Introduction

Mafic dyke swarms are found in many different geological environments on Earth, including volcanic centres, compressional plate boundaries, spreading centres and ophiolite complexes, continent-continent collision zones and in radial swarms originating from the sites of mantle plumes. Approximately 70 giant swarms have been identified worldwide that extend over distances of >300 km.

Linear magnetic anomalies caused by mafic dykes can, however, conceal useful lithological and structural information that could otherwise be determined in their absence. Such linear trends often truncate and interrupt magnetic anomalies due to magnetization contrasts, making the tracing of such features ambiguous. Similarly, any subtle magnetic texture within lithological units may be obscured by the presence of dyke anomalies. Since detectable dyke anomalies are generally caused by sources at or near the ground surface, any enhancements (e.g., calculating gradients) of the measured field are likely to exacerbate the problem. In addition, automated methods for source parameter estimation may respond preferentially to the dyke sources at the expense of other, perhaps, more useful source information.

Thus, to improve the utility of magnetic data where the obscuring effects of dykes are significant, some method to remove their signature is needed. Directional filtering either in the spatial or frequency domain is one option that allows all spectral components within some range of azimuths to be removed from the data. Many dyke swarms are parallel or sub-parallel (especially at small scales) so that standard directional filtering is appropriate for removing their signature. However, fifty percent of swarms in North America exhibit a fanning pattern over large enough angles so that applying directional filters with such a large rejection band will undoubtedly remove useful source information along with the unwanted anomalies. Furthermore, the removal of trends over a wide range of angles distorts the remaining anomaly distribution and possibly introduces new, false directional information.

Dyke anomaly removal

Overcoming these difficulties can be achieved by a coordinate transformation that can reproject the varying dyke directions onto a single azimuth (Fig. 1). Anomalies along this single trend can then be removed by standard directional filtering. First, the focal point of the swarm (latitude ao, longitude bo) is estimated from the dyke trends. The data grid is then rotated an angle of 90-ao moving the focal point to the north pole and causing the dykes to lie on lines of longitude. If the data is then regridded onto a cylindrical projection, e.g., Mercator, then lines of longitude are oriented vertically along the columns of the grid, the focal point now being at infinity. So, with the dyke anomalies now vertical, their effects can be removed with standard decorrugation or filtering techniques developed for levelling grids, i.e., those grids containing striping due to residual flight line problems. The need for filtering over a range of azimuths has been removed. Once the grid is decorrugated in the transformed coordinate system, the opposite rotations can be applied and the data values regridded at the original grid location and orientation.

Using a coordinate transformation with uni-directional filtering results in a more selective removal of specified anomaly components. A given trend will only be removed if it passes through the specified focal point. Trends along a given direction in different areas will be affected according to their location.

Removal of Mackenzie dyke swarm effects

We use the coordinate transformation approach to suppress the anomalous effects of the Mackenzie dyke swarm (Fig. 2) that contributes significantly to the magnetic field over the Slave structural province, Northwest Territories, Canada. The Mackenzie dykes constitute the largest dyke swarm known on Earth and likely originated from a mantle plume whose location has been estimated at 70o N, 118o W. The observed magnetic field over the Slave province (Fig. 3) shows the ubiquitous nature of the Mackenzie dykes. Within this area, the swarm radiates over an angle of >60o with an average north-northwesterly trend. Using the coordinate transformation and filtering procedure (Fig. 1) successfully removes the dyke anomalies (Fig. 4).

Conclusions

The suppression of variable directional trends in aeromagnetic maps has been accomplished by use of a simple coordinate transformation. One rotation on a sphere can position the unwanted trends onto meridians which, after regridding with a suitable map projection, can be made to coincide with the columns in the grid. In this fashion, techniques that exist for decorrugating gridded data sets may be used to remove the trends.

Further reading

Pilkington, M. and Roest, W.R., 1998, Removing varying directional trends in aeromagnetic data: Geophysics, v.63, 446-453.

Menu:

• Introduction
• Dyke anomaly removal
• Removal of Mackenzie dyke swarm effects
• Conclusions
• Further reading