RU2665355C2 - Method of marine high-accuracy magnetic survey - Google Patents

Abstract

FIELD: geology.SUBSTANCE: invention relates to geological mapping in water areas and mineral explorations by geophysical methods, in particular to placer mining explorations. Method for accounting for geomagnetic variations during marine magnetic surveys with high accuracy using gradiometric magnetic survey, results of which, after integration (summation of gradients), are not used as a change in the magnetic field on the profile, but are only used to calculate the variations observed on the profile, used as the zeroth approximation in the indirect method for accounting for variations, which allows achieving a very high accuracy of survey magnetic field module T, since integral graphs, in which, because of smoothing the observed field by differential basis Δx a fine structure of field T is distorted, and a decrease in the base (distance between two magnetometers) Δx leads to a large error in the measurement of weak gradients, are not used. Therefore, the gradiometric method does not comprises using the integral field obtained through gradients during studying a fine structure of the magnetic field.EFFECT: variations can be calculated and used as the zeroth approximation in the indirect method for accounting for variations.1 cl

Description

The invention relates to the field of geological mapping in water areas and the search for mineral deposits by geophysical methods, in particular the search for placer deposits.

The invention can most effectively be used in the search for deposits on the shelf.

Alluvial mineral deposits, in addition to the actually useful component (for example, gold, tin, diamonds, etc.), contain significant amounts of heavy magnetic minerals accompanying them [1].

However, the current high-precision magnetic surveys [2] are characterized by high (error up to 5 nT), but not high enough accuracy for solving specific problems when searching for placer and other mineral deposits. For example, with an error of up to 1-2 nT.

The main obstacle in conducting magnetic surveys are variations in the geomagnetic field that are especially intense at high latitudes, where they can reach tens or hundreds of nT. Magnetic variation stations (MVS) are used to evaluate them, which are installed ashore and even in the water area and their magnetic field records during surveys (t) take into account the obtained δT (t) variations in the measured on ordinary (RM) and secant (SM) routes . This method can serve as a prototype of the present invention.

However, the variations at the points of the MVS sites and on the RM and the SM at the same time can significantly differ due to anomalous variations (coastal effect, etc.), that is, introducing them into the measurements along the routes can lead to large shooting errors.

To account for abnormal variations, including To account for all types of variations, an indirect method of accounting for variations has been created [3]. This method can also serve as a prototype of the claimed invention.

However, it requires a high density of secant routes, which does not always ensure accuracy. For hydromagnetic surveys, the accuracy required to search for placers cannot be achieved.

It is also known that to improve the accuracy of marine magnetic surveys, a gradiometric survey method is used, containing two modular magnetic field sensors performing synchronous measurement of the magnetic field by two spaced apart Δx: closest to the carrier T ₁ and far T ₂ sensors [2].

The measurement of the magnetic field gradients during the hydro-magnetic survey is carried out by two spaced sensors synchronously, which leads to the absence in the resulting difference ΔТ = (T ₁ -T ₂ ) of the variations of the geomagnetic field. If we take into account that the speed of the carrier (vessel) is not large, and the length of the working routes is significant, it becomes obvious how important the problem of accounting for variations is, especially at high latitudes, where the amplitude of variations can vary by tens or hundreds of nT per day. The complete absence of variations in the measured value of the field gradient ΔT is undoubtedly an advantage of the gradientometry method. Integrating the measured field of gradients ΔT, we obtain the field T, which is not distorted by variations. This method could also serve as a prototype if it were not for the problem of integrating the field gradients, which are calculated over a large interval Δx, which is shifted by one measurement point. Thus, due to the large Δx interval, a significant gradient is provided for measuring a strong signal against a background of interference. However, in this case, in the large interval Δx, the fine field structure is lost and, accordingly, the loss of weak anomalies occurs, which carry the maximum information about the upper part of the studied section.

а его приближенное значение, осредненное на базе Δх, т.е.

Следовательно, интегрируя ΔT по х, а точнее суммируя вдоль х, мы получаем некоторое осредненное представление о поле Т.Gradients of the ΔT field are measured by two sensors spaced apart on a constant basis (Δx). The actual value of the base Δx is of the order of 100 m. Thus, in the measurement process we obtain an incorrect gradient value

and its approximate value averaged on the basis of Δx, i.e.

Therefore, integrating ΔT over x, or rather summing along x, we get some averaged representation of the field T.

In the field anomalies T _∑ = ∑ΔT there are no really existing anomalies of T having a width commensurate with Δx, as well as weak low-gradient anomalies of a higher order against the background of large gradient anomalies T. Thus, in the anomalies T _{∑ there is} no thin field structure that carries information about the structural and lithological features of the geological structure of the studied area. For example, the manifestation of placers in a magnetic field. A decrease in the Δx base leads to a deterioration in the signal-to-noise ratio, especially in the field of low-gradient weakly expressed anomalies, then the use of gradiometry loses its meaning.

The noise level during gradiometric measurements is associated with the “yaw” of nacelles with sensors on a long cable, a change in the magnitude of the carrier deviation to the near and remote sensors, violation of the Δx base and the straightness of the route. When integrating these disturbances create gradients in T _Σ new kind of noise, which must be combated.

In addition, the proposed method assumes that to summarize ΔT (integration), it is necessary to know the initial value of T ₀ , which can be obtained by linking RM and SM [3] with the participation of additionally performed secant paths (DSM). At the points of intersection of the DSM with the beginning and end of the RM and SM, zero (for integration) T ₀ values will be obtained.

