Evolution and Benefits of Gradient Magnetic Data
Evolution and Benefits of Gradient Magnetic Data
By the mid 1980’s the geophysical industry had accepted and appreciated the significant resolution of near surface features provided by the measured vertical magnetic gradient. This was quickly eclipsed by the recognition that the calculated vertical gradient is an accurate representation, in that the vertical attenuation of the magnetic field obeys strict laws of physics. More important, the calculated vertical gradient is usually devoid of noise typical of measured gradients based on closely spaced vertical sensors (~2 metres).
By the late 1980’s Terraquest had recognized the quantum leap provided by the measured horizontal magnetic gradient using widely spaced sensors, and immediately installed wing tip pods in all aircraft to independently measure the transverse and longitudinal gradients. Typical transverse sensor separations vary from 13 to 16 metres. The longitudinal magnetic gradient can be obtained from the difference between the average of the wing tip sensors and the tail sensor, or alternatively from the difference in successive readings from the tail sensor (also referred to as along-line gradient). At that time the primary utilization of the horizontal gradient was to act as a high resolution anomaly detector, specifically in the exploration of kimberlites located between the flight lines. Consider what a single magnetometer measures: the sensor is located in a tail stinger along the flight line, but any pin-point magnetic source from either side of the aircraft will always get plotted on the flight line – where it was “sensed” by the magnetometer. On the other hand the transverse gradient (being the difference between two wing tip sensors) can detect which side the magnetic field is coming from and the relative distance. The following map shows the single sensor data in traditional colour contours falsely “locating” the anomaly right over the flight line, but the fences of horizontal gradient vectors point to the real location between the flight lines. In this case the vectors appear to focus on the edges of the source. This innovation created an industry standard.
HG vectors along two flight lines pinpoint the correct location of a kimberlite between the lines, whereas the gridded colour image of the single sensor TMF incorrectly shows it centered on the flight path.
In the realm of data presentation, the horizontal gradient vectors have been replaced by enhanced gridding techniques which incorporate the horizontal gradient component right into the gridding algorithm to guide the contouring. Generally speaking this technique could be thought of as two separate lines flown only 14 metres apart.
Of course the horizontal gradient magnetic offers even more benefits in the application to general magnetic mapping. When the transverse and longitudinal gradients are gridded and plotted separately they offer very different, high resolution images which are biased toward the dimension being measured. In practice, the longitudinal gradient will preferentially improve the definition of gradients along the flight line (that is, better definition of magnetic units that strike with a high angle to the flight line). Similarly the transverse gradient improves the definition of magnetic units orientated at a small angle to the flight line. This is a powerful tool that can be used judiciously to independently examine closely spaced magnetic units that interfere with each other. For example a dyke swarm parallel or sub-parallel to the flight line will be strongly defined by the transverse gradient, but almost invisible to the longitudinal gradient, which instead enhances the resolution of magnetic stratigraphy crossing at high angles.
The following example shows the different biases presented by the longitudinal and transverse gradients. The standard Total Magnetic Field (TMI) shows a massive dyke swarm at various angles close to the north-south flight direction, any other responses are generally overwhelmed. The transverse (or lateral) gradient emphasises these dykes. The longitudinal gradient removes those dykes that are at a close angle to the survey flight line thereby allowing much better resolution of the remaining dykes and stratigraphy that occur at high angles to the flight line. A further improvement to this could be made by rotating the flight lines or grid parallel to the majority of the dykes.
Total Magnetic Field
In addition, from data acquisition, processing and interpretational perspectives there is a huge benefit to be achieved from measured magnetic gradient data because they are not subject to diurnal variations and levelling errors that continue to exist in all data sets, both of which haunt even experienced data processors. The Total Field created from measured gradients is devoid of these problems. This forms the basis of the current generation of Gradient Inverted Total Magnetic Field (we refer to it as Reconstructed Total Field or “RTF”) and it occupies a central role in our processing stream at Terraquest. Because it can be achieved very quickly and done on a flight by flight basis, it becomes the client’s first view of the TMI and is routinely delivered as a preliminary product (though we stress the fact that its natural unit is “pseudo nT” and that the data should not be used in any form of susceptibility modelling because it will not contain long wavelength features). It is a good product for near surface, high resolution data. We also use it extensively for data QC during the acquisition phase and, when required by the client, deliver interim views of the daily accumulating TMI image as the survey progresses. Ultimately, the RTF data is scaled (i.e. converted to true nT) and is used in a proprietary Terraquest process to fine level the measured TMI data (we call it Enhanced Microlevelling). As an economic incentive, this process allows a significant reduction in Tie Line coverage by a factor of 2-4 times should it be required.
As horizontal gradient specialists we believe that the RTF is not necessarily a replacement for conventional, measured TMI but an important tool to be used in the normal data reduction process. It also occupies a vital role during the data acquisition phase in that it allows an accurate visualization of the eventual, finalised measured TMI. As such, it is standard component of our Horizontal Magnetic Gradiometer surveys.
The similarity of the preliminary processed RTF and final processed TMI for a near surface high resolution survey is shown below: