Dear list members in both hemispheres; For my PhD thesis, I am studying the spatial variability of penetration resistance (a proxy for strength) in snow layers in an Alpine snow cover. Of specific interest are weak layers that are responsible for snow avalanche release. Are such weak layers less spatially variable than layers that are not critical for snow stability? To answer this, I want to compare the range, sill and nugget from model semivariograms calculated for all layers investigated. I also calculate mean and CV for each layer.
Measurements: At 113 locations on each small slope (20m x 20m), measurements of penetration resistance were made in a nested grid with a spacing of 0.5m to 2m. The penetration resistance for each layer within the grid was recorded at all locations. I have data from approximately 100 layers. Weak layers were identified with separate tests within each grid. Analyses: The penetration resistance for each layer was log10 transformed to approach normality. Grid-scale trends for each layer were investigated with a (robust) linear regression on the x-y coordinates. In most layers, this trend was statistically significant, but often in different directions even in adjacent layers. The trend was removed to do a geostatistical analysis on the normally distributed residuals. A robust experimental semivariogram was calculated for the residuals for each layer. Now I want to fit a spherical model semivariogram to the experimental semivariograms. The spherical model fits the data from most layers better than other models. Questions: - Is it possible to compare directly the range, the sill and the nugget of the spherical model semivariograms fitted to the residuals of the linearly detrended data for each layer? - Are there any pit-falls that I should be aware of? (Should I test different semivariogram models for each layer? Should I leave the linear trend in the data for the structural analyses? ...) All comments, suggestions and references are welcome and will be much appreciated. I will be happy to provide more info if needed. Best regards, Kalle PS: No one was hurt during the measurements ;-) -- * To post a message to the list, send it to [EMAIL PROTECTED] * As a general service to the users, please remember to post a summary of any useful responses to your questions. * To unsubscribe, send an email to [EMAIL PROTECTED] with no subject and "unsubscribe ai-geostats" followed by "end" on the next line in the message body. DO NOT SEND Subscribe/Unsubscribe requests to the list * Support to the list is provided at http://www.ai-geostats.org
