CN102466817A

CN102466817A - Method for picking up abnormal gravity boundary by using normalized derivative model method

Info

Publication number: CN102466817A
Application number: CN2010105431570A
Authority: CN
Inventors: 宋景明; 邓国军
Original assignee: China National Petroleum Corp; BGP Inc
Current assignee: China National Petroleum Corp; BGP Inc
Priority date: 2010-11-11
Filing date: 2010-11-11
Publication date: 2012-05-23

Abstract

A normalized derivative model method for picking up the abnormal boundary of gravity in petroleum and solid minerals exploration features that the gravity-magnetic data is used to explain the boundary of geological structure such as fracture. Weak information generated by shallow and near-surface inhomogeneities in the gravity data grid gravity anomaly is removed by using low-pass filtering, an X-axis direction derivative and a Y-axis direction derivative are respectively obtained to obtain a horizontal gradient mode, a coherent signal is determined to obtain a curvature attribute and a normalized derivative mode and an contour map, and a picked gravity anomaly boundary is determined according to a maximum connecting line of the anomaly. The abnormal information obtained by the invention not only narrows down and thins the large-scale abnormal width, but also can effectively highlight the small-scale weak abnormality.

Description

Method for picking up abnormal gravity boundary by using normalized derivative model method

Technical Field

The invention relates to a method for picking up a gravity anomaly boundary of a geological structure boundary such as fracture by using gravity and magnetic data in petroleum and solid mineral exploration.

Background

The gravity data is used for identifying the fracture mainly according to the abnormal linear stair band, abnormal distortion, abnormal partition characteristics and the like of the Bragg gravity. In recent years, it has become common to identify the location of a fracture using the location of the maximum of the total gradient of the abnormal level of gravity. In addition to the conventional methods described above, there are other methods of identifying fractures, Wang Yichang proposed the use of the horizontal second derivative to explain the reverse fault "oil geophysical prospecting" (1989.2); according to the method, a fracture structure is identified by using an improved gravity normalization total gradient and phase method, a gravity abnormal linear structure signal method and the like are provided by the Zhang organ and the like, an earth science version (2005, 35 (1)) is reported by Jilin university, and an inversion method for determining the fault breakpoint position based on a DCT Euler method is provided by the Zhang organ (2006).

These methods are either insensitive to weak signals, and may miss some smaller sized breaks; or although a weak signal is enhanced, noise is amplified at the same time, the noise is not suppressed in place, and the fracture with smaller scale cannot be identified.

Disclosure of Invention

The invention aims to provide a method for picking up the abnormal gravity boundary by a normalized derivative model method, which is used for removing noise, highlighting a linear weak signal and extracting a small-scale fractured weak signal.

The invention is realized by the following technical scheme:

1) collecting gravity data of a target area, and removing weak information generated by shallow and near-surface inhomogeneities in the Booth gravity anomaly (g) by using low-pass filtering, median filtering or small-scale upward continuation technology;

2) respectively solving X-axis direction derivative of the Booth gravity anomaly (g) subjected to denoising and information enhancement

And derivative in Y-axis direction

Then, solving horizontal gradient modules Z (X, Y) in the two directions, wherein the X axis is the north-south direction, and the Y axis is the east-west direction;

the horizontal gradient modulus (Z (x, y)) in step 2) is:

<math> <mrow> <mi>Z</mi> <mrow> <mo>(</mo> <mi>x</mi> <mo>,</mo> <mi>y</mi> <mo>)</mo> </mrow> <mo>=</mo> <msqrt> <msup> <mrow> <mo>(</mo> <mfrac> <mrow> <mo>&PartialD;</mo> <mi>g</mi> </mrow> <mrow> <mo>&PartialD;</mo> <mi>x</mi> </mrow> </mfrac> <mo>)</mo> </mrow> <mn>2</mn> </msup> <mo>+</mo> <msup> <mrow> <mo>(</mo> <mfrac> <mrow> <mo>&PartialD;</mo> <mi>g</mi> </mrow> <mrow> <mo>&PartialD;</mo> <mi>y</mi> </mrow> </mfrac> <mo>)</mo> </mrow> <mn>2</mn> </msup> </msqrt> </mrow> </math>

