# US 7,613,570 B2 · Method and apparatus for deriving a calibration filter for electromagnetic data

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- Number: US 7,613,570 B2
- Document ID: US7613570B2
- Status: Granted
- Kind: B2
- Application number: 11/547,378
- Filed: 2005-04-01
- Published: 2008-11-13
- Granted: 2009-11-03
- Earliest claimed: 2004-04-03
- Assignees: Statoil ASA
- Inventors: Tage Røsten; Lasse Amundsen
- Web page: https://patentgrep.com/patent/US7613570B2

## CPC

- G01V 3/12: operating with electromagnetic waves {(operating with millimetre waves G01V8/005)}
- G01V 3/083: {Controlled source electromagnetic [CSEM] surveying}
- G01V: GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS (means for indicating the location of accidentally buried, e.g. snow-buried, persons A63B29/02)

## Abstract

A method of deriving a calibration filter for electromagnetic data is provided. The method comprises providing a source (21, 22) of electromagnetic radiation and subsequently measuring components of the source electromagnetic field (23, 24, 25) at a receiver when the source is sufficiently close to the receiver that the down-going signal dominates over the up-going signal reflected from features below the receiver. Such a measurement can be used in frequency domain analysis (26, 28, 29) to extract the down-going signal component and derive a calibration filter for the receiver. The method is repeated (27, 30) for each receiver used in an electromagnetic sounding survey.

## Claims

### Claim 1

A method of deriving a calibration filter for electromagnetic data, comprising the steps of: generating electromagnetic radiation at a source; measuring at least one component of the electromagnetic field provided by the source at the at least one receiver when the source is sufficiently close to the at least one receiver for the magnitude of a down-going electromagnetic field to be sufficiently greater than that of an up-going electromagnetic field to allow the derivation of a calibration filter; and deriving from the measurement a calibration filter for the at least one receiver for use in processing electromagnetic data.

### Claim 2 (depends on 1)

A method as claimed in claim 1, wherein the magnitude of the up-going electromagnetic field is less than about one-third of the magnitude of the down-going electromagnetic field.

### Claim 3

A method as claimed in claimed in 1, wherein the source is positioned above the at least one receiver during the measuring step.

### Claim 4 (depends on 3)

A method as claimed in claim 3, wherein the source is positioned directly above the at least one receiver during the measuring step.

### Claim 5 (depends on 1)

A method as claimed in claim 1, wherein the at least one receiver is disposed on the seabed.

### Claim 6 (depends on 1)

A method as claimed in claim 1, wherein the electromagnetic radiation produced by the source propagates into the seabed.

### Claim 7 (depends on 1)

A method as claimed in claim 1, wherein the at least one receiver is for use in electromagnetic seabed logging (EM-SBL).

### Claim 8 (depends on 1)

A method as claimed in claim 1, wherein the calibration filter is derived for a magnetic receiver within the at least one receiver.

### Claim 9 (depends on 1)

A method as claimed in claim 1, wherein the calibration filter is derived for an electric receiver within the at least one receiver.

### Claim 10 (depends on 1)

A method as claimed in claim 1, wherein the calibration filter is obtained from a frequency domain analysis of measurements made at the at least one receiver.

### Claim 11 (depends on 1)

A method as claimed in claim 1, wherein the derived calibration filter is an effective calibration filter {tilde over (B)}(ω) obtained using the equation {tilde over (B)}(ω)={tilde over (E)}x(ω)|{tilde over (H)}y(ω), where {tilde over (E)}x(ω) and {tilde over (H)}y(ω) are measurements made at the at least one receiver of the electric and magnetic signals in the frequency domain, in orthogonal x and y directions, respectively, related by the expression
E
x
U
⁡
(
ω
)
=
1
2
⁡
[
E
~
x
⁡
(
ω
)
-
B
~
⁡
(
ω
)
⁢
H
~
y
⁡
(
ω
)
]
=
0.

### Claim 12 (depends on 11)

A method of deriving a calibration filter, wherein the calibration filter is a combination of different effective calibration filters obtained using a method according to claim 11.

### Claim 13 (depends on 1)

A method as claimed in claim 1, wherein the derived calibration filter is an effective calibration filter {tilde over (B)}(ω) obtained using the equation {tilde over (B)}(ω)=−{tilde over (E)}y(ω)|{tilde over (H)}x(ω), where {tilde over (E)}y(ω) and {tilde over (H)}x(ω) are measurements made at the at least one receiver of the electric and magnetic signals in the frequency domain, in orthogonal x and y directions, respectively, related by the expression
E
y
U
⁡
(
ω
)
=
1
2
⁡
[
E
~
y
⁡
(
ω
)
-
B
~
⁡
(
ω
)
⁢
H
~
x
⁡
(
ω
)
]
=
0.

