Declination and inclination errors in experimentally deposited specularite-bearing sand

Naturally disaggregated specularite-bearing sandstone from the Triassic Moenkopi Formation, artificially deposited in controlled magnetic fields of ～5 × 10^?2 mT, acquires a stable remanent magnetization that has systematic errors in inclination and declination. Inclinations about 12° shallower than the applied fields are produced by deposition on a horizontal surface in still water. Deposition from flowing water on a surface inclined 6–10° results in inclination errors of as much as 20°. Water flowing obliquely to the applied field results in declination errors of about 10°, with declinations systematically rotated toward the upstream direction of current flow. These experimental results indicate that specularite-bearing sediment responds to the earth's field in a manner similar to magnetite-bearing sediment, and support observational evidence for a primary magnetization of depositional origin in specularite in red beds of the Moenkopi Formation.     

Portable magnetometer theodolite with fluxgate sensor for earth's magnetic field component measurements

A new portable magnetometer theodolite with fluxgate sensor has been developed for measuring the components of the earth's magnetic field. With this instrument, used as a null detector, it is possible to make absolute measurements of declination (D) and inclination (I). The compensation of the main magnetic field allows direct intensity measurements (H, X, Y, Z components); in this case the calibration is made by reference to a proton magnetometer.The observation procedure is particularly easy and quick. The accuracy is better than 5 sec of arc forD andI and better than 1 nT for direct intensity measurements.     

Magnetic field transforms with low sensitivity to the direction of source magnetization and high centricity

Magnetic data interpretation faces difficulties due to the various shapes of magnetic anomalies and the positions of their extrema with respect to the causative bodies for different directions of the source magnetization. The well‐known transforms — reduction to the pole, pseudogravity field, and analytic signal (total gradient) — help in reducing the problem. Another way to achieve the required effect is the transformation of magnetic data, ΔT or Z, into values of the anomalous magnetic intensity T. In this respect, we have found some transforms based on differential operators such as the gradient of T and its modulus R = |?T|, the Laplacian L = ?²T, the product T ?²T and its square root Q, and the Laplacian ?²(T²) and its square root E, to be useful. They are slightly sensitive to the magnetization orientation and their extrema occur above the sources. For a 2D anomaly of a homogeneous causative body, the proposed transforms do not depend on the inclination of magnetization. In the 3D case, such independence does not exist even for the elementary field of a point dipole. The influence of the magnetization direction is estimated by an integral coefficient of sensitivity. This coefficient takes values of up to 2.0 for ΔT or Z anomalies, while their transforms T, R, E, Q and L have values of less than 0.28, 0.29, 0.24, 0.16 and 0.07, respectively, i.e. on average, 10 times less. The estimation of the centricity is carried out using the relative deviation of the principal extremum of the anomaly or its transforms from the epicentre of the model body at a depth equal to 100 units. For a ΔT anomaly this deviation is up to 67%; for the L transform it is less than 8%; for Q, E, R and T it is less than 10%, 15%, 20% and 25%, respectively. The proposed transforms take only non‐negative values. With respect to their shape, the peripheral magnetic extrema are removed, the anomalous configuration is simplified and the resolution of complicated interference patterns is improved. Their calculation does not require additional data for the direction of magnetization, which is an essential advantage over the reduction‐to‐the‐pole and pseudogravity‐field transforms. A joint analysis of the measured field and its transforms T, E and L offers possibilities for more confident separation of the anomalous effects and direct correlation to their sources. The model tests performed and the 3D field applications to real magnetic data confirm the useful properties of the transforms suggested here.     

