Patent ID: 12235401

DETAILED DESCRIPTION OF THE INVENTION

The present invention consists of a method developed to improve the imaging resulting from 2D seismic acquisition. With a more efficient dispersion of the wavefronts generated from explosive sources—preferably dynamite—, it is possible to incrementally expand the frequency content of the processed data, causing a significant improvement in the seismic sections, especially in the resolution and continuity of the reflectors.

It comprises, in its preferred configuration, at least the following steps:Step 1: Incremental distribution of charges;Step 2: Individual firing of charges; andStep 3: Processing and stacking of records performed under the previous conditions.

Step 1, Incremental Distribution of Charges, comprises the organization of the disposition of explosive charges with reduced weight amounts at each shot point. There are defined the distance and depth that will be used to separate the shot points, the number of times the initial charge will suffer reductions in the ratios that will be presented below and the maximum or minimum charge for the calculation of the increment, depending on the objective designed for the seismic program.

With this, the distribution of the charges along the seismic line is carried out according to the incremental planning, enabling the seismography team to carry out the records.C, 3/4C, C/2, C/4 . . . C/n

Wherein C is a reference charge weight value.

The value of C is preferably 1 Kg or 0.5 Kg and the configuration of the seismic line is consequently preferably arranged as follows:1 Kg; 0.75 Kg; 0.5 Kg; 0.25 Kg; or0.5 Kg; 0.25 Kg; 0.125 Kg; 0.062 Kg.

The use of dynamite with different weights allows the waves to propagate with different energies due to the elastic properties existing in rocks in the subsurface. The waves generated by smaller dynamite suffer less elastic resistance from the Earth, releasing, proportionally, greater propagation of energy. This effect is outstanding when recording reflections that have different frequency ranges and bandwidths for each charge used.

The charge positioning interval, shot point interval (SPI), is preferably 40 m, if the reference charge (C) is 1 Kg, and 80 m, if the reference charge (C) is 0.5 Kg. Its variation occurs according to the area of geophysical investigation. The depths for positioning the charges can vary from 1.5 m to 5 m, and is preferably 3 m.

In step 2, Individual Firing of Charges, the charges are detonated and the reflections of the seismic waves are recorded.

Each shot point (SP) has a unique explosive charge, detonated individually so that the recordings of reflections occur in a manner corresponding to the weights of the sources. This procedure allows obtaining images related to each charge intensity used in the acquisition, as shown inFIGS.5and6.

The recording of the reflections of the seismic waves is done by an arrangement of individual geophones with high sensitivity with a cut-off frequency, preferably, of 5 Hz. For each detonation, 150 channels are preferably connected on each side of the shot. The geophones are preferably spread symmetrically, 20 m equidistant, forming the 3000-20-0-20-3000 m recording device, with the source located at the 0 m point.

Step 3, Processing and stacking of records, allows the integration of the results obtained in the form of a seismic section for geophysical and geological interpretation, and which comprises, in its preferred configuration, the following sub-steps:sub-step 3.1: Geometry;sub-step 3.2: Static Corrections;sub-step 3.3: Attenuation of coherent noises (Shot Domain);sub-step 3.4: Deconvolution;sub-step 3.5: Velocity Analysis;sub-step 3.6: Coherent Noise Attenuation (CDP Domain);sub-step 3.7: Pre-stacking migration;sub-step 3.8: Stacking; andsub-step 3.9: Filtering.

In sub-step 3.1, Geometry, the location tables of each source (shot) and each receiver (geophones) are built up by means of their spatial positioning (x, y, and z), wherein coordinates “x” and “y” are a geographic positioning and the value of “z” is the positioning in subsurface (time or depth). This information is inserted in the header of each trace, in order to allow the reorganization and thus, the execution of all the other processing steps.

In sub-step 3.2, Static Corrections, time corrections are made that cause distortions in the focusing of reflections and can also generate false structures, due to thickness and velocity variations of the weathering layers. These layers have thicknesses, which can vary from 0 to 200 m, and have low velocities, preferably between 400 m/s and 1500 m/s. The corrections are calculated by using the seismic refraction method and applying tomography techniques in such a way that, after being calculated, they are replaced with a layer with a higher velocity, above 2000 m/s.

In sub-step 3.3, Coherent Noise Attenuation, the main noises are filtered out after transforming a seismogram in the space x time domain into the spatial frequency x temporal frequency domain via fast Fourier transform in two directions (FFT2D).

They appear in seismograms as linear events that are generated mainly by surface waves (ground roll) and by the reverberations of refracted waves in the weathering layers.

In this domain, after drawing a polygon where the noises occur, the amplitudes are zeroed and the inverse transformation is applied, returning to the space x time domain with the noises attenuated.

In sub-step 3.4, Deconvolution, it is considered that the seismic trace is the convolution of a random time series that are the reflection coefficients of the geological layers with the source signature, in this case, the seismic pulse generated by the detonation of one or more charges of dynamite.

The Wiener filter allows to calculate the inverse filter that will transform the pulse of the long duration source into a very compressed pulse from the autocorrection of the seismic trace and the Cross-correlation of the seismic traces with a desired output. At the end of this sub-step, the original seismic trace is convoluted with the inverse filter, and the pulse compression enables the separation and consequent identification of the several geological layers.

In sub-step 3.5, Velocity Analysis, the velocity analysis in the common depth point (CDP) domain is performed. Reflections for different source-receiver distances generate hyperbolic time-distance curves. Each velocity that best horizontalizes the hyperbola of a CDP and that causes greater amplitudes in the semblance panel (correlation between all traces of a CDP) is visually chosen.

