Lecture 6a - Forward and Inverse Modelling

Flashcards on forward and inverse modeling in resistivity.

What are the two main classifications of modeling approaches discussed in the lecture?

Forward and inverse modeling.

In geophysics, what does a forward model consist of?

A representation of the subsurface to simulate expected measurement values.

What is required to perform proper forward modeling?

Knowledge of the particular electrode geometry being simulated.

In resistivity sounding, what does forward modeling represent and simulate?

Represents subsurface distribution in resistivity and simulates measurement data (sounding curve).

How is inverse modeling described in lecture in contrast to forward modeling?

Inverse modeling starts with measurement data and generates a subsurface model from it.

What is inverse modeling also described as?

A process of model parameter optimization, typically done automatically using numerical algorithms.

What practical measurement aspect is recognized and included in gravity modeling?

That practical measurements only record vertical gravity changes.

How is the assessment of simulated vs. measured anomaly values typically done in forward models?

Typically assessed by eye.

What assumptions are made in a simple resistivity model?

Subsurface layering is perfectly horizontal, and there are no lateral variations in resistivity.

What are the model parameters to estimate in resistivity sounding?

Number of layers, resistivity of each layer, and thickness of each layer.

Why should models be more detailed near the surface and less detailed with depth?

To reflect the characteristics of surface-based measurements and decreasing measurement sensitivity with depth.

How does the resistivity structure affect measurement sensitivity mentioned in lecture?

Different resistivity layers have different detectabilities depending on their resistivity values, even with the same array geometry.

What does a smooth sigmoidal sounding curve typically indicate?

An indication of near-surface resistivity at one end and of deepest resistivity at the widest electrode and the other end, without clear evidence of a middle layer.

How are subsurface models usually represented?

Simplified resistivity depth plots, showing resistivity variations with depth.

What do the vertical sections and horizontal sections represent in a resistivity depth plot?

Vertical sections give the resistivity for each layer, and horizontal sections indicate the depths of the boundaries between layers.

What is something models should always reflect?

Any wider understanding that can be gained from alternate sources.

What is overparameterization?

Inclusion of too many parameters or variables within a model.

What is 'Occam's razor's principle for modeling?

To accept the most simple model which satisfies the data.

In summary what does forward models create?

Forward models use a model of the subsurface to make estimates of measured values.

What sort of representation of the subsurface do the models represent?

A very simplified representation of the subsurface, subject to the assumptions inherent within the model and the sensitivity limitations of the method itself.