Upward Continuation Technique Research Articles

Multi-Elevations Upward Continuation Method to Depth Detection of the Main Crustal Layers in Western Desert of Iraq

The upward continuation technique was applied to the gravity and RTP magnetic data to obtain the residual anomalies maps in the western part of Iraq. Twelve upward elevation residual gravity maps and another twelve upward elevation residual RTP magnetic were determined using the same twelve upward continuation elevations. The applied upward elevations range from 1000 to 55000 m. The total range of the residual anomalies maps (Color Bar range) of gravity and RTP magnetic were plotted against the upward elevations. Five segments and four intersection points were obtained for the gravity and magnetic data. The trace of these intersection points on the upward elevation axis approximately indicate the following values 4.4-5, 12.8-13.5, 27-24, and 39-39.6 km. These values may indicate the depth of the following layer boundaries, the Burj Formation ( Mid-Cambrian), basement, mid layer in the basement rocks, and the Moho depth. The authors believe it is possible to apply the upward continuation to find the main layers which show abrupt change in the physical properties

Integrated gravity and resistivity investigations of the deep Hammam Bradaa aquifer, Northeast Algeria: Implications for groundwater exploration

Recently, the Guelma region, located northeast of Algeria, has been facing an unusual water shortage. To bypass this situation, a geophysical study using gravity and resistivity has been initiated to enhance the understanding of the geology and hydrogeology of the deep structures of the study area. In order to accomplish this, a residual gravity anomaly map has been constructed by removing from the Bouguer anomaly map a regional gravity gradient using the upward continuation technique. For the purpose of validating the Vertical Electrical Sounding(VES) interpretation, two soundings were conducted near boreholes with known lithology. The Hammam Bradaa deep aquifer of the Guelma region is composed of limestones that belong to the Constantine neritic domain. The Constantine neritic domain is covered by thrust nappes and outcrops in some places of the study area as tectonic windows. The exact geometry such as depth and lateral extent of the aquifer are not well known. Since limestones are characterized by high densities and resistivities relative to the host rock, the combining of gravity and VES was able to accurately map their extension among the other lithologies composing the geological section. Multiscale edge detection of gravity data known as "worms" made it possible to highlight the principal lineaments across the study area which may contribute to the recharge of the Hammam Bradaa aquifer. The 3D Gravity inversion enabled the mapping of the spatial configuration of limestones that are characterized by positive gravity contrast relative to the host rocks. A vertical density slice extracted from the 3D gravity inversion, running from north to south, reveals a significant thickness of limestone beneath Hammam Bradaa which suggests a strong potential for groundwater resources. The geo-electric section, constructed from the interpretation of VES data and calibrated with well lithology, enabled the mapping of the depth to the top of limestone beneath conductive alluvium and marls.

Comparative analysis of the structural pattern over the Indian Ocean basins using EIGEN6C4 Bouguer gravity data

The Indian Ocean witnessed numerous seismic and tectonic activities, making the region an intricate seismotectonic and seismo-genesis potential zone. The high-resolution global satellite-derived Bouguer Gravity Anomaly (BGA) data was used for comparative analysis of the structural patterns of six Indian Ocean basins. THETA (TA) edge enhancement and upward continuation techniques were used to study the Indian Ocean crustal structure by mapping both short and longwave lineaments. Rayleigh wave group velocity (RWGV) tomography at different time periods indicates pseudo depths of ∼30 km and ∼50 km are suitable for the upward continuation (UC) of BGA that perhaps lead to understanding different contrasting boundaries in their TA map. The UC of the BGA was generated at 60 km and 100 km for highlighting different lineaments/structures/contrasting geological boundaries at the depths of about 30 km and 50 km. The orientation of TA lineaments shows NNE-SSW, NE-SW, and EES-WWN structural trends. The statistical analysis of the lineaments shows that the Mid-Indian Basin constitutes the highest values of lineament number, lineament density (LD), circular variance (CV) and circular standard deviation (CSD), indicating a slow tectonic deformation over a long period. The Somalian Basin shows the minimum lineament number and lowest LD with high values of CV and CSD, indicating a relatively stable tectonic set-up compared to other Indian Ocean basins. The Arabian and Bay of Bengal basins offer relatively low lineament statistics than the Mid-Indian, Wharton and Madagascar basins, indicating frequent tectonic activity.

