Abstract: System and methods for using nuclear magnetic resonance (NMR) T1 measurements for wireline, LWD and MWD applications and down-hole NMR fluid analyzers. The T1 measurements are characterized by insensitivity to motion, as the detrimental effects arising from tool motion or fluid flow are effectively reduced or eliminated. T1 measurements alone or in combination with other standard oil field measurements are shown to provide efficient data acquisition resulting in compact and robust data sets, the potential for substantially increased logging speeds, and simple methods for fluid typing, including direct and robust identification of gas.

Abstract: NMR spin echo signals are corrected for axial motion of the borehole logging tool. An additional correction may be applied to correct for incomplete polarization of nuclear spins due to an insufficient wait time between pulse sequences.

Abstract: A method for evaluating formation fluids includes measuring a nuclear magnetic resonance property related to a total volume of the formation fluids, measuring a dielectric property related to an electromagnetic wave travel time, measuring a bulk density, and solving a set of linear response equations representing a reservoir fluid model to determine fractional fluid volumes from the nuclear magnetic resonance property, the dielectric property, and the bulk density.

Abstract: An expert system is included in a downhole processor designed to acquire and process NMR data downhole in real time. The downhole processor controls the acquisition of the NMR data based at least in part on instructions transmitted downhole from a surface location and at least in part on evaluation of downhole conditions by the expert system. The downhole conditions include drilling operation conditions (including motion sensors) as well as lithology and fluid content of the formation obtained from other MWD data. The wait time, number of echos, number of repetitions of an echo sequence, interecho time, bandwidth and shape of the tipping and refocusing pulses may be dynamically changed. Data processing is a combination of standard evaluation techniques. Selected data and diagnostics are transmitted uphole. The expert system may be implemented as a two stage neural net. The first stage does the formation evaluation and the second stage controls the NMR pulse sequence.

Abstract: NMR spin echo signals, acquired on a MWD logging tool, are susceptible to errors magnetic flux density has a gradient and the magnet on the logging tool is moving relative to the earth. The errors can be corrected by having the excitation pulse cover a smaller or a larger volume than the refocusing pulses. Correction may also be made by selective saturation, or by echo averaging.

Abstract: A method of measuring characteristics of a flowing fluid is described including: (a) providing a flowing fluid; (b) applying a static field gradient to the flowing fluid; (c) applying a series of nuclear magnetic resonance pulses to the flowing fluid; (d) detecting signals from the flowing fluid, wherein the signals are generated in response to the nuclear magnetic resonance pulses; and (e) analyzing the real and imaginary components of the signals to determine one or more characteristics of the flowing fluid. Also described is an apparatus for implementing this method.

Abstract: A method and system for estimating native hydrocarbons from oil-based drilling muds with the aid of NMR data. Adaptive echo stacking may be used to balance between precision and sensitivity to changes in the fluid composition. Apparent T2 decay time and effective diffusion constants derived from the NMR data may be transformed into an indication of native hydrocarbon type, using known diffusion constants, fuzzy logic, and neural networks.

Abstract: A nuclear magnetic resonance (NMR) logging system for measuring fluid properties of formation penetrated by a well borehole. The system uses a conductive permanent magnet to generate a field H0. The conductive permanent magnet is disposed within a logging tool so that the H0 field is perpendicular to the axis of the borehole. Soft ferrite elements are disposed on each pole of the permanent conductive magnet and a RF coil is disposed about both the permanent magnet and the ferrite. The resulting H1 field induced by the RF coil is perpendicular to both the H0 and the axis of the borehole. The soft ferrite elements focus the conductive permanent magnetic field and increase the effectiveness of the RF antenna. A two coil embodiment is disclosed that produces a combined RF field which is substantially perpendicular to the static magnetic field external to the conductive permanent magnet, and is contained within the soft ferrite elements in the vicinity of said conductive permanent magnet.

Abstract: Apparatus and method for providing in-situ data of formation fluids at true reservoir conditions. The apparatus has NMR chamber of the flow-through type, so it is no longer necessary to divert a sample for the basic NMR analysis or transfer it uphole for laboratory analysis. The apparatus is preferably modular and is compatible with wireline tools configurable for a variety of sampling, testing and monitoring purposes. Fluid properties such as viscosity can be derived from diffusivities and relaxation times, as measured by NMR. Additional accuracy is provided in the downhole sampling process, and in the interpretation of NMR logs in real time at true reservoir conditions.

