Agree, the regional TC can be calculated for a radius less than say 200 km from
a station This 200 km number was put in as the Bouguer slab curvature
correction kicks in at about 166 km if it's remembered right. How far one wants
to take out the regional terrain correction can depend on how much relief there
is in the region, and how accurate one wants to get with their terrain
correction. Regional corrections that go out a few tens of kilometres can be
accurate enough in many cases, and subsequently require much smaller regional
correction grids that process faster in Geosoft.
Ed
On Tuesday, December 29, 2015 9:57 AM, "Rafael Rodriguez
<[email protected]>" <[email protected]> wrote:
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body.yiv6554083300hmmessage{font-size:12pt;font-family:Calibri;}-->I can see
something things. What is the scale of your work? Do you have a data line or
individual point?
You don't need a large DEM for terrain correction. You need one DEM about 20 Km
around each station. You don't need more. Then why not make DEM for a litle
number station gravity that make the work process more efficiently and less
time?Actually i have the terrain correction for the last work. I have tha data
in the higway and only make the DEM for 25 km around it. I don't need more.
Cheers
Date: Tue, 29 Dec 2015 16:00:45 +0000
From: [email protected]
To: [email protected]
Subject: Re: [geonet] Terrain Grid Problem
Hi,
not sure what has happened to take 3 days to create a large terrain grid in
your example. Perhaps the grid cell sizes of your two input DEM and output
terrain correction grids was set very fine, or the terrain correction (TC)
optimization was not set to faster? Did you mask and minimize your regional
input DEM to be a window less than <=400 km on a side?
The gravity module has been used here to create TC grids using regional 90 m
cells from SRTM data that go out to ~ 150 -200 km from a station and local DEM
25 m cells that go out to 1000 m and these took several minutes but quite a bit
less than one hour using the Geosoft gravity module (on a 4 year old quad
processor laptop). The TC optimization setting was set to "faster" in this
example. The final TC grid was a few mb in size, not into the hundreds of mb.
What grid cell sizes were used for your local and regional DEM grids that
created the TC grid?
When creating the TC grid I'd first double check the input grid cell sizes on
both your local correction DEM grid (local GPS elevations from the ground
gravity survey?) and your regional correction DEM (SRTM or other regional?)
grid. The "grid properties" tool can be used to look at both grid cell sizes
from your two DEM grids being used to create the TC grid, and used to look at
the total number of cells in your input grids. It can also be used to look at
the same properties of your large 250 mb TC grid.
I'm guessing if the two input DEM grid cell sizes are set to reasonable numbers
and if the regional DEM is a rectangle that is less than 400 km on a side, and
TC optimization is set to faster, a new TC grid will be created in several
minutes that will be quite a bit less than 250 mb in size and it should then be
viewable. Before running the TC though the "scan XY" radio button at the bottom
of the TC menu can be used to check the extent of your survey limits before the
TC is run. You can also specify numbers into your outer and inner TC correction
distances for more control in the TC menu.
If nothing shows on your original large 250 mb output TC grid and the grid
properties check indeed verifies there are TC numbers in the grid, you can try
changing the color stretch to see if anything shows up after a linear, normal
or equal area stretch is used, using either the default Geosoft color table or
the Geosoft elevation color table.
If the above does not help perhaps Geosoft could advise after they'd look at
your input grids and data.
Cheers,
Ed
Ed CunionRed Rocks Geophysical13224 W Utah CircleLakewood CO80228 USAcell
001-720-300-3641fax/phone 001-720-389-5224Skype ed.cunion
On Monday, December 28, 2015 5:51 PM, author.nameemail
<[email protected]> wrote:
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.yiv6554083300ExternalClass #yiv6554083300ecxyiv5235997781
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div.yiv6554083300ecxyiv5235997781WordSection1 {}-->Hello, I created a very
large terrain correction grid using the gravity module of Geosoft Oasis Montaj.
It took over 3 days to create the terrain correction grid. Once it was done,
it locked up my computer for another day and I finally had to shut it off. The
TC grid is about 250 megs but if I open it in Oasis Montaj, it shows a blank
screen. Is there any possibility that this grid may still be usable? If so,
what do I need to do to make it show up on the screen. Secondly, every time
I create large terrain correction grids, my computer locks up after it
completes. What can I do about this? Anything? I often end up having to shut
my computer down ungracefully and I do not like doing that. This is not an old
machine, it is actually pretty new and powerful. Thank you for your help.
Caitlin Bernier-GISPMagee Geophysical Services
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