Basement Depth Research Articles

Modeling and structural inversion from gravimetric data of the San Jorge Gulf Basin, Argentina

The subsurface structure of the San Jorge Gulf basin, Argentina, and its relationship with the regional geodynamic context have been analyzed in this contribution from gravimetric data integrated with the available geological information. Bouguer and residual anomaly maps were obtained from gravity and topographic data, and through upward continuation, respectively. From 2D models, five gravimetric lows and highs were defined in the western flank. The gravimetric minima, which were associated with a thickening of the sedimentary cover, generally have NNW-SSE and NW-SE orientations and mark an increase in the depth of the basement reaching up to 3000 m. On the other hand, the gravimetric highs were interpreted as basement horst-like structures. These configurations agree with the well data and its composite extensional structures are related to the basin's early origin and later evolution. The obtained 3D gravity model reveals the first-order geometry of the Southern Flank of the San Jorge Gulf basin eastern sector. In that sector, the anomaly obtained from structural inversion agrees with the observed anomaly within 0.4%. The 3D basement model was compared with data from 18 wells available in the area, showing low relative errors in basement depth for 13 of them. Based on the results, the use of the gravimetric method could constitute a fast, economic, robust, and reliable exploration method for the San Jorge Gulf basin. In addition, it shows that it constitutes a useful tool for the detection of structures of potential interest not yet identified.

Applying the Tilt-depth and Tilt-Euler techniques of gravity data to decipher the basement depth in Sichuan Basin, China

Applying the Tilt-depth and Tilt-Euler techniques of gravity data to decipher the basement depth in Sichuan Basin, China

Novel use of the enhanced thermal response test in crystalline bedrock

The thermophysical properties of bedrock are of primary importance when designing borehole thermal energy systems. We present a novel use of the Enhanced Thermal Response Test (ETRT) to determine bedrock thermal conductivity, natural convection, and drill hole thermal resistance as a function of depth in crystalline bedrock. Bedrock was heated with a 228-m-long hybrid cable containing copper wires and fiber optics for temperature monitoring. A reference fiber optic cable was installed along the whole length of the studied drill hole. For groundwater-filled boreholes, the ETRT offers a means to estimate the magnitude of buoyancy-driven natural convection. We estimated that the heating power in the ETRT should not exceed 20 Wm-1 for the thermal conductivities to be determined with sufficient accuracy. According our results, the accuracy of the ETRT can be significantly improved if the test is performed with a hybrid fiber optic cable combined with a reference fiber optic cable. Thermal resistance can be more accurately determined if a reference fiber optic cable is used. The most important achievement of this method is that compared to other measurement methods, the effective thermal conductivity of bedrock can be simultaneously determined along the entire length of the drill hole.

Subsurface geological model of Malay basin using free air anomaly

The aim of gravity survey is to assist in the detection and delineation of subsurface geological features such as salt domes and faults. In this study, free air anomaly (FAA) data was adopted for mapping and modelling process to delineate subsurface geological features and basement depth in Malay Basin. FAA is the measured gravity anomaly after a free air correction is applied, and it is used for elevation correction. The data of FAA in this study is obtained from Earth Gravitational Model (EGM) 2008 released by the National Geospatial-Intelligence Agency (NGA)-EGM Development Team. Oasis Montaj software was used in the mapping and modelling process whereby the base map which constructed by the Oasis Montaj is used to form the FAA map of Malay Basin. Typically, the positive anomaly is associated with the high-density intrusion at the base of the crust, while in contrast (negative anomaly), it is related to the sedimentary basin in the upper crust. On top of that, the regional-residual anomaly, total horizontal derivative (THD) and 3D Euler Deconvolution enhanced maps were produced and interpreted to acquire comprehensive insight of subsurface geological features. To conclude, this study showed 5% deviation as compared to previous reported works and the deepest basement depth encountered is 14.5 km.

