Dear all. Can anyone help me with the procedure and steps on how to use Fractal method to calculate curie surface using aeromagnetic data. Thank you. Solomon Nehemiah Yusuf PhD student (Mautech) On 19 May 2015 07:21, "solomonjanjaro <[email protected]>" < [email protected]> wrote:
> Thank you for your contributions and also sharing from your wealth of > experience. I think I will quickly revisit my window size and also follow > all these suggestions on this forum to achieve the desired result. > Grateful > Solomon Nehemiah Yusuf > PhD student > Mautech, Nigeria > On 19 May 2015 03:55, "Dhananjay Ravat <[email protected]>" < > [email protected]> wrote: > >> Thank you, Steve, for the paper recommendation :-) >> >> Here are a couple of really important things, I think, to keep in mind. >> (There are a lot more, but these are really really important, keeping the >> original question in mind.) >> >> One needs to use large windows (quite a bit larger than what people have >> been using - otherwise one ends up looking at “some” layer in the upper >> crust when the true Curie depth might be deeper. My experience with a lot >> of fractal and other model studies is that window size needs to at least 10 >> times "the expected Curie depth-to-observation elevation distance" (I have >> applications where data are at 20-30 km elevations). Upward continuing is >> not a good idea as a result with spectral methods I think - if anything one >> is making response of the magnetic bottom even longer wavelength and >> smaller in amplitude. The smaller the window size the larger the variance >> of the solutions; so with small windows (less than 200-300 km) one could >> sometimes get a close to correct result for deep magnetic bottoms, but the >> variance of the solutions is really large to be usable - but one doesn’t >> know that if one is using the centroid method. Despite large windows, one >> needs to use some tapering or other spectral estimation strategies in order >> reduce the variance of the low-wavenumber spectral estimates. >> >> Taking into account the fractal parameter of the field is very important >> (if the underlying magnetization is fractal - true in most cases). Salem >> et al. (2014, Tectonophysics, The Defractal Method) show how to go about >> estimating fractal parameter by modeling the spectral peak. But one needs >> to do model studies, and more model studies with 2D and 3D fractal models, >> to get experience on how to assess fits of fractal models - before applying >> the method (this is especially true because the RMS minimum of the fit is >> not the correct solution most times - I think this happens because it is >> difficult to pick perfect range of wavenumbers over the fit needs to be >> performed for every spectra and also because of the noise in low-wavenumber >> spectral estimates. >> >> Finally, for certain geologic situations there are multiple layers: e.g., >> magnetic, non-magnetic/less-magnetic and once again deeper magnetic layers, >> in these situations the answers are some sort of weighted average of depth >> solutions, or dipping magnetic layers, and therefore the solution may not >> correspond to a real interface. >> >> Despite all this, I am still using the methods, more and more with the >> defractal method with large windows because of the ability to estimate the >> fractal parameter by evaluating results with a range of fractal parameters, >> and selecting manually minimum/maximum solution range, and the best >> solution. Then cross-checking with heat flow or whatever other geophysical >> models available in the area. All this is fairly time-consuming…. >> >> I think I have long surpassed the length for an effective e-mail >> communication. >> >> I hope this helps. >> >> -tiku >> >> >> >> On May 18, 2015, at 3:43 PM, Steve Sheriff <[email protected]> >> <[email protected]> wrote: >> >> Take a look at Ravat et al., 2007, Geophys J. Int'l v169, p 421-434 "A >> study of spectral methods of estimating the depth to the bottom of magnetic >> sources from near-surface magnetic anomaly data" >> >> On 5/18/2015 10:10 AM, solomonjanjaro <[email protected]> >> <[email protected]> wrote: >> >> ED CUNION, >> In a situation where I have only aeromagnetic data to use.....which