Paleomagnetism of the latest Precambrian/Cambrian Unicoi basalts from the Blue Ridge, northeast Tennessee and southwest Virginia: evidence for Taconic deformation

Three components of magnetization have been observed in ninety-six samples (twelve sites) of amygdaloidal basalts and “sedimentary greenstones” of the Unicoi Formation in the Blue Ridge Province of northeast Tennessee and southwest Virginia. These components could be isolated by alternating field as well as thermal demagnetization. One component, with a direction close to that of the present-day geomagnetic field is ascribed to recent viscous remanent magnetizations; another component, with intermediate blocking temperatures and coercivities, gives a mean direction of D = 132°, I = +43°,α₉₅ = 9° for N = 10 sites before correction for tilt of the strata. This direction and the corresponding pole position are close to Ordovician/Silurian data from the North American craton and we infer this magnetization to be due to a thermal(?) remagnetization during or after the Taconic orogeny. This magnetization is of post-folding origin, which indicates that the Blue Ridge in our area was structurally affected by the Taconic deformation. The third component, with the highest blocking temperatures and coercivities, appears to reside in hematite. Its mean direction, D = 276°, I = ?17°,α₉₅ = 13.8° for N = 6 sites (after tilt correction) corresponds to a pole close to Latest Precambrian and Cambrian poles for North America. The fold test is inconclusive for this magnetization at the 95% confidence level because of the near-coincidence of the strike and the declinations. We infer this direction to be due to early high-temperature oxidation of the basalts, and argue that its magnetization may have survived the later thermal events because of its intrinsic high blocking temperatures. A detailed examination of the paleomagnetic directions from this study reveals that the Blue Ridge in this area may have undergone a small counterclockwise rotation of about 15°.     

Distortion of the geomagnetic field in volcanic terrains: an experimental study of the Mount Etna stratovolcano

The inclination (I), declination (D) and total intensity (F) of the geomagnetic field were measured on Mount Etna in 1989-1991 at a dozen sites previously sampled for archeomagnetic studies. The purpose of the work was to determine the variations of these parameters at 30 cm above ground level, and how the distortion from the main field can affect the archeomagnetic record of volcanic rocks. Ten measurements were usually performed at each site with a three-component flux-gate magnetometer, whose estimated precision is ±0.2° on direction and ±50 nT on intensity. This was considered sufficient on volcanic areas with highly magnetized rocks and where the geomagnetic gradient may be in excess of 1000 nT/m. Results averaged for each site generally show small variations in intensity (±3% of the total field) and direction (±1.5°). The averaged values of the 12 sites (I=52.6°, D=0.3°, F=44010 nT) are very close to those measured in sedimentary terrain away from the volcano (I=52.9°, D=0.35°, F=44110 nT), themselves consistent with the interpolated IGRF in eastern Sicily. The largest deviations of the geomagnetic direction have been observed on four sites, three of them located on the South flank between 1900 and 700 m elevation. It is suggested that these anomalies are mainly related to dyke swarms which are common within the South Rift Zone of Mount Etna. Our findings show that reliable archeomagnetic results can be obtained from volcanic rocks, provided that lavas of the same eruption are sampled on several sites distributed over the largest possible area.     

Continuous wavelet transform,theoretical aspects and application to aeromagnetic data at the Huanghua Depression,Dagang Oilfield,China

We use the continuous wavelet transform based on complex Morlet wavelets, which has been developed to estimate the source distribution of potential fields. For magnetic anomalies of adjacent sources, they always superimpose upon each other in space and wavenumber, making the identification of magnetic sources problematic. Therefore, a scale normalization factor, a^?n, is introduced on the wavelet coefficients to improve resolution in the scalogram. By theoretical modelling, we set up an approximate linear relationship between the pseudo‐wavenumber and source depth. The influences of background field, random noise and magnetization inclination on the continuous wavelet transform of magnetic anomalies are also discussed and compared with the short‐time Fourier transform results. Synthetic examples indicate that the regional trend has little effect on our method, while the influence of random noise is mainly imposed on shallower sources with higher wavenumbers. The source horizontal position will be affected by the change of magnetization direction, whereas the source depth remains unchanged. After discussing the performance of our method by showing the results of various synthetic tests, we use this method on the aeromagnetic data of the Huanghua depression in central China to define the distribution of volcanic rocks. The spectrum slices in different scales are used to determine horizontal positions of volcanic rocks and their source depths are estimated from the modulus maxima of complex coefficients, which is in good accordance with drilling results.     