Abstract: A method of determining the distribution of shales, sands and water in a reservoir including laminated shaly sands using vertical and horizontal conductivities derived from nuclear, NMR, and multi-component induction data such as from a Multicomponent Induction Logging Tool

Abstract: Method for data acquisition and processing, which uses time-domain averaging to increase the signal to noise ratio (SNR) in single-event measurements. In a preferred embodiment, NMR echo-trains obtained using high-speed NMR logging are provided, the echo trains are time-domain averaged, preferably in real-time, over one or more time intervals to sharpen the spatial resolution of the logging tool and/or to increase the signal to noise ratio (SNR) of the data train.

Abstract: Estimates of ringing produced by short recovery pulse sequences (CPMG or modified CPMG) are used to improve the effects of ringing in a conventional CPMG or modified CPMG sequences. This is an effective method for removal of ringing when the ringing is varying over a time interval that would be spanned by a phase alternated pair of conventional sequences. By use of frequency dithering, ringing due to the excitation pulse and the refocusing pulses can be removed.

Abstract: An apparatus and method is discussed for characterizing a fluid sample downhole of aliphatic hydrocarbon compounds, aromatic hydrocarbon compound, or connate mud filtrates containing carbon-13 isotopes using an enhanced nuclear magnetic resonance (NMR) signal on a measurement-while-drilling device. To enhance the carbon-13 NMR signal these nuclei are being hyperpolarized. Either the Overhauser Effect (OE) or the Nuclear Overhauser Effect or optical pumping and the Spin Polarization Induced Nuclear Overhauser Effect (SPINOE) can serve as a mechanism for hyperpolarization of the carbon-13 nuclei.

Abstract: A method of evaluating a characteristic of a fluid in a region, and a nuclear magnetic resonance (NMR) well logging apparatus for the implementation thereof, is disclosed. A magnetic field gradient is applied to the region, and first and second sequences of magnetic field gradient is applied to the region, with the second sequence differing in a pulse characteristic from the first sequence. A magnetization spectrum resulting from each of the sequences of pulse is captured, and a change in the magnetization spectrum resulting from the first and second sequences of pulses is analyzed to extract information about the fluid.

Abstract: A method for determining a property of fluids in formations surrounding an earth borehole includes the following steps: producing, from measurements on a multiplicity of fluid samples, a database of stored fluid property training values related to stored fluid measurement training values; deriving, from the database, radial basis function parameters; deriving formation fluid measurement values; and determining, using radial basis function interpolation, the property of formation fluids from values in the database, the parameters, and the derived formation fluid measurement values.

Abstract: A method for analyzing formations using measurements from a detector in response to energy imparted therein. The measurements have characteristics which exponentially reduce in magnitude with time. The method includes (a) determining, an N-th order integral of the value of each measurement from an initial time to the time of each measurement, wherein N represents a number of exponentially decaying characteristics; b) determining a solution to a system of linear equations relating the measurements to the integrals, the solution representing polynomials of order N related to a decay rate and an initial measurement amplitude for each component; (c) solving the polynomials to determine the decay rate and the initial amplitude for each component; (d) determining if the decay rates and initial amplitudes are within possible limits; and (e) incrementing N and repeating (a) through (d) until the decay rates or the initial amplitudes are not within possible limits.

Abstract: The present invention provides a novel use of a material having a high internal magnetostrictive damping and/or using material with explicitly low magnetostriction as a NMR probe core material. The probe structural geometry facilitates the use of material, which has a relatively low magnetic permeability.

Abstract: An MWD method and apparatus for determining parameters of interest in a formation has a sensor assembly mounted on a slidable sleeve slidably coupled to a longitudinal member, such as a section of drill pipe. When the sensor assembly is held in a non-rotating position, for instance for obtaining the measurements, the longitudinal member is free to rotate and continue drilling the borehole, wherein downhole measurements can be obtained with substantially no sensor movement or vibration. This is particularly useful in making NMR measurements due to their susceptibility to errors due caused by tool vibration. In addition, the substantially non-rotating arrangement of sensors makes it possible to efficiently carry out VSPs, reverse VSPs and looking ahead of the drill bit. A clamping device is used, for instance, to hold the sensor assembly is held in the non-rotating position.