Estimation of 2D magnetic-source parameters using analytic signals of the logarithm of different order analytic signals

The application of fast semiautomatic techniques to magnetic data is very useful for data interpretation. A new automatic method is developed to use for 2D magnetic interpretation based on combinations of analytic signals of the logarithm of different order analytic signals (CALA method). The new method provides three linear equations for estimating the structural index, the horizontal location and the depth of magnetic source independently. The CALA method is sensitive to noise owing to using higher-order derivatives, and the upward continuation technique should be used for smoothing the magnetic anomaly. The CALA method successfully estimates the structural indices and positions of the sources when tested on synthetic noise-free and noise-added magnetic data, and has higher accuracy than the AN-EUL method and the method of the derivatives of the logarithm of the analytic signal amplitude. The satisfactory results are obtained by the new method on two real magnetic data sets.

Regional Bouguer anomaly gravity data: 3-D modelling of subsurface structures of the Flores and Timor earthquake risk area

Regional anomaly gravity data are very useful for determining deep subsurface structures due correspond deep surface anomalies. This research aims at analyzing qualitatively to map the gravity anomaly of the island of Flores, modelling shallow surfaces, and interpret surface structures based on the gravity anomalies. In this research, the gravity method used was the secondary gravity data from the Bureau Gravimetric International (BGI) that would be applied using Upward Continuation techniques to interpret deep surfaces for 3D modelling of earthquake risk areas on the islands of Flores and Timor. The modelling was performed by an inversion technique applying the method of singular value decomposition (SVD) and Occam inversion. The result showed that the subsurface structure of the volcano area of the Flores island consists of sandstone, breccia, and andesite with density values of 2.42 g/cm3 - 2.62 g/cm3 and basaltic and lava with density values of 2.65 g/cm3 - 3.24 g/cm3. The most dominating rock in the study area was basaltic rocks with the average density of 2.73 g/cm3. Based on 3D models of regional Bouguer gravity anomalies, the Flores back arc fault was estimated at a depth of 30 km, the thickness of the earth’s crust in the Flores back arc fault zone was estimated to be up to 30 km while the thickness of the earth’s crust in the fore arc was estimated to be more than 60 km. The existence of magma chamber in the volcanic arc of Flores island was estimated at depth of 30 km in accordance with to the seismicity map Flores and Timor areas.

Estimation of Curie point depth and geothermal gradient in parts of the Bida Basin, Nigeria

The purpose of this work is to estimate the Curie point depth, heat flow and geothermal gradient from spectral analysis of aeromagnetic data over the Bida and Baro areas, Bida Basin, north central Nigeria. The area covered is approximately 6050 km2 and bounded by latitudes 8° 30′ and 9° 30′ north and longitudes 6° 0′ and 6° 30′ east. The aeromagnetic maps were digitised at an equal interval, and the regional values were removed using polynomial fitting. The resulting residual data were subjected to the upward continuation technique to suppress the short-wavelength components of the residual magnetic anomalies. The upward continuation residual map was divided into eight overlapping blocks, and each block was spectrally analysed. The estimated Curie point depth ranges from 13.6 to 27.2 km, the geothermal gradient ranges from 21.3 to 42.6°C/km with an average value of 34.4°C/km and the corresponding heat flow ranges from 53.2 to 106.6 mW/m2. The average value of the geothermal gradient of 34.4°C/km indicates the possibility of hydrocarbon maturation in the area. Areas of geothermal anomalies with gradients greater than 40°C/km may be prospective for geothermal energy usage in Nigeria for generation of electricity.

