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Recovering Moho parameters using gravimetric and seismic data
KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Isostasy is a key concept in geoscience to interpret the state of mass balance between the Earth’s crust and mantle. There are four well-known isostatic models: the classical models of Airy/Heiskanen (A/H), Pratt/Hayford (P/H), and Vening Meinesz (VM) and the modern model of Vening Meinesz-Moritz (VMM). The first three models assume a local and regional isostatic compensation, whereas the latter one supposes a global isostatic compensation scheme.

A more satisfactory test of isostasy is to determine the Moho interface. The Moho discontinuity (or Moho) is the surface, which marks the boundary between the Earth’s crust and upper mantle. Generally, the Moho interface can be mapped accurately by seismic observations, but limited coverage of seismic data and economic considerations make gravimetric or combined gravimetric-seismic methods a more realistic technique for imaging the Moho interface either regional or global scales.

It is the main purpose of this dissertation to investigate an isostatic model with respect to its feasibility to use in recovering the Moho parameters (i.e. Moho depth and Moho density contrast). The study is mostly limited to the VMM model and to the combined approach on regional and global scales. The thesis briefly includes various investigations with the following specific subjects:

1) to investigate the applicability and quality of satellite altimetry data (i.e. marine gravity data) in Moho determination over the oceans using the VMM model, 2) to investigate the need for methodologies using gravimetric data jointly with seismic data (i.e. combined approach) to estimate both the Moho depth and Moho density contrast over regional and global scales, 3) to investigate the spherical terrain correction and its effect on the VMM Moho determination, 4) to investigate the residual isostatic topography (RIT, i.e. difference between actual topography and isostatic topography) and its effect in the VMM Moho estimation, 5) to investigate the application of the lithospheric thermal-pressure correction and its effect on the Moho geometry using the VMM model, 6) Finally, the thesis ends with the application of the classical isostatic models for predicting the geoid height.

The main input data used in the VMM model for a Moho recovery is the gravity anomaly/disturbance corrected for the gravitational contributions of mass density variation due in different layers of the Earth’s crust (i.e. stripping gravity corrections) and for the gravity contribution from deeper masses below the crust (i.e. non-isostatic effects). The corrections are computed using the recent seismic crustal model CRUST1.0.

Our numerical investigations presented in this thesis demonstrate that 1) the VMM approach is applicable for estimating Moho geometry using a global marine gravity field derived by satellite altimetry and that the possible mean dynamic topography in the marine gravity model does not significantly affect the Moho determination, 2) the combined approach could help in filling-in the gaps in the seismic models and it also provides good fit to other global and regional models more than 90 per cent of the locations, 3) despite the fact that the lateral variation of the crustal depth is rather smooth, the terrain affects the Moho result most significantly in many areas, 4) the application of the RIT correction improves the agreement of our Moho result with some published global Moho models, 5) the application of the lithospheric thermal-pressure correction improves the agreement of VMM Moho model with some other global Moho models, 6) the geoid height cannot be successfully represented by the classical models due to many other gravitational signals from various mass variations within the Earth that affects the geoid.  

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. , xi, 56 p.
Series
TRITA-SOM, ISSN 1653-6126 ; 2016:02
Keyword [en]
crust, gravity, mantle, Moho depth, non-isostatic effect, residual isostatic topography, stripping, thermal state, Vening Meinesz-Moritz model
National Category
Engineering and Technology
Research subject
Geodesy and Geoinformatics
Identifiers
URN: urn:nbn:se:kth:diva-183577ISBN: 978-91-7595-879-8 (print)OAI: oai:DiVA.org:kth-183577DiVA: diva2:912637
Public defence
2016-04-15, Sal L1, Drottning Kristinas väg 30, KTH, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20160317

Available from: 2016-03-17 Created: 2016-03-17 Last updated: 2016-03-17Bibliographically approved
List of papers
1. Modelling Moho depth in ocean areas based on satellite altimetry using Vening Meinesz–Moritz’ method
Open this publication in new window or tab >>Modelling Moho depth in ocean areas based on satellite altimetry using Vening Meinesz–Moritz’ method
2016 (English)In: Acta Geodaetica et Geophysica Hungarica, ISSN 1217-8977, E-ISSN 1587-1037, Vol. 51, no 2, 137-149 p.Article in journal (Refereed) Published
Abstract [en]