..............①

wherein Z (x, y) is the horizontal gradient mode;

is the derivative of the Bruger gravity anomaly g in the X-axis direction;

the derivative in the Y-axis direction of the Bruger gravity anomaly g is obtained;

3) enhancing the signal by adopting a vertical second derivative method;

the signal enhancement by the vertical second derivative method in the step 3) is to solve the second derivative (Z (x, y)) in the Z-axis direction of the horizontal gradient module (Z (x, y))_zz)：

<math> <mrow> <mi>Z</mi> <msub> <mrow> <mo>(</mo> <mi>x</mi> <mo>,</mo> <mi>y</mi> <mo>)</mo> </mrow> <mi>zz</mi> </msub> <mo>=</mo> <mfrac> <mrow> <msup> <mo>&PartialD;</mo> <mn>2</mn> </msup> <mi>Z</mi> <mrow> <mo>(</mo> <mi>x</mi> <mo>,</mo> <mi>y</mi> <mo>)</mo> </mrow> </mrow> <mrow> <mo>&PartialD;</mo> <msup> <mi>z</mi> <mn>2</mn> </msup> </mrow> </mfrac> </mrow> </math>

.................②

Wherein,

the second derivative in the Z direction of the horizontal gradient modulo Z (x, y).

4) Determining a coherent signal (f) according to the following formula_g)；

f_s＝IIf(Z(x，y)_zz＞0，Z(x，y)_zz，fch)……………………③

Wherein f is_gIs a coherent signal; IIf () is a kernel function for calculating f_g(ii) a fch is a threshold factor.

The value range of the threshold value factor in the step 4) is as follows: -1. ltoreq. fch. ltoreq.1, fch 0 by default.

5) The curvature attribute (fp) and the normalized derivative modulus (f) are obtained according to the following equations:

curvature property (fp) f_p＝f_g/Z(x，y)………………………………④

Normalized derivative (f) f ═ f_p/f_pmax……………………………⑤

Wherein f is_pmax is the maximum of the curvature attribute (fp).

6) Drawing a contour map or a solid shadow map of the normalized derivative model (f), and determining a boundary of the picking gravity anomaly according to the maximum connecting line of the f anomaly.

The abnormal information obtained by the invention not only narrows down and thins the large-scale abnormal width, but also effectively highlights the small-scale weak abnormality, the abnormal grade is clear after the drawing, and the geological interpretation precision is greatly improved.

Drawings

FIG. 1 model theoretical gravity anomaly;

FIG. 2 horizontal total gradient anomaly;

FIG. 3 is a normalized derivative modulo anomaly;

FIG. 4 shows an anomaly in the global gradient of gravity level for a region of interest;

FIG. 5 shows that the gravity normalized derivative of a certain study area is abnormal.

Detailed Description

The present invention is described in detail below with reference to the accompanying drawings.

The invention highlights and enhances the change of the gravity gradient by calculating the Booth gravity anomaly (g), thereby realizing the identification and extraction of the boundary information of the gravity anomaly.

The main technical steps are as follows:

And derivative in Y-axis directionFurther, a horizontal gradient module Z (x, y) is obtained;

3) the second derivative Z (x, y) in the Z-axis direction is obtained from the horizontal gradient module Z (x, y) by using a vertical second derivative method_zzEnhancing the signal;

4) using Z (x, y)_zzThe anomaly characteristic determines a threshold factor. Determining a coherent signal (f)_g)；

5) ComputingCurvature attribute (fp) and search for maximum value f of curvature attribute (fp)_pmax，

6) And (3) calculating a normalized derivative model (f), drawing a contour map or a solid shadow map of the normalized derivative model (f), and determining a boundary of the picked gravity anomaly according to a maximum value connecting line of the f anomaly.

