Chapter 2: Acquiring rotations with magnetometers

Shuki

Ronen

Bob

Brune

Kerry

Key

Steven

Constable

• Current rotational acquisition technology for exploration seismology
• Rotations from magnetic projections
• Silver Lake magnetic-rotation survey

Chris Castillo

Requirements for a seismic sensor

2

• Durability
• Reliability
• Sensitivity
• Dynamic range (amplitude, frequency)
• Power consumption
• Availability / economy
• …...

Seismic rotation sensor technologies

3

• Ring lasers
• Fiber-Optic Gyroscopes
• Electrokinetic / Magneto-hydrodynamic
• Array-derived rotations / Rotaphone

Seismic rotation sensor technologies

4

• Ring lasers
• Fiber-Optic Gyroscopes
• Electrokinetic / Magneto-hydrodynamic
• Array-derived rotations / Rotaphone

Geodesy, Gravity waves

Inertial navigation systems

Array-derived rotations

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

​

5

Hooke’s law:

Free surface:

Array-derived rotations

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

​

6

Rotations:

Derive rotations without burying the geophones

Array-derived rotations

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

7

Rotations:

Aliasing

Noise

Array-derived rotations / Rotaphone

8

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

• Vertical and horizontal geophones attached to the same frame
• Rotation is measured by differencing geophone components
• Requires in-situ cross-calibration between geophone responses before recording data

Noise floor: 4 x 10^-9 rad/sec

Dynamic range: 120 dB

​

Brokesova and Malek, 2016

Electrokinetic rotation sensor

9

Noise floor: 0.6 x 10^-7 rad/sec

Dynamic range: 117 dB

Passband: 0.03 – 50 Hz

Power usage: 12 mA

Weight 1.5 kg

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

METR03 (AST LLC, Russia)

Electrokinetic rotation sensors for exploration seismology

10

Advantages:

• Field deployable (on land)
• ~Exploration seismology bandwidth (2-30 Hz)
• Price

Vertical

Pitch

Challenges:

• Dynamic range
• Noise floor (need < 10^-8 rad/sec)
• Availability
• Durability

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

Current sensor summary

11

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

Angela Di Virgilio, 2016

Seismic rotation sensor technologies

12

• Ring lasers
• Fiber-Optic Gyroscopes
• Electrokinetic / Magneto-hydrodynamic
• Array-derived rotations / Rotaphone

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

• Induction coil magnetometers?

Scripps Marine Electromagnetics Lab

13

Induction-coil magnetometers

EM ocean-bottom node

Field deployment

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

Controlled source EM survey

Scripps Marine Electromagnetics Lab

14

Induction-coil magnetometers

EM ocean-bottom node

Field deployment

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

Controlled source EM survey

11 years ago

15

We see seismic shots in ocean-bottom EM data.

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

I wonder why?

April 2009: Earthquake recorded by EM components

16

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

2 years ago, while working on the Kettleman 6C dataset

17

Ohad, could it be that Steve’s EM nodes were recording rotations?

Hmmm…..

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

Faraday’s law

18

Michael Faraday

Induction-coil magnetometer

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

Seismic rotations from induction-coil magnetometers

19

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

Seismic rotations from induction-coil magnetometers

20

Magnetic projections

Rotations

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

Projection null space

21

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

Ambient magnetic field direction

https://en.wikipedia.org/wiki/Euler_angles

Silver Lake magnetic-rotations experiment

22

Las Vegas

Los Angeles

Silver Lake

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

EM ambient noise floor

23

Betsy gun

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

Shot / Receiver Geometry

24

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

+ 1 remote magnetometer station to attenuate magnetotelluric waves

Rotation sensors, magnetometers and geophones

25

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

Remote magnetometer station

26

Remote Hx

Remote Hy

Remote Hz

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

27

Hy

Hx

Hz

Ry

Rx

Rz

28

Hy

Hx

Hz

Ry

Rx

Rz

29

Hy

Hx

Hz

Ry

Rx

Rz

V=130 m/s

V=230 m/s

V=1420 m/s

30

Hy

Hx

Hz

Ry

Rx

Rz

31

Hy

Hx

Hz

Ry

Rx

Rz

32

Hy

Hx

Hz

Ry

Rx

Rz

Converting magnetic projections to rotations

33

Hy

Hx

Hz

Ry

Rx

Rz

Quaternion rotation module

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

William R. Hamilton,

1844

Pitch (Ry)

Roll (Rx)

Yaw (Rz)

Rotation sensors

Pitch (Ry)

Roll (Rx)

Yaw (Rz)

Magnetometers

Pitch (Ry)

Roll (Rx)

Yaw (Rz)

Magnetometers

Noise from vertical magnetometer

Pitch (Ry)

Roll (Rx)

Yaw (Rz)

Projection null space

38

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

• Ambient magnetic field direction: Z
• Can derive rotations around X and Y
• Can’t derive rotations around Z
• Non-commutativity of rotations

https://en.wikipedia.org/wiki/Euler_angles

Pitch (Ry)

Roll (Rx)

Yaw (Rz)

