Lecture 5 & 6 - CSEM and Inversion Flashcards
What is the problem with marine CSEM surveys compared to land CSEM surveys?
In marine surveys the resisivites of seawater, marine sediments and oil and gas filled rocks have only subtle differences.
On land rocks have a v. high resistivity compared to the gas and oil filled rocks
How does EM induction methods work?
- Pass a.c. current through transmitter
- Generate an alternating primary magnetic field
- This induces alternating electric current in subsurface conductors
- Electrical current in subsurface generates secondary magnetic field (alternating)
- Secondary magnetic field detected by receiver
What is Faraday’s Law?
A time varying magnetic field induces an electric field that is proportional in magnitude to the rate of change of magnetic flux
What is Maxwell/Ampere’s Law?
A magnetic field is generated in space by the flow of current and that field is proportional to the total current
What is recorded during a CSEM survey?
- Strength of electric & magnetic field
- Phase angle Φ at different offsets
Recorded in 2 perpendicular directions
What is the phase shift?
Phase shift = π/2 + Φ
Phase shift = between the primary and secondary EM field
π/2 = how much induced EM field from primary field delayed by
Φ = Phase angle
What does a large Φ indicate?
Good conductivity
Why is there poor penetration through porous sediments filled with saline water?
Because such sediments are relatively conductive.
High conductivity = low resistivty
skin depth proportional to root(resistivity)
What is the size of the induced EM field proportional to?
Frequency
Current
What frequencies and currents are required for marine CSEM survey and why?
V. high source currents (>100A) Low frequencies (~1Hz) to reach ~1km depth
Why does the EM source need to be as towed close to the sea-bed as possible during a marine CSEM survey?
- Rapid attenuation of primary EM field in seawater
2. Reduce range over which air wave is dominant
What specifically do the receivers measure when measuring the electric field?
- Radial and azimuthal electric field
- Electric field strength (voltage)
How do you normalise the electric field measurements for comparisons with other survey?
V/A*m^2
A = current V= Voltage m^2 = length of transmitter
How long does it take to do a CSEM survey?
Can cover 40km/day
Usually 4-6 days to prospect
Max tow speed 1.5-2 knots
What are the different arrivals at a receiver? (4)
- Air Wave
- Direct wave through sea water
- Wave that has traveled through subsurface sediments
- Wave traveled through rock & reservoir
Why is deep water better?
Because air wave and noise less of an issue in deep water
What graphs do you plot in a CSEM survey?
MVO - Magnitude of Voltage vs offset
NMVO - Normalised MVO (compare to background data where is no see what data would look like without resistor)
Phase angle vs offset
How does phase angle Φ change with offset?
- Changes rapidly with offset in seawater and sea-bottom sediments
- Changes less rapidly passing through resistive reservoir
- When line becomes constant it is because the reciever is detecting the airwave only
Why is the phase of the airwave constant?
Because of air’s high resistivity
Which measured component is most sensitive to HCs during CSEM survey?
Radial component of induced EM field
What is the purpose of performing synthetic tests?
- Explore sensitivity of data to target properties
- Help design experiments
- Identify appropriate modelling strategy
- Assess resolution of models
Why are CSEM surveys better to constrain saturation than seismic surveys?
Impedance and velocity not sensitive to changes in CO2 saturations above 20%
Resistivity changed by ~3 orders of magnitude as S(CO2) changes and gradually so is a better method to constrain S(CO2)
Why is smoothing done?
To obtain the simplest, minimum structure model that fits the data reasonably well and within the level of uncertainty
What is model updates regularisation
- Parameter variation restricted to min or max known limits
2. Restrict updates to occur in a certain model areas due to a priori information
Why do we perform synthetic testing?
- Explore sensitivity to target properties
- Identify modelling strategy & inversion procedure
- Determine experimental design
Give the main geophysical survey methods in order of best to worst resoution
BEST
Seismic reflection & full wavefield, GPR (mm-10s m)
Travel-time seismiscs (10s m - 100s m)
CSEM (100s m - kms)
Resistivity, gravity magnetics have undefines resolution
WORST
What is the most commonly used method to reduce the misfit?
Least squares method.
Set up equations for the gradient of the misfit which tells us which direction to move in to improve reduce misfit
In χ^2 = Normalised(observed-calculated)^2 + λ(regularisation term)
What is λ?
λ is the trade of parameter. A large λ makes smoothness more important
How does a magnetotellurics survey work?
Utilises natural background EM fields caused by disturbances in Earth’s magnetic field
Recorded alongside CSEM with same equipment
Low amplitude so must record for weeks and then stack to reduce noise
What is a joint inversion?
Inversion of 2+ geophysical data sets simultaneously for a common model. It reduces non-uniqueness as a smaller range of models will fit the combined data set
What is the issue with a combined gravity and magnetic survey and how is it overcome?
Issue: Magnetic anomaly much larger than gravity so would dominate model.
Overcome by: weighting magnetic anomaly in misfit.
How do you weight components in the misfit when doing a joint inversion?
- Run suite of inversions and choose weightings based on combo that reduces misfit for both datasets at similar rates
- Run suite of inversions and choose weighting that similarly reduces the misfit in each dataset
How is regularization done?
- If know some properties can use to constrain
- e.g. if measured rock properties
- Set up probability density functions of mean & expected variations for each parameter
- Objective function penalizes any movement away from the probability density function
What is the observed data for ray tracing and travel-time tomography?
Usually travel-time picks for all shot-receiver pairs
What is required to produce a model from travel-time picks?
- Input velocity model
- Observed Data (arrival picks)
- Forward Step (calculates travel-times between shots & receivers through input velocity model)
- Inversion scheme - updates input velocity model to reduce misfit
How does the forward step program work?
Path of ray determined assuming wavefield travels in rays. As wave travels through model the ray is incrementally advanced. Assumes Snell’s law.
What are the disadvantages of ray tracing and travel time tomography?
- Ray theory assumes narrow ray with infinite frequency in reality wavefield has finite width and frequency. When model is complicated the assumption fails
- Assigning picks to arrivals is subjective, but if don’t you lose information from the whole waveform & secondary arrivals
- Resolution 100m - kms
What is full waveform inversion?
Invert full wavefield instead of just the travel times
What is the advantage of full waveform inversion?
Resolution much better - half wavelength = 10s-100s m
What is the disadvantage of full waveform inversion
Significant computational effort