The Congo Basin: Subsurface structure interpreted using potential field data and constrained by seismic data

The Congo Basin (CB) occupies a large part of the Congo Craton, which formed from the amalgamation of different cratonic blocks of Archean age. It records the history of deposition of up to 1 Gyr of sediments, one of the longest geological records on Earth above a crystalline basement. It is considered as a typical intracratonic basin, with its slow and long-lived subsidence history and the largely unknown formation mechanisms. In this study, we analyze the gravity field in the central area of the CB, applying two different methodologies to investigate the depth of the crystalline rock basement and crustal structures. The crustal contribution to the gravity observations is determined in one case by a data driven approach, while in the other using an isostatic compensation model that includes the effect of the rigidity of the lithosphere. Both methods are constrained by density measurements from rock samples/boreholes and interpretations of the reflection seismic profiles, crossing the CB. The results obtained are quite consistent with the aeromagnetic anomalies, showing structural highs and lows, NW-SE trending, matching the axial magnetic zone. The maximum basement depth is between 10 and 18 km, depending on the gravity method and assumptions used. Furthermore, we could identify several small basins that from the interpolation of seismic profiles are not clearly resolved, such as Busira (7–9 km deep), Lokoro (10.5–12 km deep), and Salonga basins (10–11.5 km deep), in the northwestern, southeastern, and central sector of the CB, respectively. The analysis of the gravity anomalies mainly related to the crystalline crust supposes the presence of high-density bodies located in the southern part of the CB that we interpreted as related to the extensional phases that initiated the subsidence of the basin.

Determination of depths to magnetic sources within Abakaliki–Afikpo area and environs, Southeastern Nigeria: implications for mineral and hydrocarbon prospectivity

There have been repeated calls from various quarters in Nigeria for a robust examination of Abakaliki anticlinorium and Afikpo sub-basin of the Southern Benue Trough for their mineral and hydrocarbon prospects. The Southern Benue Trough is part of a linear NE-SW trending rift system whose development is related to the breakaway of Africa from South America and the subsequent opening of the South Atlantic Ocean during the Cretaceous. Detailed geophysical studies were carried out with the high resolution aeromagnetic data using analytical techniques which include spectral analysis, analytic signal (AS) evaluations, Euler deconvolution, potential field transformation and two-dimensional potential field modelling of subsurface geology. Results reveal varying depths ranging from 0.41 to 0.67 km for depths to magnetic sources, and 1.0–5.5 km for basement depths. The presence of magnetic sources within the sedimentary basins is indicated by shallow depths to the top (DTT) of magnetic sources. Euler depth estimates ranging between 1.0 and 4.8 km for the magnetic source contacts/faults concurs with depths to the basement of between 4.5 and 4.9 km realised from the 2D modelling. Euler deconvolution of the magnetic anomalies also revealed the presence of Sill and Dyke structures at Amuzu, Enyigba, parts of Abakaliki, Utampa, Abriba, Afikpo, Ugep, and Agoi Ekpo regions with varying depths of up to 3.5 km. Structures mapped around Okposi, Amuzu, Enyigba Abakaliki, and Ishieke areas present viable prospects for hydrocarbon exploration. Structures identified at the Afikpo region could best represent mineral lode within the area. The 2D modelling shows various subsurface geological structures to include faults, fractures, sill, dyke, and considerable basement intrusion within the profile. Results from this study have identified an expanse of untapped areas with higher prospectivity which could be exploited for productive and viable mineral and hydrocarbon explorations.