An experiment for estimating Moho depth is carried out based on satellite altimetryand topographic information using the Vening Meinesz–Moritz gravimetric isostatichypothesis. In order to investigate the possibility and quality of satellite altimetry in Mohodetermination, the DNSC08GRA global marine gravity field model and the DTM2006 globaltopography model are used to obtain a global Moho depth model over the oceans with aresolution of 1 x 1 degree. The numerical results show that the estimated Bouguer gravity disturbancevaries from 86 to 767 mGal, with a global average of 747 mGal, and the estimatedMoho depth varies from 3 to 39 km with a global average of 19 km. Comparing the Bouguergravity disturbance estimated from satellite altimetry and that derived by the gravimetricsatellite-only model GOGRA04S shows that the two models agree to 13 mGal in root meansquare (RMS). Similarly, the estimated Moho depths from satellite altimetry andGOGRA04S agree to 0.69 km in RMS. It is also concluded that possible mean dynamictopography in the marine gravity model does not significantly affect the Moho determination.

Place, publisher, year, edition, pages
Springer Netherlands, 2016
Keyword
Vening Meinesz–Moritz, Moho depth, Satellite altimetry, DNSC08GRA
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
urn:nbn:se:kth:diva-166753 (URN)10.1007/s40328-015-0116-6 (DOI)000375597500001 ()2-s2.0-84971223284 (Scopus ID)
Note

QC 20160609

Available from: 2015-05-15 Created: 2015-05-15 Last updated: 2017-12-04Bibliographically approved
2. Combined Moho parameters determination using CRUST1.0 and Vening Meinesz-Moritz model
Open this publication in new window or tab >>Combined Moho parameters determination using CRUST1.0 and Vening Meinesz-Moritz model
2015 (English)In: Journal of Earth Science, ISSN 1674-487X, E-ISSN 1867-111X, Vol. 26, no 4, 607-616 p.Article in journal (Refereed) Published
Abstract [en]

According to Vening Meinesz-Moritz (VMM) global inverse isostatic problem, either the Moho density contrast (crust-mantle density contrast) or the Moho geometry can be estimated by solving a non-linear Fredholm integral equation of the first kind. Here solutions to the two Moho parameters are presented by combining the global geopotential model (GOCO-03S), topography (DTM2006) and a seismic crust model, the latter being the recent digital global crustal model (CRUST1.0) with a resolution of 1A(0)x1A(0). The numerical results show that the estimated Moho density contrast varies from 21 to 637 kg/m(3), with a global average of 321 kg/m(3), and the estimated Moho depth varies from 6 to 86 km with a global average of 24 km. Comparing the Moho density contrasts estimated using our leastsquares method and those derived by the CRUST1.0, CRUST2.0, and PREM models shows that our estimate agrees fairly well with CRUST1.0 model and rather poor with other models. The estimated Moho depths by our least-squares method and the CRUST1.0 model agree to 4.8 km in RMS and with the GEMMA1.0 based model to 6.3 km.

Keyword
CRUST1.0, density contrast, isostasy, Moho, sediment thickness
National Category
Geology
Identifiers
urn:nbn:se:kth:diva-172684 (URN)10.1007/s12583-015-0571-6 (DOI)000358666500019 ()2-s2.0-84938897847 (Scopus ID)
Note

QC 20150901

Available from: 2015-09-01 Created: 2015-08-27 Last updated: 2017-12-04Bibliographically approved
3. The spherical terrain correction and its effect on the gravimetric-isostatic Moho determination
Open this publication in new window or tab >>The spherical terrain correction and its effect on the gravimetric-isostatic Moho determination
2016 (English)In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 204, no 1, 262-273 p.Article in journal (Refereed) Published
Abstract [en]