Upward continuation and polynomial trend analysis as a gravity data decomposition, case study at Ziway-Shala basin, central Main Ethiopian rift

The first task in quantitative interpretation of a gravity data is separation of the Bouguer anomaly into its regional and residual components which are respectively related to deep and shallow subsurface geology. The decomposition process is subjective and non-unique as there is no single best approach to approximate the low frequency signature. For example, the use of spectral analysis and upward continuation require the wise choice of slope change location and continuation height respectively, which could be chosen differently by different researchers. This requires a need to work on more than one method and select the best to be applied for a given study area. The “best” choice is made based on the anomaly signature of the underlying geology. In this research, the most frequently used methods such as upward continuation and trend surface analysis methods are used and compared to approximate the regional field in Central Main Ethiopian rift bounded between 38000′-39030′E and 7000′-8030′N. The upward continuation height and the order of trend polynomial surface are first chosen, to approximate the regional gravity field signal. Accordingly, an upward continuation height of 6km and first order polynomial trend surface are chosen to be appropriate. Comparison of the two methods shows that the upward continuation technique reflects the shallow source anomalies of the area better than that of the first order linear trend surface. This outcome is verified against the result obtained based on the first vertical derivative method, spectral analysis depth estimation method, well-log data and surface geology of the area. It is therefore recommended to consider the various existing filtering techniques and choose the best candidate for the separation of the regional and residual components of the observed field.

Crustal density structure across Thailand delineated from 2D density modelling using gravity data and receiver function

This work aims to gather geological and geophysical evidences to construct the crustal density model of Thailand. The initial density models beneath 14-, 16- and 17-degree latitude were delineated from receiver function (RF) data and global crustal density model. The crustal density models were then updated to fit the Bouguer gravity anomaly data (BA) provided by the Royal Thai Survey Department using a 2-D gravity modelling software. The upward continuation technique was also applied to BA for enhancing the deep structure by omitting the influence of shallow structures. The derived crustal density models agree with Thailand’s tectonic setting. The density of the upper crust increases from 2.60 to 2.64 g/cm3 from west to east, from Sibumasu to Khorat Plateau – Indochina terrane. Between these two main terranes, the obtained model reveals the back-arc island, Sukhothai arc and Inthanon Zone, with a density of 2.55 g/cm3. The higher density of the lower crust with the derived mantle boundary from RF, ranging from 30.8 km of Sibumasu in the western to 38 km in the Khorat Plateau – Indochina terrane in the east, fit well with the filtered BA. Low BA that appear in Khorat plateau is not associated with topography in area which correspond to the high-density structure above upper mantle which need further investigation. Although, the developed crustal density models agree with BA, RF and tectonic evolution of Thailand, the additional gravity survey is suggested to enhance the crustal density models along the selected profiles.

ESTIMATION OF SOURCE PARAMETERS IN IBADAN, SOUTH – WESTERN NIGERIA USING DIGITIZED AEROMAGNETIC DATA

Airborne magnetic data covering geographical latitudes of 7000‟N to 7030‟N and longitudes of 3 30′E to 4 00′E within Ibadan area were obtained from Nigeria Geology Survey Agency. The data were ana-lyzed to map the sub surface structure and the source parameters were deduced from the quantitative and qualitative interpretation of magnetic data. The upward continuation technique was used to de-emphasize short – wavelength anomaly while the depth to magnetic sources in the area was deter-mined using local wavenumber technique, the analytic signal was also employed to obtain the depths of the magnetic basement. Analysis involving the local wavenumber, upward continuation and appar-ent magnetic susceptibility techniques significantly improves the interpretation of magnetic data in terms of delineating the geological structure, source parameter and magnetic susceptibility within Iba-dan area.. These depth ranges from 0.607km to 2.48km. The apparent susceptibility map at the cut-off wavelength of 50 m ranges from -0.00012 to 0.00079 which agree with the susceptibility value of some rock types; granite gneiss, migmatite biotite gneiss, biotite muscovite granite, hornblende granite, quartz and schists. The result of the local wavenumber suggests variation along the profiles in the surface of magnetic basement across the study area.