Improved site-dependent statistical relationships of VS and resonant frequency versus bedrock depth in Japan

Improved site-dependent statistical relationships of VS and resonant frequency versus bedrock depth in Japan

3D constrained inversion of gravity and magnetic data to image basement and plutonic bodies: A case study from Dood Arale Basin, eastern Somaliland

We have developed 3D inversion models derived from airborne gravity and magnetic data, which are constrained by seismic and well data, in eastern Somaliland. The density model reveals a northwest–southeast-trending basin, 125 km long and 25 km wide and called the Dood Arale Basin. The basin comprises two subbasins separated by a basement high and is infilled by up to 2500–3200 m of sediments. Smaller and shallower subbasins are also identified to the west of Lafaweyne and northeast of Dararweyne. The density model shows that the top basement in the platform areas is at approximately 1500–1700 m in depth and shallows to approximately 300 m at the Bur Anod, Hagraajin, and Hagrin Ranges and northwest of Eil Afwein. The basement depths in these areas are more uncertain and could be deeper because they occur in areas of high gravity anomalies caused by a combination of near-surface high-density sediments and high-density plutonic bodies within the basement. The susceptibility model indicates that the basement consists of very weakly magnetized metasediments of the Inda Ad Complex intruded by three northeast–southwest-trending magnetic bodies with upper surfaces at depths of approximately 300–3000 m. These magnetic bodies are interpreted as plutonic complexes of similar age and composition to the Lower Cretaceous syenite intrusions outcropping at Gorei in the Shilah Madu Range. Seismic reflection profiles image the sedimentary sequences, but they do not clearly map the top basement or detect any of the plutonic bodies. The plutonic bodies could have controlled the location of the basin’s border faults and contributed to the high geothermal gradient recorded at the Faro Hills-1 well. The Upper Cretaceous Gumburo and Jesomma Formations in the basin could potentially have reached maturation close to and above the plutonic bodies within the center of the basin.

Gravity mapping of basement depth in seismogenic, fault-controlled basins: The case of Middle Aterno Valley (Central Italy)

Ground-shaking phenomena in intermontane basins emplaced in seismically active areas are strongly affected by sediment thickness. There, the development of human settlements, encouraged because of the flat topography in a mostly mountainous region, implies demographic growth and an increased seismic risk, as recent earthquakes in Central Italy confirmed. The knowledge of the thickness of the basin infills and of their density distribution is critical for the ground-motion amplification analysis.We apply the recently proposed ITerative RESCaling (ITRESC) method for 3D gravity modelling to the Middle Aterno Valley (Apennines, central Italy), a fault-controlled basin where a strong seismic activity recently occurred. Although the structural framework was previously investigated through 1D or 2D geophysical studies, here for the first time a full 3D model of the carbonate basement morphology is computed by the inversion of gravity data.Differently from usual gravity modelling approaches, the ITRESC technique 1) does not assume a density contrast function, which is instead determined through a data-driven process, and 2) integrates geological or geophysical constraints to define a global “gravity/depth-to-basement” rescaling law, valid in all the investigated area.Our model integrates a number of depth constraints of different nature while at the same time honoring the gravity anomalies. The obtained gravity model of the basement depth shows several analogies with previous studies, although with significant, localized discrepancies. The results of this study are only partially consistent with a structural evolution of the Middle Aterno Valley through a polyphasic sequence, as previously hypothesized.

Reduction of Bias and Uncertainty in Regional Seismic Site Amplification Factors for Seismic Hazard and Risk Analysis

