WO2015104599A2

WO2015104599A2 - Method of determining the location and depth of magnetic sources in the earth

Info

Publication number: WO2015104599A2
Application number: PCT/IB2014/066830
Authority: WO
Inventors: Gordon Robert John COOPER
Original assignee: University Of The Witwatersrand, Johannesburg
Priority date: 2014-01-08
Filing date: 2014-12-12
Publication date: 2015-07-16
Also published as: WO2015104599A3

Abstract

The present invention relates to a method for locating magnetic bodies within the earth and in particular to a method for determining the subsurface location, geometry and depth of these bodies from aeromagnetic data. The method includes accessing aeromagnetic data and processing the data according to the described equations to determine the subsurface location, geometry and depth of these bodies.

Description

METHOD OF DETERMINING THE LOCATION AND DEPTH OF MAGNETIC SOURCES IN THE EARTH

BACKGROUND OF THE INVENTION

The present invention relates to a method for locating magnetic bodies within the earth and in particular to a method for determining the subsurface location, geometry and depth of these bodies from aeromagnetic data.

The method specifically relates to locating bodies buried in the subsurface by analysing their effect upon the ambient magnetic field of the Earth. The strength of the Earth's magnetic field has been measured across almost all of the Earth's land surface using ground and airborne based systems. Once the raw data has been collected it must be interpreted, which is performed using standard techniques such as modelling and inversion. However these techniques require initial estimates of the parameters of the magnetic bodies (such as their location, depth, dip, and susceptibility) to be effective. There are a variety of such semiautomatic interpretation techniques available, but they all have restrictions or problems, such as only working with profile data, or being restricted to a specific source type, or failing in the presence of remnant magnetisation (the magnetisation which some rocks possess even in the absence of the geomagnetic field).

The present invention provides an improved method and system to address this. SUMMARY OF THE INVENTION

According to one example embodiment, a system for interpreting aeromagnetic data, the system including: a memory for storing therein aeromagnetic data; and a data processor for accessing the data stored in the memory and processing the data according to the following formulae:

or

where Δz represents the depth of the magnetic source; angle τ is an angle selected by the data processor; and x and is the location at which the Tilt Angle of the analytic signal amplitude takes on a value τ.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a block diagram illustrating an example server to implement the present invention;

Figure 2 shows the synthetic magnetic response from the model shown in Figure 2c;

Figure 3 shows aeromagnetic data captured from a portion of the eastern limb of the Bushveld Igneous Complex in South Africa overiain with the response of dykes that are shown in Figure 3c; and

Figure 4 shows method steps carried out by the data processing module of Figure 1.

DESCRIPTION OF EMBODIMENTS

The systems and methodology described herein relate to locating magnetic bodies within the earth and in particular to a method for determining the subsurface location, geometry and depth of these bodies from aeromagnetic data.

Referring to the accompanying Figures, a system for interpreting aeromagnetic data includes a server 10 that includes a number of modules to implement the present invention and an associated memory 12.

In one example embodiment, the modules described below may be implemented by a machine-readable medium embodying instructions which, when executed by a machine, cause the machine to perform any of the methods described above.

In another example embodiment the modules may be implemented using firmware programmed specifically to execute the method described herein.

It will be appreciated that embodiments of the present invention are not limited to such architecture, and could equally well find application in a distributed, or peer-to-peer, architecture system. Thus the modules illustrated could be located on one or more servers operated by one or more institutions.

It will also be appreciated that in any of these cases the modules may form a physical apparatus with physical modules specifically for executing the steps of the method described herein. The memory 12 has stored therein aeromagnetic data.

Aeromagnetic data acquisition systems currently acquire the strength of the Earth's magnetic field over a survey area, and also the positions at which the field values were recorded. The aeromagnetic data will therefore typically include position data and magnetic field strength data with the data being time together so that it is known what magnetic field strength was measured at a particular position.

The most common positional data used is a grid which will then include an x and a y measurement. Another example of positional data is the location of an aircraft including its height when an aeromagnetic data reading was taken. it is also common nowadays to directly measure the gradients of the magnetic field i.e the df/dx, df/dy, and df/dz terms that appear in the equations below. However if they are not measured they can be calculated numerically.

In any event, a data processor 14 accesses the data stored in the memory and processes the data as will be described below.

Firstly, the data processor 14 access or calculate the gradients of the magnetic field mentioned above.

Next the data processor 14 will calculate As which is the analytic signal amplitude. The analytic signal amplitude can be thought of as the magnitude of the gradients of the magnetic field f, and is given by:

Next the tilt angle T_{AS 0}t As is calculated by the data processor 14 according to the following formulae.

The data processor 14 next substitutes into the above the equations for the derivatives of the magnetic anomaly across a contact which gives:

It will be appreciated that in this formulae ΔZ is the vertical distance to the source and Ax is the horizontal distance.

Note that it is not necessary for the contact to be vertically dipping, or for it to be vertically magnetised. Indeed, this equation is also obtained if the potential field source is a dyke, or is of any source of the type f = 1/rN.