Regional-residual separation of potential field: An example from Tunisia

We present a regional-residual separation procedure for potential field data (gravity or magnetic) based on the integration of least-squares polynomial, power density spectrum and Upward continuation techniques. The proposed procedure is composed by 6 fundamental steps: 1) gridding of the total potential field; 2) least-squares polynomial approximation to calculate polynomial regionals; 3) applying power density spectrum technique to estimate the depth of the deepest source; 4) upward continuation to attenuate the effect of shallower sources; 5) comparing the produced polynomial regional approximations and upward continuation to choose the suitable regional and finally 6) computation of the residual. The proposed procedure was applied to synthetic case and real gravity data from Tunisia.

Interpretation of the magnetic data from Shavaz iron ore using enhanced local wavenumber (ELW) and comparison with Euler deconvolution method

This paper presents an inversion method for the interpretation of 2D magnetic anomaly data, in which uses the enhanced local wavenumber function to estimate the depth and the nature (structural index) of an isolated magnetic source. However, the method is sensitive to noise. In order to lower the effect of noise, I applied upward continuation technique to smooth the anomaly. Tests on synthetic noise-free and noise-corrupted magnetic data show that the method can successfully estimate the depth and the nature of the causative source. The practical application of the technique is applied to measured magnetic anomaly data from Shavaz Iron Ore, center of Iran, and the inversion results are in agreement with the inversion results from Euler deconvolution of the analytic signal and exploratory drilling.

한반도 지역에서의 상층중력장

The free-air anomalies are computed using a data set from various types of gravity measurements in the Korean Peninsula area. The gravity values extracted from the Earth Gravitational Model 2008 are used in the surrounding region. The upward continuation technique suggested by Dragomir is used in the computation of the external free-air anomalies at various altitudes. The integration radius 10 times the altitude is used in order to keep the accuracy of results and computational resources. The direct geodesic formula developed by Bowring is employed in integration. At the 1-km altitude, the free-air anomalies vary from -41.315 to 189.327 mgal with the standard deviation of 22.612 mgal. At the 3-km altitude, they vary from -36.478 to 156.209 mgal with the standard deviation of 20.641 mgal. At the 1,000-km altitude, they vary from 3.170 to 5.864 mgal with the standard deviation of 0.670 mgal. The predicted free-air anomalies at 3-km altitude are compared to the published free-air anomalies reduced from the airborne gravity measurements at the same altitude. The rms difference is 3.88 mgal. Considering the reported 2.21-mgal airborne gravity cross-over accuracy, this rms difference is not serious. Possible causes in the difference appear to be external free-air anomaly simulation errors in this work and/or the gravity reduction errors of the other. The external gravity field is predicted by adding the external free-air anomaly to the normal gravity computed using the closed form formula for the gravity above and below the surface of the ellipsoid. The predicted external gravity field in this work is expected to reasonably present the real external gravity field. This work seems to be the first structured research on the external free-air anomaly in the Korean Peninsula area, and the external gravity field can be used to improve the accuracy of the inertial navigation system.

Tectonic control of mineralization in the Taourirt–Oujda Corridor–Horst belt (north-eastern Morocco): geophysical evidence

In this study, we present a magnetic lineaments map of the Horst belt and the Taourirt–Oujda Corridor in the north-eastern of Morocco. This map is performed by means of a multi-scale approach for detecting and analysing magnetic sources boundaries and contacts. Two filtering methods horizontal gradient coupled to upward continuation technique and Euler deconvolution have been applied to ground magnetic and aeromagnetic anomalies. The result map is consistent with many faults already recognized or supposed by the traditional structural studies and highlights new ones by specifying their layout and dips. The results show clearly the relationships between the structural setting of the Horst belt and mineralization deposit. The ore deposit is described as mainly associated with shear zones strikes in NE–SW to ENE–WSW and NW–SE direction. This significant result suggests an epigenetic origin of the mineralization.