Site amplification factors in National Building Codes are typically specified as a function of the average shear wave velocity over the first 30 m (Vs30) or site class (A, B, C, D and E) for defined ranges of Vs30 and/or ranges of depth to bedrock. However, a single set of amplification factors may not be representative of site conditions across the country, introducing a bias in seismic hazard and seismic risk analyses. This is exemplified by significant differences in geological settings between East and West coast locations in North America. Western sites are typically characterized by lower impedance contrasts between recent surface deposits and bedrock in comparison to Eastern sites. In North America, site amplification factors have been derived from a combination of field data on ground motions recorded during West Coast earthquakes and numerical models of site responses that are meant to be representative of a wide variety of soil profiles and ground motions. The bias on amplifications and their impact on seismic hazards is investigated for the Montreal area, which ranks second for seismic risks in Canada in terms of population and hazard (PGA of 0.25 g for a 2475 years return period). Representative soil profiles at several locations in Montreal are analyzed with 1-D site response models for natural and synthetic ground motions scaled between 0.1 to 0.5 g. Since bedrock depths are typically shallow (<30 m) across the island, bedrock shear wave velocities have a significant influence on the impedance contrast and amplifications. Bedrock shear wave velocity is usually very variable due to the differences in rock formations, level of weathering and fracturing. The level of this uncertainty is shown to be greatly decreased when rock quality designation (RQD) data, common information when bore hole data are logged, is available since it is highly correlated with both shear and compression wave velocities. The results are used to derive region-specific site amplification factors as a function of both Vs30 and site fundamental frequency and compared to those of the National Building Code of Canada (2015). The results of the study indicate that there are large uncertainties associated with these parameters due to variability in soil profiles, soil properties and input seismic ground motions. Average and confidence intervals for the mean and for predictions of amplification factors are calculated for each site class to quantify this uncertainty. Amplifications normalized relative to class C are obtained by accounting for the correlation between site class amplifications for given ground motions. Non-linearity in the analysis of equivalent linear 1-D site response is taken into account by introducing the non-linear G/Gmax and damping ratios curves. In this method, it is assumed that the shear strain compatible shear modulus and damping ratio values remains constant throughout the duration of the seismic excitation. This assumption is not fully applicable to a case when loose saturated soil profile undergo heavy shaking (PGA > 0.3 g). In this study, all simulations with input motion PGA >0.3 g have been performed by using the EL method instead of the NL method considering that cohesive soils (clay and silt) at Montreal sites are stiff and cohesionless soils (sand and gravel) are considerably dense. In addition, the field and laboratory data required to perform NL analyses are not currently available and may be investigated in future works.

Combining single-station microtremor and gravity surveys for deep stratigraphic mapping

Any stratigraphic reconstruction by means of surface geophysical methods is affected by the nonuniqueness of data inversion and by the resolution-depth trade-off. The combination of different geophysical techniques can reduce the number of degrees of freedom of the problem. We have focused on two low-impact single-station geophysical techniques: microtremor and gravity. These have been used by previous authors for stratigraphic mapping only by comparing the results independently. We suggest a procedure to combine microtremor and gravity data into a unique subsoil model and explore to what extent their combined use can overcome their individual weaknesses and constrain the final result. We apply the procedure to the Bolzano sedimentary basin, Northern Italy, to derive a 3D bedrock model of the basin. We use microtremor data to map the ground resonance frequencies and derive an initial 3D bedrock depth model by assuming a [Formula: see text] profile for the sediment fill. Then, we define a density model for rock and sediments and perform 3D gravity forward modeling. We then perturb the [Formula: see text] and density models and find the parameters that best fit the observed gravity anomalies. Data uncertainties are examined to explore the significance of the results. Joint use of the two techniques successfully helps interpret the stratigraphic model: Ground resonance frequencies guarantee the spatial resolution of the bedrock geometry model, whereas gravity data help constrain the frequency to depth conversion.

Determination of bedrock depth in Universitas Indonesia using the seismic refraction method

The seismic refraction method is used to determine the exact bedrock depth for placing a foundation pole. The study was conducted in Universitas Indonesia precisely at the Fasilkom Universitas Indonesia complex. The seismic survey configuration consists of 24 geophone channels with a length of 67.5 m, geophone intervals of 2.5 m, and near offset of 10 m. The wave source was generated using a hammer, and the distance between blows was 5 m. The secondary data used was geological data from SPT (Soil Penetration Test) borehole as a reference for comparison of seismic survey results. Seismic refraction data was processed using traditional techniques, namely the Hagedoorn’s Plus-Minus Method and tomographic inversion using Rayfract software. The correlation between the results of the process with the geological data from SPT drill point shows good results. However, the Plus-Minus Haggedorn method results are only able to show one refractor because of the data limitation, in contrast to the inversion method, which was able to show more than one refractor. There are two main refractors at a depth of 6 meters and 12 meters, and the adequate depth obtained only reaches 15 m. The maximum speed obtained is also around 900 m/s. It can be concluded up to a depth of 15 meters, and there is no recommended rock layer for placement of deep foundations for high rise buildings. A seismic survey with a longer seismic line is needed to get an underground picture exceeding 15 meters.