### Claim 14 (depends on 1)

A method as claimed in claim 1, wherein the derived calibration filter is an effective calibration filter {tilde over (b)}(t) obtained using the equation {tilde over (b)}(t)=ex(t)*hy−1(t), where ex(t) and hy(t) are measurements made at the at least one receiver of the electric and magnetic signals in the time domain, in orthogonal x and y directions, respectively.

### Claim 15 (depends on 14)

A method of deriving a calibration filter, wherein the calibration filter is a combination of different effective calibration filters obtained using a method according to claim 14.

### Claim 16 (depends on 1)

A method as claimed in claim 1, wherein the derived calibration filter is an effective calibration filter {tilde over (b)}(t) obtained using the equation {tilde over (b)}(t)=−ey(t)*hx−1(t), where ey(t) and hx(t) are measurements made at the at least one receiver of the electric and magnetic signals in the time domain, in orthogonal x and y directions, respectively.

### Claim 17 (depends on 1)

A method as claimed in claim 1, wherein the derived calibration filter is a true calibration filter B(ω) obtained using the equation B(ω)={tilde over (E)}x(ω)|{tilde over (H)}y(ω)|A(ω), where {tilde over (E)}x(ω) and {tilde over (H)}y(ω) are frequency domain measurements made at the at least one receiver of the electric and magnetic signals in the frequency domain, in orthogonal x and y directions, respectively, and A(ω) is a known decomposition filter, related by the expression
E
x
U
⁡
(
ω
)
=
1
2
⁡
[
E
~
x
⁡
(
ω
)
-
A
⁡
(
ω
)
⁢
B
⁡
(
ω
)
⁢
H
~
y
⁡
(
ω
)
]
=
0.

### Claim 18 (depends on 17)

A method of deriving a calibration filter, wherein the calibration filter is a combination of different effective calibration filters obtained using a method according to claim 17.

### Claim 19 (depends on 1)

A method as claimed in claim 1, wherein the derived calibration filter is a true calibration filter B(ω) obtained using the equation B(ω)=−{tilde over (E)}y(ω)|{tilde over (H)}x(ω)|A(ω), where {tilde over (E)}y(ω) and {tilde over (H)}x(ω) are measurements made at the at least one receiver of the electric and magnetic signals in the frequency domain, in orthogonal x and y directions, respectively, and A(ω) is a known decomposition filter related by the expression
E
y
U
⁡
(
ω
)
=
1
2
⁡
[
E
~
y
⁡
(
ω
)
+
A
⁡
(
ω
)
⁢
B
⁡
(
ω
)
⁢
H
~
x
⁡
(
ω
)
]
=
0.

### Claim 20 (depends on 1)

A method as claimed in claim 1, wherein the calibration filter is derived through application of a frequency domain transform to a localised window of the measured data in which the magnitude of the down-going field is greater than the magnitude of the up-going field.

### Claim 21 (depends on 1)

A program for controlling a computer to perform a method as claimed in claim 1.

### Claim 22 (depends on 21)

A program as claimed in claim 21 stored on a storage medium.

### Claim 23 (depends on 21)

Transmission of a program as claimed in claim 21 across a communications network.

### Claim 24 (depends on 1)

A computer programmed to perform a method as claimed in claim 1.

### Claim 25 (depends on 1)

A method of processing electromagnetic data using a calibration filter, the method comprising:
deriving a calibration filter in accordance with a method as claimed in claim 1;
applying the calibration filter to electromagnetic data subsequently recorded at the at least one receiver; and
separating the up-going and down-going components of the electromagnetic data.

### Claim 26 (depends on 25)

A method as claimed in claim 25, wherein the down-going component is removed.

### Claim 27 (depends on 25)

A method as claimed in claim 25, wherein the data is EM-SBL data, and the at least one receiver is disposed upon the seabed.

### Claim 28 (depends on 25)

Data obtained from a method of processing electromagnetic data as claimed in claim 25.

### Claim 29 (depends on 28)

Data as claimed in claim 28 when stored on a storage medium.

### Claim 30 (depends on 25)

A computer programmed to perform a method as claimed in claim 25.

### Claim 31

An apparatus for deriving a calibration filter for electromagnetic data, the apparatus comprising:
a source of electromagnetic radiation;
at least one receiver arranged to measure at least one component of the electromagnetic field provided by the source when the source is sufficiently close to the at least one receiver for the magnitude of a down-going electromagnetic field to be sufficiently greater than that of an up-going electromagnetic field to allow the derivation of a calibration filter; and
means for deriving from the measurement a calibration filter for the at least one receiver for use in processing electromagnetic data.

### Claim 32 (depends on 31)

A program for controlling a computer to perform a method as claimed in claim 31.

### Claim 33 (depends on 32)

A program as claimed in claim 32 stored on a storage medium.

### Claim 34 (depends on 32)

Transmission of a program as claimed in claim 32 across a communications network.