In any event, the horizontal location of the source occurs when the T_AS = πΙ2. If the 7_rtS has a value of +τ at a location x+ and a value -τ at a location x- then the depth Az to the source is given by:

or

where x_f+ and x₂+ are two adjacent locations at which

Thus the processor 14 will select any value for % between +π/2 and -π/2 and either measure the distance between the adjacent positions x+ where the TAS has the values +x, or measure the distance between the adjacent positions x-*- and x- where the TAS has the values x+ and t- respectively.

Figure 2a shows the synthetic magnetic response from the model shown in Figure 2c. Figure 2b shows the T_ASt and has the source depths overlain on it. Using the distance between the τ+ values as the source depth proved more accurate than using half the distance between the τ+ and the t- values because the τ+ values were always closer to the source than were the τ- values, and so they were less affected by interference and edge effects, interference tends to narrow the width of the T_As peaks and so reduce the depths that are determined.

Thus, Figure 2a shows a synthetic magnetic anomaly produced by the model shown in Figure 2c. The profile azimuth is south-north (left to right).

Figure 2b T_As of the data shown in Figure 2a. Overlain are the distances between the ±π/4 values and the associated depth values.

Figure 2c is the model used to generate the data shown in Figure 2a. The inclination and declination used were 0.0° and 0.0°. The susceptibilities of the bodies (from left to right) are 0.03, 0.05, 0.1 and 0.1 SI units respectively. Overlain as * are the locations of the sources. The horizontal position is taken as the midpoint of the +rt/4 TAS values, and the depth as the distance between them. The structural index N, as determined from equation 12, is displayed alongside the source location. in order to determine the susceptibility and dip of the sources, R is defined as the ratio of the vertical to the horizontal derivative of the magnetic anomaly over a contact, then:

where β = 21 -dip - 90° tan I = tan i/cos a, c - 1 - cos2l . sin2a , and a is the clockwise angle between north and the positive x axis. Hence when Δχ - 0, dip = 21 -90 - tan-1(-R\_x=0).

For a dyke, the dip becomes d

Using the expressions for the As of a contact and dyke shows that, at x = 0, the susceptibility and susceptibility*thickness product are

Contact:

Dyke:

where the geomagnetic field intensity is F, the susceptibility is k, and the width of the dyke is w. it is necessary to able to establish whether the source is a dyke or a contact before the dip and susceptibility can be calcuiated. The distance r to a potential field source of the type Λ/F is given by:

where N is the structural index of the source (Λ/=1 for a dyke and Λ/=0 for a contact, for magnetic data). This equation is not useful in determining the distance to magnetic contacts because N - 0. However this equation has been applied by the inventor to the As and obtained:

where As₂ \s the second order analytic signal amplitude given by:

where C is a constant. So if the location of the source is known then the source type can be obtained from the above two equations. The values of N are never exactly accurate due to noise, interference, etc, but since all that is required is to discriminate between contact and dyke sources, perfect accuracy is not required. Designating the source as a contact if N < 0.5 and as a dyke otherwise gave good results.

Referring to Figure 3a, this shows the application of the TAS method to an aeromagnetic data profile from the eastern Bushveld Igneous Complex, South Africa. Unlike Euler deconvoiution (Reid et ai, 1990), the T_AS method only produces a single solution per source, making interpretation easier. The forward response of the dykes is overlain on the data, and it can be seen to fit sufficiently well such that the T_AS semiautomatic interpretation method has provided a useful starting model for a full interpretation.

In this Figures, 3a shows Aeromagnetic data profile from the eastern Bushveld Igneous Complex, South Africa (dotted line), overlain with the response of the dykes that are shown in Figure 3c (red line) . The sampling interval was 15 m.

Figure 3b shows T_As of the data from Figure 3a. The horizontal dotted line shows the value of the TAs that was used to determine the source depths.

Figure 3c shows Dykes and contacts obtained from the T_As method. Their susceptibi!ity^*thickness products are displayed alongside.

In all of the above methodologies, information is displayed to a user via graphical user interface 16.

Thus it will be appreciated that a new semi-automatic interpretation technique has been introduced which can be applied to magnetic data from contacts, dykes, and sources of the type without requiring the structural index to be a priori specified. The method was shown to perform well both with synthetic data and with data from South Africa.

Claims

CLAiMS

1. A system for interpreting aeromagnetic data, the system including: a memory for storing therein aeromagnetic data; and a data processor for accessing the data stored in the memory and processing the data according to the following formulae:

or

where ΔΖ represents the depth of the magnetic source; angle τ is an angle selected by the data processor; and x and is the location at which the Tilt Angle of the analytic signal amplitude takes on a value x.

2. A system according to claim 1 wherein the data processor retrieves a value of f from the memory and uses the retrieved value of f to compute gradients of a magnetic field being df/dx, df/dy and df/dz.

3. A system according to claim 1 wherein the data processor retrieves the values of df/dx, df/dy, and df/dz.from the memory.

4. A system according to claim 3 or claim 4 wherein the data processor uses the gradients df/dx, df/dy, and df/dz to compute an analytic signal amplitude As.

5. A system according to claim 4 wherein the data processor computes the tilt angle of the analytic signal amplitude As_.