According to the values of T ₀ on each route, and reduced to the distance between the nearest points, the field gradients measured at a distance Δx, we summarize the gradients

where i is the number of points on the profile; T ₀ - field at the starting point of integration after preliminary linking RM, SM and DSM on the second sensor farther from the carrier. Then, T _j is subtracted from this field and we obtain the estimate of the variations ΔδT (t).

Due to the fact that the gradiometry data contain errors, the difference ΔδT (t) may contain a certain trend, which must be taken as a linear trend or higher orders until the fields at the start and end points of intersection with the DSM with a specific route and / or even at intermediate intersections of RM and SM. The resulting trend is subtracted from ΔδТ and it does not participate in further coordination.

Subtract RM, SM and DSM from the above T ₂ = T ₂ (t) and identify the difference with the variations ΔδT (t) on each of the routes, which can be used as the zeroth approximation for repeated linking [3], i.e. to link RM and SM with allowance for DSM by the sum of the variations obtained, including by gradiometry at intermediate points between the DSM on the RM and SM.

After the second linking, the received corrections are taken into account in the fields measured on the route, including between the intersection points of RM and SM, DSM. This is facilitated by gradiometric photography.

Thus, the process of evaluating δT variations during marine high-precision magnetic surveys with high accuracy is as follows:

1) a second sensor is installed to measure the magnetic field gradients;

2) the magnetic field is linked to the PM and SM, taking into account the DSM (performed at the ends of the RM and SM routes) to obtain a linked field T ₀ at the initial points of integration of the field gradients;

3) a linear trend is subtracted from the sum of the gradients located between the first and last integration points on the profile, provided that the deviations from the trend at the first and last points differ from the linked values of the initial field T ₀ and do not exceed the specified error (for example, measurement error or 1 / 3 of the expected accuracy of the survey) with the exception of the trend (for example, due to deviation), you can use the linked fields at the points of intersection of RM and SM on the studied profile. In this case, the trend (according to the relevant criteria [4]) can be estimated in the form of higher order polynomials;

4) deviations from the trend on each route are entered into the initial linked field (item 2) from the first to the last point;

5) the field obtained at the first iteration is subtracted from the initial unlinked field, and the difference obtained is used as the zero approximation of variations during repeated (second iteration) matching of the initial field;

6) the integral variation is added to the results of the second linking of the initial field with a zero approximation for variations, i.e., the gradient obtained on each of the profiles and the linking process can be completed if the discrepancy at the intersection points of all the PM and the SM satisfies the specified shooting error, or is continued to achieve this accuracy. There can be 3-4 iterations in total.

EFFECT: increased accuracy of hydro-magnetic surveys, which leads to increased efficiency when searching for alluvial and other solid minerals from related magnetic minerals and when searching for hydrocarbon deposits by magnetic anomalies that are indicators of hydrocarbon deposits.

LITERATURE

1. Smirnov A.N., Palamarchuk V.K., Glinskaya N.V., Burdakova E.V., Mishchenko O.N., Popova E.S. Methodological aspects of the search for placer deposits on the shelf of the Arctic and Far Eastern seas using the magnetoacoustic method // Arctic. Ecology and Economics, 2015. No. 1 (17) S. 47-51.

2. Instructions for magnetic exploration. - L .: "Nedra", 1981.

3. V.K. Palamarchuk. Taking into account variations in the geomagnetic field and linking observations with high-precision aeromagnetic surveys. Novosibirsk, Publishing House "Science" Siberian Branch: Geology and Geophysics, No. 10, 1983, p. 107-114.

4. V.K. Palamarchuk. Experience in separating anomalies by the trend method, Novosibirsk, Nauka Publishing House Siberian Branch: Geology and Geophysics, No. 4, 1972.

Claims (1)

A method of high-precision hydro-magnetic surveying, comprising a magnetometer on a long cable and high-precision satellite coordination in (x, y, h), performing a hydro-magnetic survey in coordinates and in time t from marine mobile carriers, along a network of ordinary (RM) and planned secant (SM) routes characterized in that a second magnetometer is installed, forming, with the first, two modular sensors of the gradiometer magnetometer, performing synchronous measurement of the Earth’s magnetic field with two nearby T ₁ (t) and far T ₂ (t) sensors Then, at the ends of the RM and the SM, additional secant paths (DSM) pass, iterate the field to the RM and the SM, including the DSM, find the zero approximation of the field for integrating the gradients starting from the intersection points of the RM and the SM with the DSM, calculate the gradient ΔT ₁₂ (t ) = T ₁ (t) -T ₂ (t), bring the difference ΔT ₁₂ (t) to the distance between the two closest measuring points by the first and second sensors (Δ (t)), sum (integrate) the difference Δ (t) from the beginning each route RM and CM since legislation bound T field ₀ of the first point (the intersection of PM and SM and MPA or MPA), calculated time Itza among linked and integrated fields removed from this difference linear trend to match (at a given error σ ₀₎ values at the first and last points of the route, calculate the deviation from the trend and identify them with zero approximation for additional variation δT ₀ (t), summarize δT ₀ (t) with the corrections obtained after linking the initial field and use these variations (δT ₁ (t)) as the zeroth approximation to take into account the indirect indirect variations from the initial data; after reconciling the observed field, we obtain

on RM, SM and DSM with zero approximation δT ₂ (t), a new integration begins with corrected (linked) values

at the first points of intersection of RM and SM with DSM, eliminate the trend from integrated values, calculate the difference between the initial T ₂ (t) and corrected

use them as an estimate of variations

and make them the final account of the variations, incl. in intermediate intervals between the intersection points of RM and SM on each profile.