An analytic signal approach to the interpretation of total field magnetic anomalies1

The analytic signal (AS) is defined as the square root of the sum of the squares of the vertical and the two horizontal derivatives of the total magnetic field ΔT. This paper verifies theoretically that peaks of the AS correlate directly with their magnetic causative bodies and are positioned symmetrically over them, i.e. the main feature of the AS is that it is independent of the inclination of the magnetic field. This avoids the difficulties that are often faced in the conventional process of reduction to pole for ΔT, when the direction of magnetization of the causative bodies is not known. In addition, the AS has characteristics similar to the derivative features of the magnetic field, so that it is very sensitive to edge effects of the causative magnetic bodies. The theoretical derivations are tested by comparison with calculations on models, and, in a field example from Hunan Province, China, the AS is applied successfully to the interpretation of ΔT, whereas the conventional process of reduction to pole fails, due to the reverse magnetization of the causative body.     

Gravity and magnetic signatures of volcanic plugs related to Deccan volcanism in Saurashtra, India and their physical and geochemical properties

The Bouguer anomaly and the total intensity magnetic maps of Saurashtra have delineated six circular gravity highs and magnetic anomalies of 40-60 mGal (10⁻⁵m/s²) and 800-1000 nT, respectively. Three of them in western Saurashtra coincide with known volcanic plugs associated with Deccan Volcanic Province (DVP), while the other three in SE Saurashtra coincide with rather concealed plugs exposed partially. The DVP represents different phases of eruption during 65.5±2.5 Ma from the Reunion plume. The geochemical data of the exposed rock samples from these plugs exhibit a wide variation in source composition, which varies from ultramafic/mafic to felsic composition of volcanic plugs in western Saurashtra and an alkaline composition for those in SE Saurashtra. Detailed studies of granophyres and alkaline rocks from these volcanic plugs reveal a calc-alkaline differentiation trend and a continental tectonic setting of emplacement. The alkaline plugs of SE Saurashtra are associated with NE-SW oriented structural trends, related to the Gulf of Cambay and the Cambay rift basin along the track of the Reunion plume. This indicates a deeper source for these plugs compared to those in the western part and may represent the primary source magma. The Junagadh plug with well differentiated ring complexes in western Saurashtra shows well defined centers of magnetic anomaly while the magnetic anomalies due to other plugs are diffused though of the same amplitude. This implies that other plugs are also associated with mafic/ultramafic components, which may not be differentiated and may be present at subsurface levels. Paleomagnetic measurements on surface rock samples from DVP in Saurashtra suggest a susceptibility of 5.5×10⁻² SI units with an average Koenigsberger ratio (Q_n) of almost one and average direction of remanent magnetization of D=147.4° and I=+56.1°. The virtual geomagnetic pole (VGP) position computed from the mean direction of magnetization for the volcanic plugs and Deccan basalt of Saurashtra is 30°N and 74°W, which is close to the VGP position corresponding to the early phases of Deccan eruption. Modeling of gravity and magnetic anomalies along two representative profiles across Junagadh and Barda volcanic plugs suggest a bulk density of 2900 and 2880 kg/m³, respectively and susceptibility of 3.14×10⁻² SI units with a Q_n ratio of 0.56 which are within the range of their values obtained from laboratory measurements on exposed rock samples. The same order of gravity and magnetic anomalies observed over the volcanic plugs of Saurashtra indicates almost similar bulk physical properties for them. The inferred directions of magnetization from magnetic anomalies, however, are D=337° and 340° and I=−38° and −50° which represent the bulk direction of magnetization and also indicate a reversal of the magnetic field during the eruption of these plugs. Some of these plugs are associated with seismic activities of magnitude ≤4 at their contacts. Based on this analysis, other circular/semi-circular gravity highs of NW India can be qualitatively attributed to similar subsurface volcanic plugs.     