In this study, the Moho depth is estimated based on the refined spherical Bouguer gravity disturbance and DTM2006 topographic data using the Vening Meinesz-Moritz gravimetric-isostatic hypothesis. In this context, we compute the refined spherical Bouguer gravity disturbances in a set of 1 degrees x 1 degrees blocks. The spherical terrain correction, a residual correction to each Bouguer shell, is computed using rock heights and ice sheet thicknesses from the DTM2006 and Earth2014 models. The study illustrates that the defined simple Bouguer gravity disturbance corrected for the density variations of the oceans, ice sheets and sediment basins and also the non-isostatic effects needs a significant terrain correction to become the refined Bouguer gravity disturbance, and that the isostatic gravity disturbance is significantly better defined by the latter disturbance plus a compensation attraction. Our study shows that despite the fact that the lateral variation of the crustal depth is rather smooth, the terrain affects the result most significantly in many areas. The global numerical results show that the estimated Moho depths by the simple and refined spherical Bouguer gravity disturbances and the seismic CRUST1.0 model agree to 5.6 and 2.7 km in RMS, respectively. Also, the mean value differences are 1.7 and 0.2 km, respectively. Two regional numerical studies show that the RMS differences between the Moho depths estimated based on the simple and refined spherical Bouguer gravity disturbance and that using CRUST1.0 model yield fits of 4.9 and 3.2 km in South America and yield 3.2 and 3.4 km in Fennoscandia, respectively.

Place, publisher, year, edition, pages
Oxford University Press, 2016
Keyword
Satellite gravity, Gravity anomalies and Earth structure, Mantle processes, Mid-ocean ridge processes, Rheology: crust and lithosphere, Rheology: mantle
National Category
Geophysics
Identifiers
urn:nbn:se:kth:diva-182164 (URN)10.1093/gji/ggv450 (DOI)000368427000016 ()2-s2.0-84958610069 (Scopus ID)
Note

QC 20160218

Available from: 2016-02-18 Created: 2016-02-16 Last updated: 2017-11-30Bibliographically approved
4. Modelling Moho parameters and their uncertainties from the combination of the seismic and satellite gravity data
Open this publication in new window or tab >>Modelling Moho parameters and their uncertainties from the combination of the seismic and satellite gravity data
(English)Manuscript (preprint) (Other academic)
Abstract [en]

We present a method for estimating a new global Moho model (KTH15C), containing Moho depth and density contrast, from a combination of global models of gravity (GOCO05S), topography (DTM2006) and seismic information (CRUST1.0 and MDN07) to a resolution of 1°×1° based on a solution of Vening Meinesz-Moritz’ inverse problem of isostasy. Particularly, this article has its emphasis on the modelling of the observation standard errors propagated from the Vening Meinesz-Moritz and CRUST1.0 models in estimating the uncertainty of the final Moho model. The numerical results yield Moho depths ranging from 6.5 to 70.1 km, with a global average of 23.4 ± 13 km. The estimated Moho density contrasts range from 21 to 680 kg/m3, with a global average of 345.4 ± 112 kg/m3. Moreover, test computations display that in most areas estimated uncertainties in the parameters are less than 3 km and 50 kg/m3, respectively, but they reach to more significant values under Gulf of Mexico, Chile, Eeastern Mediterranean, Timor sea and parts of polar regions. Comparing the Moho depths estimated by KTH15C and those derived by KTH11C, GEMMA2012C, CRUST1.0, KTH14C, CRUST14 and GEMMA1.0 models shows that KTH15C agree fairly well with CRUST1.0 but rather poor with other models. The Moho density contrasts estimated by KTH15C and those of the KTH11C and KTH14C model agree to 120 and 80 kg/m3 in RMS. The regional numerical studies show that the RMS differences between KTH15C and Moho depths from seismic information yields fits of 2 to 4 km in South and North America, Africa, Europe, Asia, Australia and Antarctica, respectively.    

