Geotechnical Layer (GTL)
The geotechnical characteristics of soils are an important aspect of the construction of buildings and structures such as tunnels, bridges, etc. These low velocity upper surface regions have significant effect on ground motion simulations, particularly at higher frequencies, above 0.5Hz and higher. For that reason, ground motion simulation modelers often want material properties from CVMs to contain accurate Vs values in the top 300-500 meters.
The basic ucvm_query method provides a command line (and API) option to add GTL values to returned material properties. The effect of this option is to modify the native velocity model material properties in the top 350m (user defined) using information about the near surface geology. The end effect is to return lower Vs values in many areas which better reflect actual Vs values in many areas of California.
ucvm_query returns both the original model Vs values, and the modified Vs values when this option is used. The modified values are returned in the "combined" data columns of the ucvm_query return string.
In this example, the native material properties are returned in columns 6,7,8 and the modified (or combined) values are returned in columns 14,15,16. In nearly all cases, the GTL modifications are made for material properties in top 300-500m, and do not effect properties below that.
ucvm_query return values are:
Col 0: lon - Input lon in decimal degrees Col 1: lat - Input latitude in decimal degree Col 2: Z - Input depth (or elevation) in meters Col 3: elevation - Returned elevation (m) from UCVMC Digital Elevation Map Col 4: vs30_from_Map - Returned Vs30 (m/s) from a Wills-Wald site type to Vs30 relationship (for points in California), or from Wald topography relation (outside of California) Col 5: cvm_name - Input name of user specified CVM that was queried Col 6: cvm_vp(m/s) - Returned Vp (m/s) from specified CVM Col 7: cvm_vs(m/s) - Returned Vs (m/s) from specified CVM Col 8: cvm_rho(kg/m3) - Returned density (kg/m3) from specified CVM Col 9: gtl_name - Geotechnical used to modify values in top 350m Col 10: gtl_vp - Geotechnical layer Vp Col 11: gtl_vs - Geotechnical Layer Vs Col 12: gtl_rho - Geotechincal layer rho Col 13: combining_algorithm_name - Name of algorigthm used to combine base model and GTL Col 14: combined_vp - Returned Vp based on base model and GTL Col 15: combined_vs - Returned Vs based on base model and GTL Col 16: combined_rho - Returned rho based on base model and GTL
-118.0000 34.0000 0.000 280.896 390.000 1d 5000.000 2886.751 2654.500 elygtl 0.000 390.000 0.000 ely 824.177 195.000 1084.062
UCVM provides a method for integrating low surface Vs values into the top 0-350m (user defined) of existing velocity models. This is useful because in some cases, existing velocity models do not provide detailed near surface velocity values. The UCVM supported optional GTL is based on the Ely-Jordan method that uses the surface site characteristics as an observation that can be used to provide an estimated near surface Vs value. The method then smoothly merges this surface-based Vs estimate into the underlying model Vs using a interpolation scheme developed by Ely.
The CVM-S4 model provides a rule-based GTL for regions in Los Angeles Basin, and in the Salton Sea region of southern California that was derived from various data sources including borehole and oil well-logs. When you query cvm-s4, the GTL information is added to the returned data by the underlying model. It provides Vs values below 200m/s in some regions. There is no way to remove the GTL without modifying the underlying CVM-S4 software.
CVM-H model can deliver a GTL. The GTL is controlled by the USE_GTL flag in the conf/ucvm.conf file. Set this flag to true, and the model will return material properties in the top 350m based on the Ely-Jordan method.
CVM-S4.26 model can deliver a GTL. The GTL is controlled by a flag in the model/cvms5/data/config file. By default, this flag is set to off, in which case the lowest Vs values the model will return are about 850m/s.
Users can edit this flag, set it to gtl=on, and the model will return lower velocities based on the Ely-Jordan method.
When the model returns low Vs values, the combined_data columns of ucvm_query are no different from the model data columns. This is true for both the CVM-S4.26 and CVM-H models. If the user configures these models to return a GTL, UCVM treats the GTL values as native model values, and the model columns, and combined_data columns are identical.
However, when the user specifies that UCVM should add a GTL, the native model values are returned in the model_values colums, and the combined_model plus GTL values are returned in the combined_data columns.
The GTL works if you explicitly specify where the GTL starts and ends (which for all practical purposes is 0m depth start and 350m depth end for Ely). You have to specify both the GTL and the interpolation scheme, which for the Ely GTL is “ely”.
Therefore, the command to query the 1d model with the Ely GTL using the Ely interpolation function from 0m to 350m depth is:
./ucvm_query -f ../conf/ucvm.conf -m 1d,elygtl:ely -z 0,350 Using Geo Depth coordinates as default mode. -118 34 0 -118 34 100 -118 34 200 -118 34 300 -118 34 400 -118 34 500 -118.0000 34.0000 0.000 280.896 390.000 1d 5000.000 2886.751 2654.500 elygtl 0.000 390.000 0.000 ely 824.177 195.000 1084.062 -118.0000 34.0000 100.000 280.896 390.000 1d 5000.000 2886.751 2654.500 elygtl 0.000 390.000 0.000 ely 3423.315 1528.545 2305.443 -118.0000 34.0000 200.000 280.896 390.000 1d 5000.000 2886.751 2654.500 elygtl 0.000 390.000 0.000 ely 4333.536 2277.050 2439.286 -118.0000 34.0000 300.000 280.896 390.000 1d 5000.000 2886.751 2654.500 elygtl 0.000 390.000 0.000 ely 4848.471 2746.588 2511.898 -118.0000 34.0000 400.000 280.896 390.000 1d 5000.000 2886.751 2654.500 none 0.000 0.000 0.000 crust 5000.000 2886.751 2654.500 -118.0000 34.0000 500.000 280.896 390.000 1d 5000.000 2886.751 2654.500 none 0.000 0.000 0.000 crust 5000.000 2886.751 2654.500
The GTL is calculated small surface velocities and gradually increasing them by depth using the “ely” interpolation function and after 350m the interpolation and GTL are no longer used.
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