Magnetic Basement Depth Inversion in the Space Domain

We present a total-field anomaly inversion method to determine both the basement relief and the magnetization direction (inclination and declination) of a 2D sedimentary basin presuming negligible sediment magnetization. Our method assumes that the magnetic intensity contrast is constant and known. We use a nonspectral approach based on approximating the vertical cross section of the sedimentary basin by a polygon, whose uppermost vertices are forced to coincide with the basin outcrop, which are presumably known. For fixed values of the x coordinates our method estimates the z coordinates of the unknown polygon vertices. To obtain the magnetization direction we assume that besides the total-field anomaly, information about the basement’s outcrops at the basin borders and the basement depths at a few points is available. To obtain stable depth-to-basement estimates we impose overall smoothness and positivity constraints on the parameter estimates. Tests on synthetic data showed that the simultaneous estimation of the irregular basement relief and the magnetization direction yields good estimates for the relief despite the mild instability in the magnetization direction. The inversion of aeromagnetic data from the onshore Almada Basin, Brazil, revealed a shallow, eastward-dipping basement basin.     

On the interpretation of magnetic anomalies for strong remanent magnetizations

In this paper some aspects concerning the interpretation of magnetic anomalies are treated, particularly when the remanent magnetization intensity is strong. In this case, since total and induced magnetization vectors can have very different directions, a correct anomaly interpretation strictly depends on the knowledge of their declinations and inclinations.Thus, a specific procedure is described to determine such parameters from well-known semi-empirical techniques and vectorial relations.Furthermore, the classical definition of apparent susceptibility is shown to be inadequate to this problem and a more general formulation is suggested, which is not only related to the true susceptibility and to the Koenisberger ratio, but also to the declinations and inclinations of the induced and remanent magnetization vectors.The two apparent susceptibilities are then compared for some synthetic magnetic anomalies and significant differences are found.     

New Cambrian paleomagnetic pole for Yangtze Block

A total of 120 samples from 12 sites were collected from two flanks of a fold. Stepwise thermal demagnetization has successfully revealed characteristic magnetization components from the rocks in each case. A well-defined component determined from red fine-grained sandstone is clustered in the northeasterly direction with shallow upward inclination (D = 29.3°,I= -19.2°,k = 283.7, α₉₅ = 7.3°. tilt-corrected). The pole position (39.5°N, 247.3°E,dp = 4.0°,dm = 7.6°) derived from this component is close to the Permian pole for the Yangtze Block, indicating that the red fine-grained sandstone has been overprinted. The red mudstone reveals two characteristic components Component A with lower unblocking temperature, characterized by northerly declination and moderate to steep inclination corresponds to a pole position overlay with the present North Pole. Component B (D = 129.1°,I=-23.6°,k = 44.6, α₉₅ = 7.8°, tilt-corrected) with higher unblocking temperature, passes fold test, and yields a pole position (39.5°S, 185.l°E,dp = 4.4°,dm = 8.3°) different from the other poles for the Yangtze Block. It is therefore suggested that component B was probably a primary magnetization and the Yangtze Block was situated at low latitudes in the Southern Hemisphere in the Middle Cambrian.     

Paleomagnetic results of the Cretaceous marine sediments in Tongyouluke, southwest Tarim

Southwest Tarim (hereafter SW Tarim) is one of afew areas that well developed Cretaceous marinesedimentary rocks in China [1]. The Cretaceous marinesediments are stretched in front area along the Tian-shan and Kunlun Mountains. Toward the center ofTarim Basin, the Cretaceous sediments are buried bygreat thickness of Tertiary and Quaternary sedimentswith little exposure. Compared with the Cretaceousterrestrial strata of north Tarim, the Cretaceous marinestrata of SW Tarim continue and d…     