MATLAB programs for the synthetic models

The separation of residual gravity anomaly from regional gravity has considerably been studied for many years in gravity explorations. In addition, it is considered as a critical step in gravity data inversion. Some techniques have been developed for regional–residual anomaly separation both in space and frequency domains. One of these techniques for computing the regional anomaly is nonlinear filtering. In this paper, some techniques such as low-pass filtering, Butterworth, upward continuation, and nonlinear filtering are used to on synthetic gravity data in present of random noise and noise free for the purpose of residual–regional anomaly separation. The obtained results of techniques are compared with each other. The results have shown that separation methods are so efficient where synthetic models are located in shallow depth. Moreover, it is found that in comparison with other separation techniques, nonlinear filtering is more efficient in residual–regional anomaly separation and upward continuation technique is more efficient than Butterworth filter and low-pass filter. In addition, all of the obtained results have shown that Butterworth and low-pass filters are the same.

An Improved Analytic Signal Technique for the Depth and Structural Index from 2D Magnetic Anomaly Data

This paper presents a new inversion method for the interpretation of 2D magnetic anomaly data, which uses the combination of the analytic signal and its total gradient to estimate the depth and the nature (structural index) of an isolated magnetic source. However, our proposed method is sensitive to noise. In order to lower the effect of noise, we apply upward continuation technique to smooth the anomaly. Tests on synthetic noise-free and noise corrupted magnetic data show that the new method can successfully estimate the depth and the nature of the causative source. The practical application of the technique is applied to measured magnetic anomaly data from Jurh area, northeast China, and the inversion results are in agreement with the inversion results from Euler deconvolution of the analytic signal.

Subsurface structural mapping of Northern Nasser Lake region, Aswan, Egypt, using Bouguer data

In this study, we attempt to delineate the subsurface structures for thetectonic active region of Northern Nasser Lake using integrated interpretation techniquesof gravity data with seismicity. The depths to the gravity sources, and the locations of thecontacts of density contrast were estimated. Two methods were used for estimating sourcedepths and contact locations: horizontal gradient (HG) and Euler deconvolution methods.Moreover, power spectral analysis, bandpass and upward continuation techniques wereapplied to evaluate the shallow and deep seated structures. Shallow depth structureswere ranging between 0.30 km and 0.80 km. However, two average levels (interfaces) atdepth 3.1 km and 7.2 km below the measuring level were revealed for the intermediateand deep seated structures respectively. Results of Euler deconvolution method suggestedthat, in the eastern part of the area, the basement could be observed on the ground and hasbecome deeper in the central part. The interpreted structural map reveals that the area isaffected by a set of faults trending mainly in the NW, E–W, N–S and NE–SW directions.Actually, this map has confirmed the idea that the intersections between the N–S and E–W striking faults along Nasser Lake area have generated seismic pulses. Moreover, threeseismic zones (Z1, Z2 and Z3) are well correlated with the fault trends of the subsurfacestructures as derived from the horizontal gradient map. The present results suggest thatthere exist seismically-active fault east of High Dam, passing throughout Aswan reservoirfrom north to south. This fault is occupying region of high stress values which maygenerate large earthquakes in future, as it has long extension over several kilometers.Furthermore, the evaluated intruded volcanic bodies are found almost at the intersectionsbetween the E–W and NW oriented faults. 

Contribution de la gravimétrie à l'étude de la structure du bassin de Tadla (Maroc) : Implications hydrogéologiques

This study is based on the analysis and the interpretation of the gravity data of the Tadla basin. Its purpose is to increase the knowledge of this basin structure. A residual anomaly map was first calculated from the Bouguer anomaly data witch are strongly affected by a regional gradient. The computed map provides information on the ground density variation but it does not bring enough of new elements. Data filtering allows us to emphasize the structures affecting the basin. We chose the horizontal gradient coupled to the upward continuation techniques that permit to highlight news structures and to give information on their dip. The elaborated structural map of the study area constitutes a useful document for rationalizing the future groundwater exploration in the Tadla basin. To cite this article: A. Najine et al., C. R. Geoscience 338 (2006).