Keyword
VMM, CRUST1.0, MDN07, Moho, density contrast, uncertainty
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-183552 (URN)
Note

QS 2016

Available from: 2016-03-16 Created: 2016-03-16 Last updated: 2016-03-17Bibliographically approved
5. A new Fennoscandian crustal thickness model based on CRUST1. 0 and a gravimetric–isostatic approach
Open this publication in new window or tab >>A new Fennoscandian crustal thickness model based on CRUST1. 0 and a gravimetric–isostatic approach
2015 (English)In: Earth-Science Reviews, ISSN 0012-8252, E-ISSN 1872-6828, Vol. 145, 132-145 p.Article, review/survey (Refereed) Published
Abstract [en]

In this paper a new gravimetric–isostatic crustal thickness model (VMM14_FEN) is estimated for Fennoscandia. The main motivation is to investigate the relations between geological and geophysical properties, the Moho depth and crust–mantle density contrast at the crust–mantle discontinuity. For this purpose the Bouguer gravity disturbance data is corrected in two main ways namely for the gravitational contributions of mass density variation due to the different layers of the Earth's crust such as ice and sediments, as well as for the gravitational contribution from deeper masses below the crust. This second correction (for non-isostatic effects) is necessary because in general the crust is not in complete isostatic equilibrium and the observed gravity data are not only generated by the topographic/isostatic masses but also from those in the deep Earth interior. The correction for non-isostatic effects is mainly attributed to unmodeled mantle and core boundary density heterogeneities. These corrections are determined using the recent seismic crustal thickness model CRUST1.0. We compare our modeling results with previous studies in the area and test the fitness. The comparison with the external Moho model EuCRUST-07 shows a 3.3 km RMS agreement for the Moho depth in Fennoscandia. We also illustrate how the above corrections improve the Moho depth estimation. Finally, the signatures of geological structures and isostatic equilibrium are studied using VMM14_FEN, showing how main geological unit structures attribute in isostatic balance by affecting the Moho geometry. The main geological features are also discussed in the context of the complete and incomplete isostatic equilibrium.

Keyword
CRUST1.0, Density contrast, ESCM180, Isostasy, Moho, Topographic compensation
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
urn:nbn:se:kth:diva-166638 (URN)10.1016/j.earscirev.2015.03.003 (DOI)000355885300009 ()2-s2.0-84927166194 (Scopus ID)
Funder
Swedish National Space Board, 116/12
Note

QC 20150513

Available from: 2015-05-12 Created: 2015-05-12 Last updated: 2017-12-04Bibliographically approved
6. On the residual isostatic topography effect in the gravimetric Moho determination
Open this publication in new window or tab >>On the residual isostatic topography effect in the gravimetric Moho determination
2015 (English)In: Journal of Geodynamics, ISSN 0264-3707, E-ISSN 1879-1670, Vol. 83, 28-36 p.Article in journal (Refereed) Published
Abstract [en]

In classical isostatic models, a uniform crustal density is typically assumed, while disregarding the crustal density heterogeneities. This assumption, however, yields large errors in the Moho geometry determined from gravity data, because the actual topography is not fully isostatically compensated. Moreover, the sub-crustal density structures and additional geodynamic processes contribute to the overall isostatic balance. In this study we investigate the effects of unmodelled density structures and geodynamic processes on the gravity anomaly and the Moho geometry. For this purpose, we define the residual isostatic topography as the difference between actual topography and isostatic topography, which is computed based on utilizing the Vening Meinesz-Moritz isostatic theory. We show that the isostatic gravity bias due to disagreement between the actual and isostatically compensated topography varies between 382 and 596 mGal. This gravity bias corresponds to the Moho correction term of 16 to 25 km. Numerical results reveal that the application of this Moho correction to the gravimetrically determined Moho depths significantly improves the RMS fit of our result with some published global seismic and gravimetric Moho models. We also demonstrate that the isostatic equilibrium at long-to-medium wavelengths (up to degree of about 40) is mainly controlled by a variable Moho depth, while the topographic mass balance at a higher-frequency spectrum is mainly attained by a variable crustal density.