Abstract: A method for evaluating formation fluids includes measuring a nuclear magnetic resonance property related to a total volume of the formation fluids, measuring a dielectric property related to an electromagnetic wave travel time, measuring a bulk density, and solving a set of linear response equations representing a reservoir fluid model to determine fractional fluid volumes from the nuclear magnetic resonance property, the dielectric property, and the bulk density.

Abstract: The invention relates to methods for prospecting wells using nuclear magnetic resonance. The inventive method consists in generating a static magnetic field near a well with the aid of a magnet which is made of a conductive rear-earth material in the form of an elongated parallelepiped which is magnetized in a direction perpendicular to a large side surface and longitudinal axis thereof. The width of the magnet is of at least twice as large as the narrow side of said parallelepiped. A radio-frequency coil is arranged on a frame whose diameter is equal to or higher than a diagonal of the cross-section section of a magnet arranged inside the frame. The coil elements are arranged on planes which are parallel to the longitudinal axis of the magnet and vertical with respect to the smaller side thereof arranged in symmetrical sectors which are opposite to the large side surface of the magnet. A compensating unit is arranged along the large side surfaces of the magnet parallel to the longitudinal axis thereof.

Abstract: A method for determining a formation fluid property includes acquiring a suite of nuclear magnetic resonance (NMR) measurements of a fluid sample using a pulse sequence that includes pulsed field gradient pulses for encoding diffusion information, wherein each NMR measurement in the suite is acquired with a different value in a parameter in the pulsed field gradient pulses for producing a different diffusion effect, wherein the acquiring is performed in a formation fluid sampling tool in a borehole; inverting the suite of NMR measurements to produce a distribution function that relates diffusion properties of the fluid sample with an NMR property of the fluid sample; and determining the formation fluid property from the distribution function.

Abstract: Pulse sequences are applied to a fluid in an earth formation in a static magnetic field having internal gradients. From the received signals, relaxation and diffusion characteristics of the fluid are determined. The determination takes into account the internal field gradients.

Abstract: A method is disclosed for interpretation of multi-dimensional nuclear magnetic resonance data taken on a sample of an earth formation. Specifically, a set of NMR data is acquired for a fluid sample located either in a borehole or in a laboratory environment. From the set of NMR data, a multi-dimensional distribution is calculated using a mathematical inversion that is independent of prior knowledge of fluid sample properties. The multi-dimensional distribution is graphically displayed on a multi-dimensional map. Each fluid instance or artifact visible on the graph is identified as representing a probable existence of a detected fluid. One or more quantitative formation evaluation answers for one or more fluid instances is computed based on the multi-dimensional distribution associated with the respective fluid instance.

Abstract: A phase alternated Carr Purcell (PACP) sequence is effective in removing DC offset from spin echo signals in earth formations. Modifications of PACP sequences, possibly in combination with other pulse sequences, can remove both DC offset and ringing. Such sequences may be used where the ringing is non-repeatable.

Abstract: A method for tuning a nuclear magnetic resonance (NMR) tool having an operating frequency and equipped with an antenna, is disclosed comprising: (a) transmitting a rf magnetic field to a sample under investigation; (b) receiving an NMR signal from the sample within a detection window; (c) determining mistuning of said antenna relative to said operating frequency; (d) analyzing the received echo signal to determine mistuning of the received signal from the operating frequency. The mistuning of the received signals from the operating frequency may be determined by analyzing any changes in phase of the echo along the echo signal. The antenna tuning process may be automated by measuring calibrated signal amplitudes at more than one frequency and identifying a maximum amplitude. The system tuning may be maintained by repeating (a) through-(d) while operating the tool and implementing a feedback loop.

Abstract: Stochastic pulsing using random or pseudo-random sequences is used for determination of nuclear spin properties of earth formations. This reduces the power requirements. Processing of the received signals may include linear or nonlinear cross-correlation with the excitation pulse sequence. Inversion of the results of the processing gives the desired nuclear spin property.

