# Tag Info

14

Several things can indicate that a seismic event is an explosion and not an earthquake. Firstly, the depth of the source. The shallowest earthquakes tend to have a hypocentre that is at least a couple of km below the ground surface and some earthquakes in certain parts of the world can go down to about 700 km. Artificial explosions on the other hand, will ...

8

It depends on the implementation, but bare-bones reverse time migration is usually not amplitude friendly. The problem is that the ideal imaging condition — deconvolution — is difficult to apply or unstable in the time domain. So cross-correlation is used instead, and this loses the relative amplitude information... so amplitudes are no longer necessarily ...

7

SEG-Y files are a general purpose archival format. So they can be "anything". 2D seismic, 3D seismic, raw, processed, unstacked or even derivatives (seismic attributes). How to get information about your data in the SEG-Y file: SEG-Y contain 3 types of headers that describe the data. The EBCDIC header is a 40 by 80 character sheet that is a remnant from ...

6

SEG-D is a specialized format, while SEG-Y is a general-purpose format. In general, SEG-D is intended for field recordings of seismic data, and SEG-Y is intended for 'seismic data exchange'. Having said this, SEG-Y is so general-purpose, and so ubiquitous, that I'm not surprised to hear (anecdotally) that people are using it for data acquisition. Certainly, ...

6

The answer to most of your questions is: it depends. Is amplitude good for mapping channels? Amplitude maps can be great for finding and interpreting channel systems. It depends on the geology and on the seismic data's characteristics. Sometimes you can just see the channels, which might be qualitatively useful, and sometimes you can infer quantitative ...

5

It is a kind of spectral shaping, intended to increase the vertical resolution of seismic reflection data. The logic goes like this: Seismic data is band-limited and lacks high frequencies. This limits its vertical (travel time, and thus thickness) resolution. This is annoying because we often care about thin beds. The spectral peak of seismic data tends ...

5

The best reference on this is: J.A. Hudson and J.R. Heritage, 1981, The use of the Born approximation in seismic scattering problems, Geophys. J. R. Astr. Soc., v.66, 221-240. Basically, the Born approximation is a governing principle of the standard imaging condition that treats seismic wavefields from a single point scatterer. This is the justification ...

5

Here is a link to a colored inversion operator design code written and provided by Peter Zahuczki. This code requires that you provide a P-impedance log amplitude spectrum; he uses the open-source interpretation software OpendTect to compute this spectrum. Peter has a easy to follow tutorial on how to use his code with OpendTect. Good luck!

5

Typically seismic data should oscillate around 0 amplitude in the time domain, i.e. there should be just as much positive amplitude as there is negative (otherwise the ground would have had a net displacement in some direction). This is the energy at 0 Hz in the amp spectrum. It is like a bulk shift (DC shift) on the data. So... unless the ground was ...

4

Having experienced many minor earthquakes (Mag 3 to 5 in the East African Rift Valley, Samoa, Solomon Islands, Philippines and Indonesia), and one major life-threatening earthquake (Magnitude 8, Nepal), I can make a few personal observations about physiological reactions. First, You are quite correct about the body picking up any swaying feeling, and ...

4

Accelerometers, preferably triaxial (i.e., those that record horizontal and vertical motion). Beyond that your question is indeed a bit too open-ended. Some use many cheap MEMS (http://onlinelibrary.wiley.com/doi/10.1029/2008GL036572/epdf; https://www.theverge.com/circuitbreaker/2017/7/13/15963996/raspberry-pi-seismograph-early-warning-earthquakes; these are ...

4

The information you provide is rather limited, however, I want to try and disentangle this. The way I see it, you are mixing three different concepts in your questions. I will work through the semi-circle, through the elliptical towards the hyperbolic assumption. Semi-Circle This is a GIF of a central source emiting a wave in a homogeneous space: You can ...

4

The propagation of seismic waves is described by the wave equation: $$\mathrm{\nabla \cdot \sigma = \frac{\partial ^2 u}{\partial t^2}}$$ Where $\sigma$ is the stress and $u$ is the displacement. Geometric ray theory is an approximation to the full wave equation where the length scale of variation in seismic wavespeed is much larger than the seismic ...

4

Oh boy, a thorough understanding will be difficult, because reading these things to their full potential can take 10+ years of experience... If you really want to dive into it, read a book on seismic (exploration) geophysics -- this is the main reference book in the field: https://wiki.seg.org/wiki/Seismic_Data_Analysis . The best way to understand the ...

4

The Fourier transform of a Gaussian is a Gaussian. If your signal is given by $$g(t)=\frac{1}{\sqrt{2 \pi \sigma^2}} \exp\left( -\frac{(t-t_0)^2}{2 \sigma^2}\right)$$ then your frequency spectrum is $$G(f) = \exp(-2 \pi^2 \sigma^2 f^2),$$ where f is the frequency ($2 \pi \omega).$ I'l let you do the substitution with your particular numbers. Enjoy! ...