Rock-magnetic and archeomagnetic survey from some classical settlements at Chapultepec archeological site (western Mesoamerica)

Many archaeological artifacts contain magnetic minerals that may record the direction and strength of the Earth’s magnetic field. The geomagnetic field parameters (declination, inclination and intensity) change through time and then may be used as a dating tool. Over the last three decades, the archeomagnetic method was successfully applied in Europe. Still, no systematic studies are devoted to the American sites. We report here, a detailed rock-magnetic and archeomagnetic investigation of some pre-Columbian settlements at Chapultepec archeological site (western Mesoamerica). Continuous low-field susceptibility vs. temperature curves performed in air point to Ti-poor titanomagnetites as remanence carriers. Hysteresis ratios fall essentially in the pseudo-single-domain region. The twelve oriented samples taken from the furnace located in the habitation area were demagnetized applying pick alternative fields. In most cases, a stable uni-vectorial remanent magnetization was found yielding a reasonably well defined mean characteristic direction: inclination I = 32.8°, declination D = 353.4°, and parameters of Fisherian statistics α₉₅ = 10.4° and k = 14. Still poorly defined directional master curves for Mesoamerica, together with relatively dispersed mean directions obtained in this study, make dating targets difficult. Archaeomagnetic dating was carried out on the basis of Bayessian statistics. This established a most probable time interval from 512 to 634 A.D. with an average of 573 ± 61 A.D.     

Secular variation of the direction of the ancient geomagnetic field for Loyang region,China

Curves of the variation of the inclination and declination of the geomagnetic field for the Loyang region during the last 2400 years are presented. An I-D diagram of secular variation, like that for London, has been drawn for Loyang.     

Estimating magnetic dike parameters using a non‐linear constrained inversion technique: an example from the Särna area,west central Sweden

In this paper, we describe a non‐linear constrained inversion technique for 2D interpretation of high resolution magnetic field data along flight lines using a simple dike model. We first estimate the strike direction of a quasi 2D structure based on the eigenvector corresponding to the minimum eigenvalue of the pseudogravity gradient tensor derived from gridded, low‐pass filtered magnetic field anomalies, assuming that the magnetization direction is known. Then the measured magnetic field can be transformed into the strike coordinate system and all magnetic dike parameters – horizontal position, depth to the top, dip angle, width and susceptibility contrast – can be estimated by non‐linear least squares inversion of the high resolution magnetic field data along the flight lines. We use the Levenberg‐Marquardt algorithm together with the trust‐region‐reflective method enabling users to define inequality constraints on model parameters such that the estimated parameters are always in a trust region. Assuming that the maximum of the calculated g_zz (vertical gradient of the pseudogravity field) is approximately located above the causative body, data points enclosed by a window, along the profile, centred at the maximum of g_zz are used in the inversion scheme for estimating the dike parameters. The size of the window is increased until it exceeds a predefined limit. Then the solution corresponding to the minimum data fit error is chosen as the most reliable one. Using synthetic data we study the effect of random noise and interfering sources on the estimated models and we apply our method to a new aeromagnetic data set from the Särna area, west central Sweden including constraints from laboratory measurements on rock samples from the area.     

Time and spatial variations in the ratio of nighttime and daytime critical frequencies of the F 2 layer

The ratio of daytime and nighttime values of the foF2 critical frequency is analyzed on the basis of the data of 28 ionospheric stations in the Eastern Hemisphere. It is found that three types of time variations in this ratio are observed after 1980: an increase with time (a positive trend), a decrease with time (a negative trend), and the absence of pronounced changes (a zero trend). The sign of this trend is shown to be governed by the signs of the magnetic declination D and magnetic inclination I at the given ionospheric station. This fact makes it possible to assume that the above trend is caused by long-term variations in the zonal component V _ny of the horizontal wind in the thermosphere, the latter component contributing into the vertical drift velocity W. The causes of the systematic changes in the thermospheric circulation regime after 1980 are still unknown; however, it is quite probable that they are related to anthropogenic changes in the atmosphere.     