Keyword
Compensation, Crustal thickness, Isostasy, Moho, Isostatic topography
National Category
Geochemistry Geophysics
Identifiers
urn:nbn:se:kth:diva-160755 (URN)10.1016/j.jog.2014.11.002 (DOI)000348083500003 ()2-s2.0-84916918950 (Scopus ID)
Funder
Swedish National Space Board, 76/10:1 116/12
Note

QC 20150302

Available from: 2015-03-02 Created: 2015-02-27 Last updated: 2017-12-04Bibliographically approved
7. Effect of the lithospheric thermal state on the Moho geometry
Open this publication in new window or tab >>Effect of the lithospheric thermal state on the Moho geometry
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Gravimetric methods applied for a Moho recovery in areas with sparse and irregular distribution of seismic data often assume only a constant crustal density. Results of the latest studies, however, indicate that corrections for the crustal density heterogeneities could improve the gravimetric result especially in regions with a complex geologic/tectonic structure. Moreover, the isostatic mass balance reflects also the density structure within the mantle. The gravimetric methods should therefore incorporate an additional correction for the sub-crustal density heterogeneities. Following this principle, we solve the Vening Meinesz-Moritz (VMM) inverse problem of isostasy constrained on seismic data to determine the Moho depth of the South American tectonic plate including surrounding oceans, while taking into consideration the crustal and mantle density heterogeneities. Our numerical result confirms that the contribution of sediments significantly modifies the Moho geometry especially along the continental margins with large sediment deposits. To account for the mantle density heterogeneities we develop and apply a method of correcting the Moho geometry for the contribution of the lithospheric thermal state (i.e., the lithospheric thermal-pressure correction). In addition, the misfit between the isostatic and seismic Moho models, attributed mainly to deep mantle density heterogeneities and other geophysical phenomena, is corrected for by applying the non-isostatic correction. The results reveal that the application of the lithospheric thermal-pressure correction improves the RMS fit of the VMM gravimetric Moho solution to the CRUST1.0 seismic model and the point-wise seismic data in South America about 40% and 7% respectively.  

Keyword
Crust, gravity, lithosphere, Moho, thermal state.
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-183556 (URN)
Note

QS 2016

Available from: 2016-03-16 Created: 2016-03-16 Last updated: 2016-03-17Bibliographically approved
8. The observed geoid height versus Airy's and Pratt's isostatic models using matched asymptotic expansions
Open this publication in new window or tab >>The observed geoid height versus Airy's and Pratt's isostatic models using matched asymptotic expansions
2014 (English)In: Acta Geodaetica et Geophysica, ISSN 2213-5812, Vol. 49, no 4, 473-490 p.Article in journal (Refereed) Published
Abstract [en]

Isostasy is a key concept in geodesy and geophysics. The classical isostatic models of Airy/Heiskanen and Pratt/Hayford imply that the topographic mass surplus and ocean mass deficit are balanced by mountain roots and anti-roots in the former model and by density variations in the topography and the compensation layer below sea bottom in the latter model. In geophysics gravity inversion is an essential topic where isostasy comes to play. The main objective of this study is to compare the prediction of geoid heights from the above isostatic models based on matched asymptotic expansion with geoid heights observed by the Earth Gravitational Model 2008. Numerical computations were carried out both globally and in several regions, showing poor agreements between the theoretical and observed geoid heights. As an alternative, multiple regression analysis including several non-isostatic terms in addition to the isostatic terms was tested providing only slightly better success rates. Our main conclusion is that the geoid height cannot generally be represented by the simple formulas based on matched asymptotic expansions. This is because (a) both the geoid and isostatic compensation of the topography have regional to global contributions in addition to the pure local signal considered in the classical isostatic models, and (b) geodynamic phenomena are still likely to significantly blur the results despite that all spherical harmonic low-degree (below degree 11) gravity signals were excluded from the study.

National Category
Geochemistry
Identifiers
urn:nbn:se:kth:diva-159122 (URN)10.1007/s40328-014-0064-6 (DOI)000346798500007 ()2-s2.0-84920280503 (Scopus ID)
Note

QC 20150127

Available from: 2015-01-27 Created: 2015-01-22 Last updated: 2016-03-17Bibliographically approved

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