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Advanced Processing and Interpretation of Gravity and Magnetic Data Prepared by GETECH GETECH Kitson House Elmete Hall Leeds LS8 2LJ UK Phone +44 113 322 2200 Fax +44 113 273 5236 E-mail [email protected] www.getech.com Processing and Interpretation of G&M Data This short document is intended to p...

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Advanced Processing and Interpretation of Gravity and Magnetic Data

Prepared by

GETECH

GETECH Kitson House Elmete Hall Leeds LS8 2LJ UK Phone +44 113 322 2200 Fax +44 113 273 5236 E-mail [email protected] www.getech.com

Processing and Interpretation of G&M Data

This short document is intended to provide background theory and methodology of the uses of gravity and magnetic data in exploration. Section 1 discusses the interpretation process itself, outlining the importance of qualitative interpretation and the complementary roles that gravity and magnetic data offer. Section 2 provides examples of the various types of enhancements (or transforms) applied to gravity and magnetic data to highlight particular characteristics or features to aid qualitative interpretation. Section 3 describes additional advanced methods of quantitative processing in support of interpretation that can be applied to gravity and magnetic data, including 3D gravity inversion, depth to source estimation and 2D modelling.

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Processing and Interpretation of G&M Data 1.

The Interpretation Process

As with all geophysical interpretation, the analysis of gravity and magnetic data has two distinct aspects: qualitative and quantitative. The qualitative process is largely map-based and dominates the early stages of a study. The resultant preliminary structural element map is the cornerstone of the interpretation. Qualitative interpretation involves recognition of: •

the nature of discrete anomalous bodies including intrusions, faults and lenticular intrasedimentary bodies - often aided by reference to characteristic magnetic response charts

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•

•

•

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and perhaps performing simple test models disruptive cross-cutting features such as strike-slip faults effects of mutual interference relative ages of intersecting faults structural styles unifying tectonic features/events that integrate seemingly unrelated interpreted features

The most important element in this preliminary qualitative stage surprisingly is not the interpretation of anomalous bodies themselves (that follows later) but rather the network of discontinuities e.g. lines of truncation and strike-slip faults that serve to compartmentalise and delimit discrete anomalies that at first sight may appear as a confused pattern of unravellable anomalies. Strike-slip faults/shear zones, small and large-scale, are commonplace particularly within intra-continental situations where crust is old, bearing witness to countless fault reactivations. They provide the principal means by which major structures are truncated and crustal stress is decoupled (fully or partially) from one crustal block to another. The quantitative process. Putting lines on maps during the qualitative process is the start of quantitative phase. Refinement of these locations begins with the determination of z i.e. depth values. For example, depth estimates to tops of anomalous magnetic bodies are generated by a number of means including: slope measurement methods, analytic methods such as Euler and Werner. Gravity and magnetic modelling (ideally seismically controlled) including forward and inversion approaches contribute significantly to location in x, y and z. Accurate results of all these rely upon sensible qualitative recognition of body types.

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Processing and Interpretation of G&M Data Interplay of the qualitative and the quantitative soon develops, particularly as computer modelling proceeds. Not infrequently, the results of modelling alert the interpreter to unexpected geological scenarios that necessitate a qualitative re-appraisal of certain anomalies, perhaps for example even alluding to a change in the interpreted structural style for an entire study area. The likelihood of this happening depends on whether the study zone lies within an under-explored frontier area or is mature. The greater the seismic control within the modelling process, the less ambiguous the model will be. Once modelling is complete, the qualitative process reasserts itself on the basis of the mapped gravity and magnetic data alone, by the interpolation and extrapolation of modelled features into regions that do not benefit from modelling / seismic / well control. In this way, body geometries can be more accurately defined on a map-wide basis, more precise xyz location of bodies determined, and, interfering/overprinted bodies better recognised for what they are. Depth to basement contour maps can also be generated, conditioning the contours manually to the interpreted structural framework. 1.1

The supporting roles of gravity and magnetic data

Interpretation of magnetic data is theoretically more complex than the corresponding gravity data due to: •

• •

the dipolar nature of the magnetic field, in contrast with the simpler monopolar gravity field the latitude/longitude dependent nature of the induced magnetic response for a given body due to the variability of the geomagnetic field over the Earth’s surface However, in practice it is often simpler than that of gravity due to the smaller number of contributory sources. Often, though not always, there is just one source - the magnetic crystalline basement.

The gravity response is, by contrast, generated by the entire

geologic section. In the case of intrasedimentary bodies, the dipolar nature of the magnetic response is particularly diagnostic of the disposition (e.g. dip) of the source. It is for this reason that it is important for the interpreter to be familiar with a wide range of induced magnetic responses produced by simple geological bodies at the geomagnetic field inclination for the region. Seeking mutual consistency

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Processing and Interpretation of G&M Data of both gravity and magnetic interpretations ensures that ambiguities within the interpretation are minimized.