In order to carry out theoretical verification on the technology and compare the technology with a conventional method, namely a horizontal total gradient method, a cube model is given by utilizing a three-dimensional forward modeling, so that the theoretical gravity anomaly of the cube (figure 1) is obtained, and the higher the contour line mark value in the figure is, the higher the gravity value is. The gravity anomaly is shown as having a center height that decreases around. Calculating the horizontal total gradient abnormality (figure 2) of the theoretical gravity abnormality by utilizing conventional gravity abnormality processing, wherein the horizontal total gradient abnormality contour line is more intensively distributed near the theoretical model boundary (square), is mild in abnormality and is reduced towards two sides; and (3) calculating the abnormal normalized derivative model (figure 3) of the theoretical gravity anomaly by utilizing the normalized derivative model technology, wherein the abnormal characteristics of the normalized derivative model are consistent with the abnormal horizontal total gradient, the abnormal contour lines are more concentrated, and the abnormal maximum value range of the normalized derivative model is narrowed. The delineation of the abnormal boundary (or break) is more subtle.

The effect of the normalized derivative mode on picking up anomalous boundary information can be seen from the following example.

FIGS. 4 and 5 are anomaly contour plots calculated from a Booth's gravity anomaly for the same study area reflecting gravity anomaly boundaries that are geologically interpretable as fractures. The traditional gravity level total gradient anomaly map (fig. 4) mainly reflects large-scale linear information in a research area, namely reflects the existence of large-scale fracture and cannot reflect small-scale fracture or weak information. The gravity normalization derivative mode abnormal boundary picking-up graph (figure 5) can well reflect main fracture in a research area and can also well reflect small fracture or weak information in the research area. The number of fractures explained with reference to fig. 4 is small, and the contour line near C, B is almost blank, no abnormal display is shown, and the feeling on the whole is messy. From fig. 5, it is possible to explain both large major fractures and secondary minor fractures. Meanwhile, the control relation between the main fracture and the secondary fracture can be reflected, and the secondary fracture has certain regularity in a local area. For example, the west small fracture direction of a is mainly NWW direction, and the east fracture direction of a is mainly NE direction.

Claims

1. A method for picking up the abnormal boundary of gravity by normalized derivative model method is characterized by comprising the following steps:

1) collecting gravity data of a target area, and removing weak information generated by shallow and near-surface inhomogeneities in the bump gravity anomaly by using low-pass filtering, median filtering or small-scale upward continuation technology;

2) respectively solving X-axis direction derivative of the Booth gravity anomaly subjected to denoising and information enhancement

And derivative in Y-axis direction

Then, the horizontal gradient modulus Z (x, y) of the two directions is obtained,

the X axis is in the north-south direction, and the Y axis is in the east-west direction;

3) enhancing the signal by adopting a vertical second derivative method;

4) determining a coherent signal (f) according to the following formula_g)；

f_g＝IIf(Z(x，y)_zz＞0，Z(x，y)_zz，fch)……………………③

Wherein f is_gFor coherent signals, IIf () is the kernel function, fcg is the threshold factor;

5) obtaining the curvature attribute and the normalized derivative according to the following formula:

curvature property f_p：f_p＝f_g/Z(x，y)…………………………④

Normalization pilot number f: f ═ f_p/f_p max………………………⑤

Wherein f is_pmax is the maximum of the curvature property.

6) Drawing a contour map or a solid shadow map of the normalized derivative model f, and determining a boundary of the pickup gravity anomaly according to a maximum value connecting line of the normalized derivative model f anomaly.

2. The method according to claim 1, characterized in that the horizontal gradient norm (Z (x, y)) of step 2) is:

..................①

wherein Z (x, y) is the horizontal gradient mode;

is the derivative of the Bruger gravity anomaly g in the X-axis direction;

the derivative of the direction of the Y axis of the Breger force anomaly g.

3. The method according to claim 1, characterized in that the vertical second derivative method for enhancing the signal in step 3) is to obtain the second derivative (Z (x, y)) in the Z-axis direction for the horizontal gradient module (Z (x, y))_zz)：

...............②

Wherein,the second derivative in the Z direction of the horizontal gradient modulo Z (x, y).

4. The method according to claim 1, characterized in that the threshold factor value range in step 4) is: -1. ltoreq. fch. ltoreq.1, fch 0 by default.