4

My understanding is the following (NB: it could be wrong!). The assumption is that on a common shot gather, your travel time follows a hyperbolic curve: $$f(x)=t^2=t_0^2 + \frac{x^2}{v^2},$$ where $t_0$ is the zero-offset travel time, $x$ the offset and $v$ the speed of the medium above the interface. The hyperbolic travel-times, for example, create ...

3

A bit more information - total plagiarism from Q&A: what earthquake science can tell us about North Korea’s nuclear test. Any sort of earthquake or explosion, whether natural or man-made, produces different sorts of shock waves which travel through the Earth and can be detected by seismometers, which can measure very small ground movements. The ...

3

Perhaps an easier question would be - what percentage of the earth has discovered oil/gas fields! This data should be easier to find than the surveys themselves, and can probably give you a rough estimate. For an idea of the publically available seismic surveys conducted around western Australia, this site (https://wapims.dmp.wa.gov.au/wapims) can show you ...

3

I too have found this nomenclature rather confusing. I am pretty sure that trace-based inversion is merely where each seismic trace is inverted independently of surrounding traces. This would take into account most deterministic/probabilistic inversion types (e.g. model, coloured, sparse, etc). In contrast, geostatistical inversion would not strictly be ...

3

The assumption that the earth's reflectivity is random (i.e. has a white spectrum) is only necessary for the statistical extraction of a wavelet from the data itself via the amplitude spectrum of the autocorrelation function. The convolutional model still works regardless of the reflectivity pattern, although getting a wavelet may be more difficult if the "...

3

When a bus or truck goes by, you typically hear it before you feel it, so you already know what it is before any noticeable ground motion starts, and your brain accepts the shaking as being due to the vehicle. In an earthquake, on the other hand, unless it's strong enough to actually start noticeably moving objects around (and I'm assuming that, when you ...

3

You may want to investigate the Delivery package available from http://www-old.dpr.csiro.au/StochasticSeismicInversion/ 'Delivery is an open-source, trace-based Bayesian seismic inversion code for use in oil reservoir characterisation at the early development and appraisal stage. See the Computers and Geosciences paper for most of the details, and the ...

3

I hope I understand your question right, I think there are some confusion about frequency range and wave-types. Seismic waves are acceleration of matter that transfer kinetic energy from a seismic source (e.g. an earthquake, an explosion or ambient noise) to heat. It's similar to sound waves, vibrations from the surface of e.g. a music instrument set the ...

3

The concept of pore pressure is identical to the concept of pore fluid pressure, as you identified yourself. The article rightfully identifies two pressure types: the confining pressure (those forces that try to compress the soil, like the weight of the overburden pushing on the soil) and the pore fluid pressure (the forces that resist compression; water, ...

3

After a more elaborate literature survey, it turns out that there are two types of quality control that can be performed on seismic data. One is of the acquisition parameters [1], which are just a few bytes per minute. Meanwhile, in [2], the entire data can be screened through QC software before being sent to a central server. This would help report errors ...

3

Answer 1 The horizontal axis is indeed time (don't forget units and note how long those periods are!). As for the vertical axis, I'm certain that this is displaying vertical (or 'Z' or 'V') displacement. Keep in mind - like you mention - that we are only looking at a 1-D picure of the data, but there are other perspectives we are missing! For example, how ...

3

tldr; Increased density corresponds with decreased P-wave velocity The P-wave velocity for an isotropic medium is: $$V_p = \sqrt\frac{K+4/3\mu}{\rho}$$ where $K$ is the Bulk Modulus, $\mu$ is the shear modulus, and $\rho$ is the density (https://en.wikipedia.org/wiki/Elastic_modulus, see $M$ the P-wave modulus). Isotropy just means that the strain on ...

3

If waves would travel only through a medium while keeping their form (i.e., P-waves remain P-waves and S-waves remain S-waves), you would obtain something a bit like this video: https://www.youtube.com/watch?v=YctV5crEXyM . However, standard models for wave propagation are more complicated than that. Upon hitting an interface, P-waves both reflect off and ...

2

Crossline coverage is 1/2 the spread width. A survey is usually designed to deliver perfectly abutted crossline coverage (overlapping the spreads as the previous correspondent said). Common midpoint (CMP) bin width is 1/2 the cable spacing and is sampled by one source-to-receiver ray path so crossline fold by definition is one. Easiest thing is to draw a ...

2

By midpoint range he refers to the size (length in the crossline direction) of the area in the subsurface that is illuminated by one pass of the boat. In a simple case of flat and parallel layers, that range is half of the size of the "spread" or receiver patch. BTW, this is true both in inline or crossline directions. By coverage he means how the ...

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