Characteristic geomagnetically induced magnetic responses for regions close to the geomagnetic equator. Modelling of potential field data is an important aspect of the interpretation, and is often performed using a “bottom-up / outside-in / magnetics-first” approach. This ensures that deep magnetic basement sources, which impact regionally on the study area, are understood first, before attention is focused on the detail within the area of interest. The interpreter should always be aware of the potential confusion generated by overprinting of similar wavelength responses caused by: (i) deep crustal features, (ii) laterally distal crustal features, and, (iii) broad centrally located shallow crustal features. Resolving this confusion is invariably achieved by seeking consistency between the modelled gravity and magnetic data, while adhering to sensible geological principles and experience. The following expands on this process. A “magnetics first” approach recognises that the sedimentary section often possesses little significant magnetic susceptibility. The major proportion of magnetic signal is generated at crystalline (igneous or metamorphic) basement level. This is useful, because unlike gravity where the entire section contributes to the observed field, all but the shortest wavelength magnetic responses can be ascribed to the underlying basement. If shallow intra-sedimentary magnetic sources do exist, these are usually of short wavelength and sufficiently discrete to be recognised for what they are. The modelling of the magnetic data is particularly important for extending interpretation below the effective level of seismic penetration. Once the magnetic data have

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Processing and Interpretation of G&M Data been interpreted in this way, consistency is then sought with the longer wavelength gravity features. Any remaining long wavelength gravity anomalies may be more properly ascribed to broad shallow sources, rather than to deep sources. 1.2

Magnetic response of N-S orientated features located at the Equator

Interpretation of magnetic anomalies close to the magnetic equator is complicated for several reasons: •

•

•

Ambient field is horizontal Ambient field is weak (~35,000 nT compared to up to 70,000 nT in higher latitudes) Structures striking N-S are difficult to identify

Magnetic anomalies are generated when the flux density cuts the boundary of a structure. If the structure strikes parallel with the field then in Equatorial areas the flux stays within the structure and no anomaly is generated.

Induced magnetic response of a 2D rifted basin striking W-E and N-S at or near the geomagnetic equator. The sediments are assumed to have low susceptibility and the basement high susceptibility. Small arrows show the induced magnetisation vector directions.

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Processing and Interpretation of G&M Data A similar effect is seen when a magnetic field is reduced to the equator (RTE) instead of to the pole (RTP), where N-S structures are difficult or impossible to identify in RTE maps. This is shown below. In this example the TMI has Inclination = –62 and Declination = –12, thereby allowing both stable RTP and RTE anomalies to be derived. In magnetic equatorial regions where Inclination is less than say 15 then RTP is generally unstable and can not be derived.

Since faults and many structures have irregular shapes, albeit in regional form they may be 2D, then parts of the structure will be magnetically imaged where the flux cuts the structural interface generating dipole shape anomalies. Thus N-S striking structures may be identified by a ‘string of pearls’ i.e. line of magnetic dipole anomalies. The Analytic Signal is the best derivative to recover the N-S contacts in equatorial regions as is shown by the diagram below.

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Processing and Interpretation of G&M Data 2.

Enhancements and Transformations of Potential Field Data

The area used in this series of images is a region of Poland traversed (NW-SE) by the TeisseyreTornquist Zone which divides the shallower crystalline basement of the East European platform to the NE, from the deeper West European platform to the SW. The thick Palaeozoic and Mesozoic sedimentary cover of central Poland has undergone significant deformation (folding and faulting) during the Caledonian, Variscan and Alpine orogenic phases. This has generated a set of clear magnetic and gravity responses from basement and the sedimentary section that allow similarities and differences to be clearly observed in the images generated. The gravity images are on the left hand side of the page and the magnetic images are on the right. All the techniques described in this section were generated using GETECH’s own ‘GETgrid’ software package. The software utilises FFT and spatial domain operators and has a host of additional features (e.g. boolean logic, vector overlays, grid arithmetic).

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Processing and Interpretation of G&M Data Reduction to the Pole (RTP)

This technique transforms induced magnetic responses to those that would arise were the sources placed at the magnetic pole (vertical field). This simplifies the interpretation because for sub-vertical prisms or sub-vertical contacts (including faults), it transforms their asymmetric responses to simpler symmetric and anti-symmetric forms. The symmetric ‘highs’ are directly centred on the body, while the maximum gradient of the anti-symmetric dipolar anomalies coincides exactly with the body edge. Pole reduction is difficult at low magnetic latitudes, since N-S bodies have no detectable induced magnetic anomaly at zero geomagnetic inclination. Pole reduction is not a valid technique where there are appreciable remanence effects. Pseudo-Gravity and Pseudo-Magnetic Fields

A magnetic grid may be transformed into a grid of pseudo-gravity. The process requires pole reduction, but adds a further procedure which converts the essentially dipolar nature of a magnetic field to its equivalent monopolar form. The result, with suitable scaling, is comparable with the gravity map. It shows the gravity map that would have been observed if density were proportional to magnetisation (or susceptibility). Comparison of gravity and pseudo-gravity maps can reveal a good deal about the local geology. Where anomalies coincide, the source of the gravity and magnetic disturbances is likely to be the same geological structure. (see Automatic Lineament Tracing). Similarly, a gravity grid can be transformed into a pseudo-magnetic grid, although this is a less common practice.