Abstract: Wireline NMR well logging measurements suffer from disadvantages of poor vertical resolution, logging speeds less than 20 ft/min, and power consumption in excess of 200 W. In spite of these disadvantages, NMR well logging is used because it is capable of providing estimates for a number of petrophysical parameters that are difficult to obtain from other wireline data. These include estimates of the bulk volume irreducible (BVI) of fluids in the formation. The present invention targets BVI and clay bound water (CBW) measurements. Logging speeds of up to 60 ft/min are attainable with little or no loss of resolution. In one preferred embodiment, the tool has four sensors circumferentially distributed around the logging tool and in contact with the borehole wall. A horseshoe like magnet is used to generate the static magnetic field.

Abstract: The invention relates to a nuclear magnetic resonance method of detecting and monitoring the flocculation kinetics of high molecular weight fractions of a complex fluid. The inventive method applies the following to the fluid: a first static polarization magnetic field and, subsequently, at least a second oscillating pulsed magnetic field which generates a nuclear magnetic resonance for the nuclei considered and the acquisition of relaxation signals from the nuclei in the fluid. Moreover, the method detects, in the relaxation signals, a first part P1 which is representative of the relaxation of the flocculated fractions in the fluid and a second part P2 which is representative of the relaxation of the liquid fraction of the fluid; and determines the flocculation rate of the fractions by comparison with values M<sb>x</sb>(t=0) and M<sb>x1</sb>(t=0) which were extrapolated at the start of the acquisition times of the two parts.

Abstract: A logging while drilling (LWD) and measuring while drilling (MWD) method is disclosed for analyzing the porosity of geologic formations containing carbonate-formations. The method is based on a combination of T1 and T2 measurements that enables detection of diffusivity coupling and permits more accurate determination of pore size distribution, formation permeability and irreducible water saturation.

Abstract: A loop-gap resonator for providing an excitation pulse in a down hole nuclear magnetic resonating (NMR) tool for determining a parameter of interest in a formation adjacent a borehole. The loop-gap resonator is constructed having one or more capacitive gaps formed in a nonmagnetic conductive loop. The loop-gap resonator may be deployed down a bore in a measurement while drilling (MWD) configuration or in a wire line configuration. The MWD configuration may utilize a non-rotating sleeve rotationally associated with the drill string.

Abstract: An apparatus and method for determining a parameter of interest of a formation composed of carbonate rock. A nuclear magnetic resonance (NMR) sensor assembly produces a pulsed RF field designed for obtaining measurements indicative of the parameter of interest of the formation. A downhole processor processes the measurements for obtaining BVI and BVM using a cutoff time based on classification of the carbonate. Further processing is done to estimate the permeability of the carbonate.

Abstract: A system and method for calculating the lateral velocity of a rotating drilling tool within a borehole, the system comprising: a pair of accelerometers placed oppositely across the drilling tool axis of rotation and two magnetometers. The method comprises: reading tool radial acceleration signals ar1, ar2, and tangential acceleration signals, at1 and at3, obtained with a quadrature accelerometer detection system; reading Bx and By, the tool's magnetic phase data, from two orthogonally placed magnetometers; determining the rotational phase angle of the drilling tool relative to the earth's gravity field by first determining the tool's magnetic phase, and the phase shift between the tool's magnetic phase and gravity phase; and processing the foregoing data to yield a lateral tool velocity which optionally compensates for the effect of gravity on the accelerometers, or converts the lateral tool velocity from the tool reference frame to the borehole reference frame, or does both.

Abstract: An objective oriented NMR logging method selects pulse sequences over a plurality of frequencies from a set of building blocks. The building blocks include trainlet sequences wherein each trainlet comprises an excitation pulse and a plurality of refocusing pulses, the total length of a trainlet being typically less than 10 ms. Another building block is a short CPMG or modified CPMG sequence and yet another building block is a regular CPMG or modified CPMG sequence. The modified CPMG sequences may have refocusing pulses with a tipping angle less than 180° to reduce the power consumption. Based on the logging objective (formation evaluation or FE, FE plus hydrocarbon typing, FE plus gas evaluation) the building blocks are combined at a plurality of frequencies with different wait times and TEs.