Summary

40

1. Robust rotation sensor technology is not (yet) widely available for industry-scale exploration seismology
2. 3-component magnetometers can be used as 3-component seismic rotation sensors
3. Need to remove noise from ambient local magnetic fields
4. Consider null space of projections

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

Prospects

41

1. 6C earthquakes from existing magnetometer data

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

2. 6C ocean-bottom seismic surveys

• Steve Constable, Kerry Key, and Scripps students
• Shuki Ronen
• The sponsors of the Scripps Marine EM Consortium
• John Archer, Patryusz Batchelda – Geokinetics
• Chris Castillo

Acknowledgements

Spare slides

43

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

What are rotation data�Acquisition and application of 6C data�Recording rotations without rotation sensors�Silver Lake magnetic-rotation survey

Steven Constable, Kerry Key

Rotations from magnetometers

44

Time (sec)

Geophones

Induction-coil magnetometers

mm / s

nanoTesla

What are rotation data�Acquisition and application of 6C data�Recording rotations without rotation sensors�Silver Lake magnetic-rotation survey

Six-component Earthquake data from combined 3C geophones and 3C magnetometer

45

Is this reliable?�Active seismic field experiment to validate rotations from magnetic

Transverse

Pitch

Radial

EM ambient noise floor

46

Ring Lasers

47

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

Based on the “Sagnac” effect:

Phase shift of two light beams propagating in opposite directions

Source: Wikipedia

Ring Lasers

48

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

Based on the “Sagnac” effect:

Phase shift of two light beams propagating in opposite directions

• Earth’s rotation
• Gravitational waves

​

Seismic = noise

Source: Wikipedia

Ring Lasers

49

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

Down to 10^-13 rad/s at low frequencies

Wetzel, Germany

Ring Lasers

50

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

Angela Di Virgilio, 2016

Ring Lasers

51

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

Angela Di Virgilio, 2016

Ring Lasers

52

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

ROMY:

Tetrahedral ring laser currently under construction near Munich

Fiber-optic Gyros

53

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

Based on the “Sagnac” effect:

Phase shift of two light beams propagating in opposite directions

Need to ensure that polarization of the two light beams is identical

Source: Wikipedia

Fiber-optic Gyros

54

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

iXBlue (France)

​

20 nrad/s < 0.01 Hz

60 nrad/s 0.01-10 Hz

1 urad/s 10-100 Hz

​

High power usage: ~8 w

​

High sensor self-noise

Source: iXBlue

Magneto-hydrodynamic rotation sensor

55

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

Advanced Technology Associates (Albuquerque, New Mexico)

Magneto-hydrodynamic rotation sensor

56

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

Advanced Technology Associates (Albuquerque, New Mexico)

Magneto-hydrodynamic rotation sensor

57

AST (Moscow)

Magneto-hydrodynamic rotation sensor

58

AST (Eentec)

The seismoelectric effect

59

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

S. Haines, 2004

The seismoelectric effect

60

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

S. Haines, 2004

Pride, 1994, seismoelectric effect in porous medium

61

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

E: Electric field, H: Magnetic field, D: dielectric displacement, B: Magnetic flux density, J: Electric current density, u: particle displacement, w: relative grain/fluid displacement, p_f: fluid pressure, tau: bulk stress tensor, G: shear modulus, rho: bulk density

rho_f: pore fluid density, Omega: angular frequency, sigma: electrical conductivity, epsilon: electrical permittivity, mu: magnetic permeability, eta: fluid viscosity, K: hydraulic permeability, L: relation between grain/fluid motion, K_u: undrained bulk modulus, K_s: solid bulk modulus, K_f: fluid bulk modulus, phi: porosity

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

Pride, 1994, seismoelectric effect in porous medium

62

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

E: Electric field, H: Magnetic field, D: dielectric displacement, B: Magnetic flux density, J: Electric current density, u: particle displacement, w: relative grain/fluid displacement, p_f: fluid pressure, tau: bulk stress tensor, G: shear modulus, rho: bulk density

rho_f: pore fluid density, Omega: angular frequency, sigma: electrical conductivity, epsilon: electrical permittivity, mu: magnetic permeability, eta: fluid viscosity, K: hydraulic permeability, L: relation between grain/fluid motion, K_u: undrained bulk modulus, K_s: solid bulk modulus, K_f: fluid bulk modulus, phi: porosity

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

Water table at Silver Lake is ~500 meters down

63

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

Vertical geophone frequency content during active shooting at Silver Lake

EM rotations / 6C earthquakes / Seismoelectric effect / EM rotations + seismoelectric field test

64

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

B_z

B_y

B_x

R_x

R_y

R_z

R_x

R_y

R_z

65

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

B_z

B_y

B_x

R_x

R_y

R_z

R_x

R_y

R_z

Non-commutativity of rotations

66

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

67

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

B_z

B_y

B_x

R_x

R_y

R_z

R_x

R_y

R_z

68

Current rotational acquisition / Rotations from magnetic projections / Silver Lake magnetic-rotation experiment

B_z

B_y

B_x

R_x

R_y

R_z

R_x

R_y

R_z