Sedimentary Basins of Indonesia: Outline and Thickness Variation Understanding

Offshore petroleum exploration in Indonesia began in late 1960’s and thereafter a significant number of seismic data sets were acquired. Based on these data, several basin outline maps were generated such as those published by Hamilton (1974), BEICIP FRANLAP (1992), and Sujanto (1997). Based on these data sets, 60 sedimentary basins were officially recognized by the Government of Indonesia (Sunarjanto et al., 2007 included in 2008 publication). The outlines of the basins were used as a reference by government officials and the petroleum industry. Recently, the Geological Agency published a map which shows 128 sedimentary basin outlines in Indonesia. Unfortunately, these maps were not accompanied with supporting subsurface data.The understanding of those sedimentary basins is very important for petroleum exploration, as they are basically the places to find hydrocarbons. Petroleum potential within a basin is related to its sediment accumulation and tectonic history. Critical petroleum system elements such as source rocks, reservoir and seal mainly comprise sedimentary rocks. The order of deposition, quantity of sediments and basin history will control the effectiveness and quantity of hydrocarbon generation in a particular basin.This article will go through the history of various basin outline maps and aim to provide additional information, such as basement depth, to give further detail on the basins in Indonesia. There are some detailed maps which show the distribution of oil and gas fields, which are obviously related to sediment thickness.

Evaluation of engineering site conditions in North Gyeongsang Province, South Korea, using a large database of borehole and standard penetration test data

In this study, the distributions of seismic response parameters and engineering site classes were quantified using a large database of borehole and standard penetration test (SPT) data for the North Gyeongsang Province, Korea. Empirical shear wave velocity (VS) profiles were constructed for 17,180 boreholes by applying a series of development methodologies based on combining the SPT–N value (blow count) and the boring log. N–VS relationship for each stratum was used and representative VS values were assigned for the lower rock layer without N value. If necessary, extrapolation and estimation were additionally applied to obtain the mean VS up to a depth of 30 m (VS,30). Seismic response parameters, such as VS,30 and the mean VS of soils above the engineering bedrock (VS,Soil), were derived and analyzed based on the empirical VS profile. Engineering site classification was performed using international and Korean seismic design codes. The validity of the empirical VS acquisition methodology and the related engineering bedrock depth determination were examined by comparing the derived bedrock depth to the appearance ratio and depth of the rock layer in the boring log. The effects of limiting the maximum converted N value, which is applied to the N–VS relationship, and of defining the engineering bedrock as a stratum with VS > 760 m/s were analyzed. Finally, the spatial bias of the borehole locations was reviewed by analyzing the distribution characteristics of the borehole elevations using a digital elevation model. The results may be used as basic data to assess the adequacy of site classification systems, to derive randomized input parameters, and as reference data for international geospatial zonation verification.

Assessment of Seismic Bedrock in Deep Alluvial Plains. Case Studies from the Emilia-Romagna Plain

The estimation of seismic shaking is essential for a realistic assessment of the local seismic hazard and the implementation of effective strategies for prevention and mitigation of the seismic risk. One of the most important aspects in the analysis of the site seismic assessment is the recognition of the seismic bedrock and its depth. Unfortunately, these data are not always easy to evaluate, especially in areas where the thickness of loose or poorly consolidated sediments is high. This article illustrates data and case studies from the Emilia-Romagna sector of the Po Plain, in order to provide examples and suggestions for the recognition of the seismic bedrock in alluvial and coastal areas characterised by significant thicknesses of unconsolidated sediments, using available data and not expensive geophysical surveys. The application of the proposed method indicates that the study area can be divided into four domains characterized by different depths of the seismic bedrock: the marginal or pede-Apennine belt, the high structural zones, the syncline/minor anticline zones, and the Po delta-coast zone.