other >> methods do I use? Please suggest. >> Solomon >> On 18 May 2015 17:05, "edcunion <[email protected]>" < >> [email protected]> wrote: >> >>> Beside longer wavelength magnetic data other available datasets that >>> might be tried to crosscheck the mag result depths are regional gravity, >>> satellite gravity datasets are freely available on the web with Grace being >>> just one example, seismic surface wave shear velocity models (coarse in >>> some areas) may also help, some of these data too are obtainable on the >>> web, and regional MT studies, Alan Jones and his teams have published >>> several papers with images from different continents, there may be some >>> regional depth inferences in these MT papers. >>> >>> Ed Cunion >>> Red Rocks Geophysical Consulting >>> 13224 W Utah Circle >>> Lakewood, CO 80228 USA >>> email: [email protected] >>> >>> >>> >>> On Monday, May 18, 2015 6:49 AM, "solomonjanjaro < >>> [email protected]>" <[email protected]> wrote: >>> >>> >>> Dear all. >>> Please I need help on different methods of determining crustal thickness >>> using aeromagnetic data. >>> 1. I have upward continued filter on my residual data and window a block >>> of 66x66 km and I have 16 blocks of this size over my study area and I have >>> also used this formula Zb=2Zo - Zt >>> Where Zb = curie depth >>> Zo = depth to centroid >>> Zt= depth to top >>> But want other methods to confirm result. >>> Thank you >>> Solomon Nehemiah Yusuf >>> PhD student( Mautech Nigeria) >>> --- >>> Forum archives can be accessed here: >>> http://lyris.geosoft.com/read/?forum=geonet >>> <http://lyris.geosoft.com/read/?fourm=geonet> >>> You are currently subscribed to geonet as: [email protected]. >>> To subscribe or unsubscribe from any of our forums, select the User >>> Forum selection on the Geosoft Community page: >>> http://www.geosoft.com/support/community/forums/register/ >>> >>> >>> --- >>> >>> Forum archives can be accessed here: >>> http://lyris.geosoft.com/read/?forum=geonet >>> <http://lyris.geosoft.com/read/?fourm=geonet> >>> >>> You are currently subscribed to geonet as: [email protected]. >>> >>> To subscribe or unsubscribe from any of our forums, select the User >>> Forum selection on the Geosoft Community page: >>> http://www.geosoft.com/support/community/forums/register/ >>> >> --- >> >> Forum archives can be accessed here: >> http://lyris.geosoft.com/read/?forum=geonet >> <http://lyris.geosoft.com/read/?fourm=geonet> >> >> You are currently subscribed to geonet as: [email protected]. >> >> To subscribe or unsubscribe from any of our forums, select the User Forum >> selection on the Geosoft Community page: >> http://www.geosoft.com/support/community/forums/register/ >> >> >> -- >> >> Steven Sheriff >> Professor Emeritus of Geophysics >> University of Montana >> Missoula, MT USA 59812www.umt.edu/geosciences >> >> --- >> >> Forum archives can be accessed here: >> http://lyris.geosoft.com/read/?forum=geonet >> <http://lyris.geosoft.com/read/?fourm=geonet> >> >> You are currently subscribed to geonet as: [email protected]. >> >> To subscribe or unsubscribe from any of our forums, select the User Forum >> selection on the Geosoft Community page: >> http://www.geosoft.com/support/community/forums/register/ >> >> >> --- >> >> Forum archives can be accessed here: >> http://lyris.geosoft.com/read/?forum=geonet >> <http://lyris.geosoft.com/read/?fourm=geonet> >> >> You are currently subscribed to geonet as: [email protected]. >> >> To subscribe or unsubscribe from any of our forums, select the User Forum >> selection on the Geosoft Community page: >> http://www.geosoft.com/support/community/forums/register/ >> > --- > > Forum archives can be accessed here: > http://lyris.geosoft.com/read/?forum=geonet > <http://lyris.geosoft.com/read/?fourm=geonet> > > You are currently subscribed to geonet as: [email protected]. > > To subscribe or unsubscribe from any of our forums, select the User Forum > selection on the Geosoft Community page: > http://www.geosoft.com/support/community/forums/register/ > --- Forum archives can be accessed here: http://lyris.geosoft.com/read/?forum=geonet You are currently subscribed to geonet as: [email protected]. 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