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Processing and Interpretation of G&M Data Traditional Filtering

Filtering is a way of separating signals of different wavelength to isolate and hence enhance anomalous features with a certain wavelength. A rule of thumb is that the wavelength of an anomaly divided by three or four is approximately equal to the depth at which the body producing the anomaly is buried. Thus filtering can be used to enhance anomalies produced by features in a given depth range.

Traditional filtering can be either low pass (Regional) or high pass (Residual). Thus the technique is sometimes referred to as Regional-Residual Separation. Bandpass filtering isolates wavelengths between user-defined upper and lower cut-off limits.

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Processing and Interpretation of G&M Data Pseudo-depth Slicing

A potential field grid may be considered to represent a series of components of different wavelength and direction. The logarithm of the power of the signal at each wavelength can be plotted against wavelength, regardless of direction, to produce a power spectrum. The power spectrum is often observed to be broken up into a series of straight line segments. Each line segment represents the cumulative response of a discrete ensemble of sources at a given depth. The depth is directly proportional to the slope of the line segment. Filtering such that the power spectrum is a single straight line can thus enhance the effects from sources at any chosen depth at the expense of effects from deeper or shallower sources. It is a data-adaptive process involving spectral shaping. As such, it performs significantly better than arbitrary traditional filtering techniques described above. When gravity and magnetic depth slices coincide it is a good indication that the causative bodies are one and the same.

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Processing and Interpretation of G&M Data First Vertical Derivative

VDR = −

∂Α ∂z

This enhancement sharpens up anomalies over bodies and tends to reduce anomaly complexity, allowing a clearer imaging of the causative structures. The transformation can be noisy since it will amplify short wavelength noise. In our example it clearly delineates areas of different data resolution in the magnetic grid.

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Processing and Interpretation of G&M Data ⎛ ∂Α ⎞ ⎛ ∂Α ⎞ ⎟⎟ HDR = ⎜ ⎟ + ⎜⎜ ⎝ ∂x ⎠ ⎝ ∂y ⎠ 2

Total Horizontal Derivative

2

This enhancement is also designed to look at fault and contact features. Maxima in the mapped enhancement indicate source edges.

It is complementary to the filtered and first vertical

derivative enhancements above. It usually produces a more exact location for faults than the first vertical derivative, but for magnetic data it must be used in conjunction with the other transformations e.g. reduction to pole (RTP) or pseudo-gravity. Specific directional horizontal derivatives can also be generated to highlight features with known strikes. This technique can be applied to pseudo-depth slices to image structure at different depths.

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Processing and Interpretation of G&M Data ∂2Α 2VD = ∂Ζ 2

Second Vertical Derivative

The second vertical derivative serves much the same purpose as ‘residual’ filtering in gravity and magnetic maps, in that it emphasises the expressions of local features, and removes the effects of large anomalies or regional influences. The principal usefulness of this enhancement is that the zero value for gravity data in particular closely follows sub-vertical edges of intrabasement blocks, or the edges of suprabasement disturbances or faults. As with other derivative displays, it is particularly helpful in the processing stage where it can be used to highlight line noise or mislevelling. ⎛ ∂Α ⎞ ⎛ ∂Α ⎞ ⎛ ∂Α ⎞ ⎟⎟ + ⎜ AS = ⎜ ⎟ + ⎜⎜ ⎟ ⎝ ∂x ⎠ ⎝ ∂y ⎠ ⎝ ∂z ⎠ 2

Analytic Signal (Total Gradient)

2

2

The analytic signal, although often more discontinuous than the simple horizontal gradient, has the property that it generates a maximum directly over discrete bodies as well as their edges. The width of a maximum, or ridge, is an indicator of depth of the contact, as long as the signal arising from a single contact can be resolved. This transformation is often useful at low magnetic latitudes because of the inherent problems with RTP, (at such low latitudes).

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Processing and Interpretation of G&M Data Automatic Lineament Tracing

The automatic lineament detection algorithm requires the data to have been processed (or transformed) such that the edge of a causative body is located beneath a maximum in the grid. Several transforms satisfy this requirement e.g. horizontal derivative of gravity (or of pseudogravity, for magnetic data) and also analytic signal. The results help to quantify the different gravity and magnetic responses of structures located in the shallow and deep sedimentary sections and in the basement.

A significance factor N, ranging in value from 0 to 4, is assigned to each grid cell depending on the relation to its neighbours. N=1 might represent a point on a spur, N=2 and N=3 a point on a ridge and N=4 a point on a peak. The values of N are colour coded and displayed as a grid. These lineament grids can then be displayed on top of any other grid.

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Processing and Interpretation of G&M Data Grid Display

Aside from the data transformations applied to grids it is often beneficial to display grids themselves in a variety of ways. This ensures that the maximum amount of information contained in the transforms can be utilised in the interpretation phase. The following three grids of gravity data show the same data displayed in grey-scale shaded relief, colour shaded relief and in a dipazimuth display. Vector data (station locations, flight lines, coastlines etc.) can be added as an overlay. The di...