Subsurface Geological Evaluation of the Central Sumatra Basin in Relation to the Presence of Heavy Oil

Central Sumatra Basin has been proven as a mature basin that produces large amounts of conventional oil. In fact, some of the existing oil fi elds are heavy oil containing such as Duri, Sebanga, Rantau Bais, and Kulin fi elds with their API Gravity values of lower than 25o . Apart from those oil fi elds the Central Sumatra Basin is expected to bear signifi cant heavy oil potential. In this light, this paper emphasizes discussion of subsurface geological evaluation on suspected fi elds/areas that contain heavy oil. This evaluation serves as a preliminary step in investigation of heavy oil resources/reserves in the basin. Analysis results on stratigraphic sequence and seismic interpretation provide information support facts over presence of heavy oil that are usually associated to main faults of Dalu-Dalu, Rokan, Sebanga, Petapahan, Pulau Gadang, and Kotabatak. Large tectonic events as a compression phase in the Middle Miocene – recent developed regional uplift and formed main thrust faults system, anticline structures due to the creature of basement highs, during which the F3 was deposited. The thrust faults system are important in the process of heavy oil generation in which surface water encroached into uplifted oil traps hence triggering heavy oil transformation mechanisms of biodegradation and water washing. This study provides illustration over sequences the heavy oil is generated in and their dimension in relation to area of structural anticlines. Based on available data, evaluation on subsurface geology has shown that anticlinal structures containing heavy oil tend to be characterized by near surface uplift (Basement up to 500 - 750 ms), whereas structures with lesser certainty in heavy oil containment tend show lower degrees of uplift marked by basement depth around 1000 ms or deeper. In general, seismic interpretation has shown that heavy oil is contained some sequences within sequences of 4 to 7 (equivalent to Menggala, Bekasap, Bangko, and Duri-Telisa formations).

Geophysical interpretation for delineating groundwater and subsurface structure in the East of Cairo City, Egypt

This study aimed to delineate groundwater and the structural trends in the East of Cairo City. For this aim, we carried out different geophysical approaches such as gravity, magnetic, and electric methods. For the gravity and magnetic data analysis, we used filtering, Euler deconvolution, and radially power spectrum techniques. In addition, 3-D modeling was performed on the magnetic data only. The radial power spectrum technique shows that the depths of the surface anomalies corresponding to the short wavelength are 0.48 and 0.5 km. While, the depths of the surface anomalies corresponding to the long wavelength are ranging from 2.74 to 2.33 km from gravity and magnetic interpretation, respectively. The Euler deconvolution revealed that the investigation area was under the effect of several fault trends; a major NW-SE trend and NE-SW and NW-SE as minor trends for gravity data. While the fault trends are in N-S and NE-SW directions for RTP magnetic data. The 3-D modeling results show that the basement depth was fluctuating between 1.186 to 1.657 Km. The principal tectonic deformations of the investigating area are towards the E-W, N-S, and NE-SW trends. The gravity and magnetic techniques in this study revealed that different faults dissect the area of trends NE-SW matching the Gulf of Aqaba direction, NW-SE matching the Gulf of Suez, N-S matching the River Nile system, and E-W matching the Mediterranean tectonics. The geo-electrical method was used to investigate groundwater presence. Regarding the seismicity, the area can be considered moderate to highly active. Most of the area faults are active except in the area's southern part. So, this part can be considered the most suitable and safe part for construction.

