Search found 619 matches

by GuyM
Mon Sep 07, 2020 11:37 pm
Forum: General Geophysics and Geoscience
Topic: Diffraction sensitive to velocity
Replies: 4
Views: 156

Re: Diffraction sensitive to velocity

What is the difference between time and depth when it comes to diffraction and sensitivity to velocity ? Well you'd never normally depth convert a stacked (normal ray) section, so it doesn't really come up. Or rather, I'd be asking what your goal is when you do it(!) Mostly depth conversion is for ...
by GuyM
Sun Sep 06, 2020 7:17 pm
Forum: General Geophysics and Geoscience
Topic: Diffraction sensitive to velocity
Replies: 4
Views: 156

Re: Diffraction sensitive to velosity

Is this question tied to depth conversion at all? One of the approaches you can use is: - interpret horizons in two-way-time on a pre-stack time migration - develop a layer based interval velocity model using the V(rms) velocities from Pre-stack time migration and well control - depth convert layer ...
by GuyM
Sun Sep 06, 2020 6:06 am
Forum: General Geophysics and Geoscience
Topic: The Kirchhoff migration
Replies: 3
Views: 106

Re: The Kirchhoff migration

Well - I'm mostly pointing you to the SEG Wiki, which is mostly Oz Yimaz's book. If you can get hold of a copy it will be a big help.
by GuyM
Sun Sep 06, 2020 4:30 am
Forum: General Geophysics and Geoscience
Topic: The Kirchhoff migration
Replies: 3
Views: 106

Re: The Kirchhoff migration

The SEG Wiki Pages may help : https://wiki.seg.org/wiki/Kirchhoff_summation "The diffraction summation that incorporates the obliquity, spherical spreading and wavelet shaping factors is called the Kirchhoff summation, and the migration method based on this summation is called the Kirchhoff migratio...
by GuyM
Sun Sep 06, 2020 2:24 am
Forum: General Geophysics and Geoscience
Topic: lateral velocity variations
Replies: 1
Views: 67

Re: lateral velocity variations

It's how fast the velocity varies horizontally. If you had very boring geology - just horizontal, flat layers, deposited and compacted normally - then you would see a vertical variation in seismic velocity, but no lateral variations. On the other hand, when you have those same flatish layers overlyi...
by GuyM
Fri Sep 04, 2020 7:52 pm
Forum: General Geophysics and Geoscience
Topic: Particle Swarm Optimization (PSO) CODE
Replies: 3
Views: 81

Re: Particle Swarm Optimization (PSO) CODE

Ah - you misunderstand, I just found this library : https://pyswarms.readthedocs.io/en/latest/ and this thread on research gate: https://www.researchgate.net/post/Is_it_easy_to_implement_Genetic_algorithm_and_Particle_swarm_optimization_on_Python My observation was that we tended to find better main...
by GuyM
Fri Sep 04, 2020 6:04 pm
Forum: General Geophysics and Geoscience
Topic: Particle Swarm Optimization (PSO) CODE
Replies: 3
Views: 81

Re: Particle Swarm Optimization (PSO) CODE

Well, no, but you might want to consider making the leap to Python at some stage. A quick google shows a bit more activity on PSO in terms of open libraries in that community. More or less anything we looked for we found very quickly in Python. Even stuff like single-click outputs of Powerpoint file...
by GuyM
Fri Sep 04, 2020 5:31 pm
Forum: General Geophysics and Geoscience
Topic: huygens' principle
Replies: 3
Views: 127

Re: huygens' principle

I used to have a nice powerpoint slide illustrating this; it was animated and added in more and more circles centered on the same straight line until a new "wavefront" formed; that's back when I used to run seismic processing training courses :-)
by GuyM
Fri Sep 04, 2020 5:29 pm
Forum: Oil and Gas Exploration
Topic: North Carolina sues over federal decision on seismic testing
Replies: 4
Views: 153

Re: North Carolina sues over federal decision on seismic testing

Hmm - I'm not sure how critical this one area is to the industry survival; WesternGeco have no boats, and I'm prepared to wager the other companies are all in the "data library" game as well. They like new real estate (ie under explored areas) for the speculative potential, rather than the actual po...
by GuyM
Fri Sep 04, 2020 4:55 pm
Forum: General Geophysics and Geoscience
Topic: diffraction-in seismic
Replies: 4
Views: 107

Re: diffraction-in seismic

Not come across that term but it seems to be used when there's complex back-scatter patterns. Back-scatter is another way of saying diffractions. Where you have a rough surface - for example a seafloor that has a small amount of soft sediment over a steeply dippling, uplifted, harder and layered roc...
by GuyM
Fri Sep 04, 2020 11:17 am
Forum: General Geophysics and Geoscience
Topic: diffraction-in seismic
Replies: 4
Views: 107

Re: diffraction-in seismic

What will happen for diffraction? You get a curve shape on an CDP gather; this can be "flattened" using NMO, but the velocity needed to flatten the diffraction will be higher than the true stacking velocity that flattens the reflections. Not that diffractions and reflections can appear at the same ...
by GuyM
Thu Sep 03, 2020 5:20 pm
Forum: Oil and Gas Exploration
Topic: North Carolina sues over federal decision on seismic testing
Replies: 4
Views: 153