2D and 3D potential‐field upward continuation using splines

ABSTRACTThe dominant upward‐continuation technique used in the potential‐field geophysics industry is the fast Fourier transform (FFT) technique. However, the spline‐based upward‐continuation technique presented in this paper has some advantages over the FFT technique. The spline technique can be used to carry out level‐to‐uneven surface 2D and 3D potential‐field upward continuation. An example of level‐to‐uneven surface upward continuation of 3D magnetic data using the spline technique is shown, and it is evident that the continued anomalies are very close to the theoretical values. The spacing can be irregular. Synthetic examples using the spline technique to continue noise‐contaminated gravity and magnetic data upward to an altitude of 15 km on irregular grids are shown. Gaussian noise with a zero mean and a standard deviation of 1% does not cause much error and can readily be tolerated. Through comparison with the FFT technique, it is found that for low‐altitude gravity and magnetic upward continuation, both the FFT technique and the spline technique are suitable; for high‐altitude upward continuation, the FFT technique is inaccurate, whereas the spline technique works very well. Also, upward continuation by the spline technique has a smaller edge effect than upward continuation by the FFT technique. The spline‐based upward continuation technique works fairly well even when the periphery of a grid is not quiet: it is rather robust in general. A real example shows that the spline technique can be employed to perform upward continuation of total‐field magnetic data and to de‐emphasize near‐surface noise.

Gravity study of the structure of Suswa volcano and basement in the Kenya Rift

The most remarkable gravity feature in the Kenya Rift Valley is a positive gravity lineation that accords with the spatial distribution of the microseismic, volcanic and geothermal activity. It is interpreted in terms of a hot mantle intrusion zone, 10–20 km wide, representing the heat source for many geothermal systems in the rift. Filtering and upward-continuation techniques have led to the identification of the gravity components associated with the complex caldera structure of Suswa volcano and sub-volcano formation, by modelling of the residual gravity data. Geophysical data indicate a model where ascending masses of material from the mantle are intimately coupled to the accumulation of a mafic melt at shallow depth in the area of Naivasha tectonic segment, where a geothermal field at Olkaria is under exploitation. High-frequency gravity data are consistent with volcanological and geophysical evidence in suggesting that the area of Late Quaternary caldera volcanoes, Olkaria, Suswa and Longonot, is a thermally privileged site.

Some aspects of regional‐residual separation of gravity anomalies in a Precambrian terrain

The Bouguer gravity field measured over two Archean greenstone belts of northwestern Ontario is analyzed using three different regional‐residual separation techniques. The purpose of the analysis is to obtain a residual map suitable for gravity modeling studies to help define the subsurface characteristics of the greenstone belts and associated granitic areas. The methods used to derive the regional and residual maps are spectral factorization, upward continuation, and graphical smoothing. The substantial differences in the three sets of maps emphasize the ambiguity and subjectivity of the separation process. Each method may yield nonunique results. For example, in the spectral factorization technique, the filter design is dictated by the clarity of the slope change between the short‐ and long‐wavelength features and, in the case of the upward continuation technique, by the choice of the continuation height. The graphical method is empirical clearly nonunique. The regional map obtained through graphical smoothing is the most satisfactory for the purpose stated since it bas been designed to have minimal contributions from the shallow and broad greenstone masses outcropping at the surface. These features are clearly visible in the spectrum‐based regional map and to a lesser extent in the upward‐continued regional map. All three types of residual maps follow the general outline of the geologic units and thus are probably equally useful for a qualitative study of the anomaly shapes. However, for quantitative modeling purposes, the graphically produced residual is most suitable, since it can be successfully fitted by models that are consistent with the known surface geology and measured density values. The location, spatial extent, and the amplitudes of the analytically produced residual anomalies, in many areas, show poor correlation with the surface measurements, rendering these maps less satisfactory.