Mapping Marine Ferrous Targets Using the SeaQuest Gradiometer System
The figures below show the results of a target survey conducted by the United States Naval Undersea Warfare Center (NUWC) – Keyport, WA. The images show the striking contrast between conventional magnetometer (total field) data and the high resolution gradient data obtainable with SeaQuest.
Figure 1 shows the total magnetic field data collected by the top sensor of the SeaQuest platform. This image represents data that would be obtainable by a conventional total field survey and is presented for comparison purposes. The total field image is dominated by north-south trending curvilinear anomalies, which are likely related to magnetic susceptibility variations in the bedrock. This strong background magnetic response makes it difficult to quickly identify anomalies associated with ferrous objects. Presenting the total field grid with a ‘stretched’ colour-scale allows identification of at least four potential ferrous targets in the western half of the survey site.
In contrast, the total gradient map (Figure 2) allows easy identification of at least 12 (high-confidence) ferro-magnetic objects within the survey block. The wavelengths associated with the geological magnetic effects are effectively suppressed in this image in comparison to the total field image. Targets are defined by simple ‘bulls-eye type’ positive anomalies, which are centered over the target position. In the western part of the survey block, a low amplitude NNW-trending linear anomaly is present. This anomaly corresponds to a known pipeline marked on the marine charts of the area. It is worth noting that the amplitude of the pipeline anomaly is less than 0.5nT/m, and yet it is clearly visible in the total gradient map.
Also of interest is the large anomaly east of the center of the map. Despite its size, the anomaly is obscured by geology in the total field data, yet it shows up prominently in the total gradient data.
It is easy to see that the total gradient (Analytic Signal) directly measured by SeaQuest provides the clearest results, effectively creating an intuituve magnetic ‘image’ of the sea bottom. While the singleaxis gradient results enhance only certain types of anomalies based on their geographic direction, the total gradient is effectively a direction-independent result, enhancing all near-surface anomaliesequally, and suppressing deep geology evenly.
Magnetic gradient is commonly used to enhance the signals from small, relatively close sources typical of iron manmade objects, and to suppress the signals from large distant sources associated with geological variation. The total gradient technique goes even further by eliminating the directional dependence of conventional gradiometer methods. This produces an easily interpreted magnetic ‘image’ of the sea floor, with target positions unambiguously marked by ‘bulls-eye’ type anomalies. Also, the total gradient anomalies are expressed with a higher signal-to-background-noise ratio than with conventional techniques, enabling the identification of tiny targets that would otherwise be invisible.
The SeaQuest gradiometer platform enables the acquisiton of high-quality total gradient data because of its hydrodynamic stability and the high absolute accuracy of its sensors, producing clean results free from heading errors and offsets. Despite high currents and demanding conditions, SeaQuest provided consistent results that did not require the filtering or level-shifting that are necessary steps, yet large sources of calculation error, for other gradiometer instruments.