Re: North Carolina sues over federal decision on seismic testing

Seismic testing uses powerful airguns that blast sounds at the ocean floor repeatedly for months at a time. Marine experts say these sounds can harm marine and coastal resources – and could have significant impacts on the state’s fishing and tourism industries. After a lot of R+D it looks like effe...
by GuyM
Thu Sep 03, 2020 3:01 am
Forum: General Geophysics and Geoscience
Topic: coherent event
Replies: 1
Views: 62

Re: coherent event

Coherence is how similar two traces are to each other.
There's a few ways of calculating this (or terms it can mean) : https://wiki.seg.org/wiki/Coherence
by GuyM
Thu Sep 03, 2020 2:57 am
Forum: General Geophysics and Geoscience
Topic: grid in geophysics
Replies: 3
Views: 84

Re: grid in geophysics

A surface in a sub-surface modelling package can be multi-valued; like the top of a salt dome, that can bend over. Or a thrut fault. By multi-valued, I mean that at give X,Y coordinates the surface is present in the model more than once. With a grid, you have an z, y and z value. There's only one z ...
by GuyM
Thu Sep 03, 2020 2:53 am
Forum: General Geophysics and Geoscience
Topic: meaning of redundancy
Replies: 4
Views: 114

Re: meaning of redundancy

Well - I have run a written and run a few seismic processing courses in my time ;-)
by GuyM
Tue Sep 01, 2020 3:15 pm
Forum: General Geophysics and Geoscience
Topic: meaning of redundancy
Replies: 4
Views: 114

Re: meaning of redundancy

Oh - thank you for asking these questions! I've stepped back from geophysics after >25 years in seismic and while I'm very happy with my new career direction, it is also nice that all of the stuff that I have learned over the last quarter of a century is not sitting in my head not helping anyone :-)...
by GuyM
Tue Sep 01, 2020 3:11 pm
Forum: General Geophysics and Geoscience
Topic: meaning of redundancy
Replies: 4
Views: 114

Re: meaning of redundancy

When you have lot of seismic data containing the same information. For example, modern marine seismic is often collected at a 2ms sample interval, and with a receiver group interval of 12.5m This would allow for frequencies up to 250Hz (Nyquist) and a CMP bin spacing of 6.25m Given the filtering eff...
by GuyM
Sat Aug 29, 2020 8:06 pm
Forum: General Geophysics and Geoscience
Topic: nmo eq
Replies: 7
Views: 243

Re: nmo eq

How do you relate the NMO equation to Pythagorean theorem? In the picture, you have a right angled triangle formed by : - half the source receiver distance; ie from the source to the mid point - the vertical depth of the reflector below that midpoint - the ray-path from the source to the mind point...
by GuyM
Fri Aug 28, 2020 10:01 pm
Forum: General Geophysics and Geoscience
Topic: nmo eq
Replies: 7
Views: 243

Re: nmo eq

I'll keep that in mind, if I can.

It is hard when there is so much technical language at play.
by GuyM
Fri Aug 28, 2020 7:01 pm
Forum: General Geophysics and Geoscience
Topic: nmo eq
Replies: 7
Views: 243

Re: nmo eq

Did you read the second link? That walks you through how DMO and NMO are combined when it comes to a dipping interface. Remember we are working with events in two-way-time; if you want to consider angles, then you have to be dealing with distances (or depths), not times. Think about the method of di...
by GuyM
Fri Aug 28, 2020 5:38 pm
Forum: General Geophysics and Geoscience
Topic: nmo eq
Replies: 7
Views: 243

Re: nmo eq

You are confusing NMO (equation 1) and DMO (equation 2) https://wiki.seg.org/wiki/NMO_for_a_flat_reflector https://wiki.seg.org/wiki/Principles_of_dip_moveout_correction DMO is a partial prestack time migration with a weak velocity dependence, that corrects the velocity for dip and reducing CMP smea...
by GuyM
Fri Aug 28, 2020 8:35 am
Forum: General Geophysics and Geoscience
Topic: wave theory - kirchhoff migration
Replies: 4
Views: 173

Re: wave theory - kirchhoff migration

"Sudden change in facies" means that the rock type changes very quickly. "near surface scattering" is where small faults and heterogeneities in the weathered layer diffract seismic energy; this is often under-sampled spatially and so appears as noise. It'sthe reason why you don't see further signal ...
by GuyM
Thu Aug 27, 2020 2:29 pm
Forum: General Geophysics and Geoscience
Topic: wave theory - kirchhoff migration
Replies: 4
Views: 173

Re: wave theory - kirchhoff migration

Seismic wave theory.

You are using an integral form of the wave equation.

SEG Wiki has you covered :
https://wiki.seg.org/wiki/Dictionary:Wave_equation
https://wiki.seg.org/wiki/Dictionary:Ki ... s_equation
by GuyM
Thu Aug 27, 2020 2:25 pm
Forum: General Geophysics and Geoscience
Topic: huygens' principle
Replies: 3
Views: 127

Re: huygens' principle

Huygens' Principle says that every point in a reflector can be considered to be a source of secondary wavelets. So try this. Get a pencil, a ruler, a coin and a piece of paper. - draw a straight line with the ruler, about 15cm long - place the coin on that line, so that half is on one side, and half...