Sub-step 3.6, Coherent Noise Attenuation in the CDP domain, aims to attenuate multiple reflections whose trajectories are reflected at least once on the free surface and twice on the same reflector appearing at the time corresponding to twice of the primary reflection time. As at this time the velocity is higher, the primary reflection will appear horizontalized inside the CDP while the multiple reflection will appear undercorrected being the target of the filters. The most used filters are those that use the 2D Fourier transform or the Radon transform.

In sub-step 3.7, Pre-stacking migration, the objective is to focus the diffracted energies at the position of the diffractor point, due to the fact that, during the process of propagation of the seismic energy, each point in the subsurface works as a spreader (diffractor), deflecting energy in different directions. Methods that migrate recorded data with any source-receiver offset are called pre-stack migration methods.

In sub-step 3.8, Stacking, the redundancy provided by the CMP technique allows to have a statistical sampling of several seismic attributes, the amplitude of the reflection being one of them. The most used calculation method to find this value, in view of the various measurements performed (one for each CMP trace) is the arithmetic mean of the samples at a given time. To calculate this mean, it is necessary to horizontalize the hyperbolas for each reflector (or migrate the same before the stacking), by using the velocities obtained in the process of Velocity Analysis. With this procedure, the signal-to-noise ratio increases and, the greater the redundancy, the greater the attenuation of random noise.

In sub-step 3.9, Filtering, the objective is to eliminate or attenuate the residual noise still present in the data. There are several widely known tools in the field of geology and geophysics, commonly used for this purpose, wherein there can be highlighted: FXDecon, which attenuates random noise, FK filter, which attenuates linear noise, and the frequency filter.

The seismic section, consisting of records made with the methodology proposed in this document, results in an image with good quality due to the composition of the frequency content provided by each one of the charges with different weights.

The present invention enables an improvement in the quality of seismic data, providing to the interpreters more precision that allows more control, helping to reduce the uncertainties existing in the study of the geology of the area.

TESTS

Two comparative tests were performed: the first test, between the traditional arrangement method, with equal and equidistant charges in 37.5% of the line, and the present invention with alternating equidistant charges with 4 different weights in 62.5% of the line. The second test, between a method composed of alternating equidistant charges with 2 different weights on 66.6% of the line, and the present invention with alternating equidistant charges with 4 different weights on 33.4% of the line. The results of the final processed sections can be seen inFIGS.7and8.

The parameters used in the first test, related to the traditional method (FIG.1) were: dynamite with a charge of 1 Kg, equidistant 80 m, coupled in holes with a depth of 3 m and distributed over 6 Km of a seismic line with 16 Km in total.

In the method of the present invention, dynamites were used with alternate charges, equidistant 40 m, distributed in a sequential decreasing way: 1 Kg, 0.75 Kg, 0.5 Kg, 0.25 Kg, coupled in 3 m depth holes and distributed over the 10 Km remaining of a line with 16 Km in total. This means that redundancy of charges of the same weight occurred every 160 meters, allowing each shot of equal weight to be tested 100 times.

The upper part ofFIG.3shows four records corresponding to each charge weight used. As expected, there is a notable difference in frequency content, whereinFIG.3D, obtained with 1000 g charge, has lower dominant frequencies, andFIG.3A, which corresponds to 250 g, has higher dominant frequencies. Note that the ground roll, characterized by low frequencies and high amplitudes, is more evident inFIG.3Dthan inFIG.3A.

The lower part ofFIG.3shows the frequency contents corresponding to the four shots containing different charges. There is a reduction in the lower frequencies content, represented by the green color amplitudes at the top of the section, seen from the heavier to the lighter source weight.

The 1000 g charge has a higher content of amplitude and more abrupt limits of values within the window of 1 to 10 Hz (green colors) and another in 15 Hz (purple colors) (FIG.3D).

The 250 g charge has frequency content from 5 Hz to 40 Hz distributed more smoothly (FIG.3A) than heavier sources. The smooth transition of recorded frequency content from the firing of lighter charges generates a more uniform frequency spectrum.

To compare the quality of the migrated sections, we have applied the same standard processing flow to the five datasets: tomographic static correction, F-K filter, deconvolution, gain, velocity analysis, residual static, migration, post-stacking filters.

FIGS.5and6show the migrated seismic sections, individually, obtained from the four data subsets with equivalent source weights for each test.FIGS.5A,5B,5C and5Dcorrespond to the source charges with 1000 g, 750 g, 500 g and 250 g, respectively, andFIGS.6A,6B,6C and6Dcorrespond to 500 g, 250 g, 125 g and 62 g, respectively.FIGS.7and8show the migrated seismic sections, full stack, using the records from all source weights for the two tests performed.

Considering the frequency content and continuity of the reflectors, there is a significant difference at the bottom of images A, B, C and D inFIG.3. As noted earlier, smaller sources provide a more uniform record of a distributed frequency content in the spectrum. The image quality of the existing geological structure at the time interval of 300 ms to 600 ms on the right side of the section is better for 500 g (FIG.3B) and 250 g (FIG.3A) when compared to the sections related to charges of 1000 g and 750 g. Although the SPI is 160 m for each subset of equivalent charges, all migrated sections are of good quality (FIG.7).

When comparing the energy ratio between the four data subsets, it can be noted that doubling the source weight does not result in doubling the energy:500 g/250 g 125% increase in energy;750 g/250 g 144% increase in energy;750 g/500 g 115% increase in energy;1000 g/250 g 154% increase in energy;1000 g/500 g 123% increase in energy;1000 g/750 g 106% increase in energy.

Those skilled in the art will value the knowledge presented herein and can reproduce the invention in the presented embodiments and in other variants, all encompassed by the scope of the appended claims.