-D which are professionally judged reliability designations.
The resulting 3-D alignments are to be considered Quality Level B in accordance with ASCE/CI 38 standard guidelines, which means positional data are derived by means of geophysical detection methods and are not direct observations of the exposed facility. A full synopsis of this methodology is available from the manufacturer Vivax – Metrotech. UMS systematic use of the SPAR 300 since 2012 has demonstrated that the results, when properly assessed and with appropriate ground truth data (i.e., test holes) and/or multichannel GPR, can provide very useful, contiguous 3D alignment information which can grossly reduce requirements for vacuum excavated test hole observations.
Electromagnetic based utility position observations are adversely affected by distortion in the underground environment, such as might result from adjacent or crossing conductors. Magnetic field distortion is defined as any effect that causes the expected circular field around a long cable or pipeline to be non-circular. The results can be heavily influenced by utility congestion and adjacent metallic objects within the project area (i.e. vehicles, junction boxes). Discontinuities in the pipeline (i.e. valves, “tees”, etc.) can also distort the signal. In addition, the signal to noise ratio drops off significantly with depth. Model-based positioning methods measure the circularity of the magnetic field, from any perspective in the field, not just over the centerline. The degree of non-circularity is an effective metric on the reliability of the model-based solution at each observation point.