Paleosecular variations 12–20 Kyr as recorded by sediments from Lake Moreno (Southern Argentina)

We present results of paleomagnetic and sedimentological studies carried out on three cores Lmor1, Lmo98-1, Lmor98-2 from bottom sediments of Lake Moreno (south-western Argentina), and integrate them with data from our previous studies. Measurements of directions (declination D and inclination I) and mass specific intensity of natural remanent magnetization (NRM intensity), magnetic susceptibility (specific, χ and volumetric, κ), isothermal remanent magnetization (IRM), saturation of isothermal remanent magnetization (SIRM), and back field remanent coercivity (B_0CR) were performed. The stability of the NRM was investigated using alternating-field demagnetization. The results show that these sediments meet the criteria required to construct a reliable paleomagnetic record. The cores were correlated very well based on magnetic parameters, such as χ and NRM intensity, as well as with lithological features. Tephra layers were identified from the lithological profiles and magnetic susceptibility logs. We obtained the D and I logs of the characteristic remanent magnetization for the cores as a function of shortened depth. The data from the three cores were combined to form a composite record using the Fisher method. A comparison between stacked inclination and declination records of Lake Moreno and those obtained in previous works on Lake Escondido and Lake El Trébol shows good agreement. This agreement made it possible to transform the stacked curves into time series spanning the interval 12–20 kyr. The results obtained improved our knowledge of SV and the behaviour of the geomagnetic field and also allowed us to determine the range of past inclination variations from −70° to −45° for the southern hemisphere, where data are scarce.     

The recent secular variation of declination and inclination in Kenya

Oriented specimens of very young lavas have been collected from 3 volcanic sites in Kenya in order to determine the declination and inclination angles at the times the lavas cooled. TheseD andI values have been tentatively fitted to a time scale extending over the past 1000 years based on historical, geological and vegetational evidence. A plot of declination against inclination for this period takes the form of a partial loop, somewhat similar to those obtained at other places.     

Analytic signal approach and its applicability in environmental magnetic investigations

We investigate the analytic signal method and its applicability in obtaining source locations of compact environmental magnetic objects. Previous investigations have shown that, for two-dimensional magnetic sources, the shape and location of the maxima of the amplitude of the analytic signal (AAS) are independent of the magnetization direction. In this study, we show that the shape of the AAS over magnetic dipole or sphere source is dependent on the direction of magnetization and, consequently, the maxima of the AAS are not always located directly over the dipolar sources. Maximum shift in the horizontal location is obtained for magnetic inclination of 30°. The shifts of the maxima are a function of the source-to-observation distance and they can be up to 30% of the distance. We also present a method of estimating the depths of compact magnetic objects based on the ratio of the AAS of the magnetic anomaly to the AAS of the vertical gradient of the magnetic anomaly. The estimated depths are independent of the magnetization direction. With the help of magnetic anomalies over environmental targets of buried steel drums, we show that the depths can be reliably estimated in most cases. Therefore, the analytic signal approach can be useful in estimating source locations of compact magnetic objects. However, horizontal locations of the targets derived from the maximum values of the AAS must be verified using other techniques.     

Preliminary Results from Paleomagnetic Records on Lake Sediments from South America

The preliminary results of paleomagnetic and radiocarbon dating of late pleistocene-holocene sediments from two lakes of south-western Argentina (41°S, 71.5°W) are presented. The magnetic susceptibility, intensity and direction of the natural remanent magnetisation were measured. The stability of the natural remanent magnetisation was investigated by alternating field demagnetisation. The magnetic parameters allowed the cores within each lake to be correlated. ¹³ C analysis, total organic content measurements and C ¹⁴ dating were carried out. A model of sedimentation is suggested. Using this model and the correlation, curves of variations of magnetic inclination and declination in time are shown.