Prof Ravat, Thank you for having my issue in mind. Thank you Solomon On 18 Jun 2015 20:29, "Dhananjay Ravat <[email protected]>" < [email protected]> wrote:
> See references in Salem et al. (2014, Tectonophysics). > > See Bansal et al. (2011? Geophysics) on how to use fractal model for > computations. They use small windows with the previously specified > windows. Also, if you use centroid method, you cannot determine fractal > parameter of the “field", you have to assume one. The higher the fractal > parameter, the lower the obtained depth. > > See Bouligand et al. (2009, JGR) to see how one may construct a fractal > model to test whether the method is working. Bouligand et al. also have > another parameter search method for fractal modeling, but once again the > fractal parameter of “magnetization” (in this case), must be assumed. The > minimum RMS between observed and modeled spectra depends generally > underestimates the depth. > > Carol Finn of USGS is trying to get the Geosoft’s Curie depth/magnetic GX > make available publicly (Claire Bouligand and I gave our previous programs > for this). She said you need to ask Dima at Geosoft and she will request > the permission for making the module available on a case by case basis. > (Both Dima and Carol are going to hate me for writing this in a reply on > the listserver since this could mean added work for it - so if anyone > doesn’t anticipate using this in the next year or so might want to wait. > But I would presume the rest of you will love me for it. :-) > > These methods cannot determine fractal parameter. The only one that does > is the Salem et al (2014, Tectonophysics). My model studies show that > windows need to be about 500 km unless the magnetic bottom is on the order > of 10 km (in which case it could be lower, like 300 km). There is a lot of > spectral averaging involved in getting these estimates and one needs to > “interpret” the estimates further in complex geological situations - many > factors come into play). It takes me almost an hour (sometimes more) to > be happy with a single estimate and one also needs the knowledge of > regional geology and geophysics to make sense of it (or “interpret” the > result). > > Long wavelength anomaly fidelity is also very important in all this, which > means if one is processing data pre-2005 IGRF, you need to strip off IGRF > and use the Comprehensive Model of the magnetic field (CM3, and CM4, Sabaka > et al., 2002, 2004, GJI). Removing degree 15 main field in the CM is very > useful. > > If one is not willing to do all this (including model studies to > understand the limitations), one should avoid this problem. > > Cheers, > > tiku > > > On May 19, 2015, at 5:25 AM, solomonjanjaro <[email protected]> > wrote: > > 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 > <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]. 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/