Probabilistic Cover‐Basement Interface Map in Cloncurry, Australia, Using Magnetotelluric Soundings

AbstractIn this article we propose a workflow for creating geologically realistic depth to basement maps and apply it to the undercover area of Cloncurry, located in the Mount Isa province, Queensland. A magnetotelluric (MT) survey consisting of 457 soundings was inverted using a 1D trans‐dimensional Bayesian algorithm designed to be robust to non‐1D effects present in the data. Automated change‐point analysis was then used to derive probability distributions on basement depth for each site independently. These were laterally combined, along with drill hole data and a structural model derived from aeromagnetic and geological data, using a Bayesian estimate fusion algorithm to create a region wide depth to basement probabilistic map. Combining many different constraints in this way lead to a significant reduction in posterior uncertainty. Individual MT site change‐point posteriors were highly multi‐modal in certain areas, with multiple resistivity transitions that could correspond to the cover‐basement interface. The estimate fusion process correlates these uncertainties and the combined posterior was thus much less multi‐modal. Our results show that the sedimentary cover gradually thickens toward the north, while toward the east its thickening is controlled by a two‐steps fault system. This workflow highlights the value gained from integrating different types of geoscientific data but also shows the capability of the MT method used within a probabilistic workflow to accurately image depth to basement even using limited constraints and assumptions.

Evaluation of Uncertainties in Site Response Analysis of Deep Soil Profiles in South Carolina Coastal Plain

ABSTRACTThe South Carolina Coastal Plain consists of deep soil sediments over basement bedrock. The depth of basement bedrock varies from being present at the surface to a depth of more than 1200 m at the southern tip of the state. A large variation exists in the thickness of the sediment, which impacts the seismic site response analyses of the Coastal Plain, particularly in areas where the availability of deep shear-wave velocity profiles is limited. This study evaluates the impact of variations in the shear-wave velocity profiles for two sites in the South Carolina Coastal Plain. The shear-wave velocity profiles were measured using different geophysical methods, including a combined multichannel analysis of surface waves and microtremor array measurement (MASW-MAM) method and P–S suspension logging. The equivalent-linear site response analyses were conducted by applying a synthetic earthquake motion at the depth of the B–C boundary (a depth of competent rock in which the shear-wave velocity is 760 m/s). The results are presented in terms of the amplification factor and its standard deviation. Results show that the average shear-wave velocity at the first 30 m (VS30), the shear-wave velocity contrast at the interface of the base layer and the B–C boundary, and the depth to the B–C boundary have a significant impact on the amplification factor and its variability, particularly for the amplification factor at periods higher than 0.1 s. The MASW-MAM method provided significantly lower VS30 values than the P–S suspension logging method at one of the two sites. Consequently, an additional peak in the amplification factor was observed for the site that had a low VS30, and the corresponding period was close to the resonant period of the loose, surface deposit.

Grav3CH_inv: A GUI-based MATLAB code for estimating the 3-D basement depth structure of sedimentary basins with vertical and horizontal density variation

This study presents a software tool Grav3CH_inv developed to estimate the three-dimensional depth structure of sedimentary basins from their gravity data through an iterative process. The algorithm linked to the developed code operates recursively both in the wavenumber domain and in the space domain based on a triple method combination. The modelling strategy allows considerations of the model space with an exponential increase in density with depth and also with density variations in the horizontal direction. The accuracy of computation of gravity anomalies in the FFT-based forward procedure is also increased by using the shift-sampling technique which minimizes discretization effects during transformations between the space and wavenumber domains. Given the observed gravity anomalies and the density design of the basin, the iterative procedure performs automatically until the goodness of the fit between the observed and modeled anomaly, either the root-mean-square error or the largest error is below its pre-assigned value. As an advantage, the computing time is acceptably short for such a kind of an modelling problem. The GUI-enabled interactive control functions of the Grav3CH_inv code allow users to set optional settings, style of outputs and export formats, and facilitate operations without requiring coding expertise to perform the relevant procedures of the algorithm. The feasibility and accuracy of the proposed software is demonstrated by evaluating the synthetically produced gravity anomalies of various 3D basin models and also by analyzing an actual gravity data from the Los Angeles basin, California.