Abstract: A method is disclosed for extracting information about a system of nuclear spins from a region of an earth formation. Specifically, a set of NMR data is acquired for a fluid sample located either in a borehole or in a laboratory environment. From the set of NMR data, a multi-dimensional distribution is calculated using a mathematical inversion that is independent of prior knowledge of fluid sample properties.

Abstract: A modification to the Coates-Timur relationship to produce a more coherent relationship applicable to carbonate formations is disclosed. In this method, permeability may be determined using porosity and the ratio of bound fluid volume to (1—bound fluid volume). This method also allows for improved estimation of irreducible water saturation of a carbonate formation using the ratio of kc and (e?f+kc). Likewise, the bound fluid volume of a carbonate formation may be determined using the ratio of ?kc and (e?f+kc). In these relationships, e, x, and f are constants according to the following relationships e=xc, f=bc+1, x is between 1 and 100 mD (preferably 10 mD).

Abstract: A method for obtaining nuclear magnetic resonance measurements includes inducing a static magnetic field in a formation fluid sample; applying an oscillating magnetic field to the fluid sample according to a preparation pulse sequence that comprises a J-edit pulse sequence for developing J modulation; and acquiring the nuclear magnetic resonance measurements using a detection sequence, wherein the detection sequence comprises at least one 180-degree pulse. The method may further include acquiring the nuclear magnetic resonance measurements a plurality of times each with a different value in a variable delay in the J-edit pulse sequence; and analyzing amplitudes of the plurality of nuclear magnetic resonance measurements as a function of the variable delay to provide J coupling information.

Abstract: A method and system are disclosed for NMR echo-train data acquisition and processing for enhanced tracking of the error in the signal due to magnetoacoustic effects, known as ringing, that are not cancelled out or otherwise corrected. In one aspect, the method enables correction of ringing signals in a single pass by way of collecting a composite signal that includes echoes due to pulses at least at two different frequencies. The choice of one or more of the frequencies, the phase of the pulses, and the recovery period enable cancellation of the ringing signal from the echoes, including the echo from an excitation pulse. In another aspect, the method encompasses providing protection against errors due to tool motion by saturating a relatively wide sensitive region with the echo signals collected from within this relatively wide sensitive signal. Preferred embodiments of the invention include one or more of the aforementioned aspects.

Abstract: A method of reducing ringing artifacts in a nuclear magnetic resonance measurement is provided. The method utilizes a new pulse sequence that offers ringing cancellation opportunities as well as compensation for otherwise accumulating spin dynamics errors. Ringing cancellation is accomplished by forming linear combinations of spin echoes induced as a result of the pulse sequence. Because the linear combinations are formed between closely spaced spin echoes (i.e., echoes within the same sequence), the likelihood that the ringing artifact will have changed between measurements is diminished.

Abstract: Nuclear magnetic resonance well logging apparatus and method of use for geophysical examination of a borehole as the borehole is drilled. The apparatus is connected to the drill bit to follow it through the borehole as the borehole is formed. The apparatus comprises a non-magnetic protective collar with lateral protrusions or wings, which provide enhanced abrasion resistance, strength, and rigidity to the probe section. This collar is twisted about its longitudinal axis. The apparatus also comprises one or more permanent magnets within the metal collar and an antenna mounted outside of the collar. Both the magnet and the antenna are twisted about the longitudinal axis of the probe section to produce a generally helical static magnetic field and a radio frequency magnetic field that is substantially orthogonal to the static magnetic field.

Abstract: A method and apparatus for determining abnormal pressure zones of a geologic formation using NMR measurements, preferably for logging-while-drilling applications. In a preferred embodiment, a normal compaction trend is constructed using NMR-derived clay bound water volume (CBW) content for non-consolidated subsurface formations or bulk volume irreducible (BVI) and CBW for consolidated formations versus depth. Deviations from this normal compaction trend are used to indicate the presence of shale sections with higher porosity that directly corresponds to an overpressured top seal or transitional zone.

Abstract: A method for determining a property of a flowing fluid by nuclear magnetic resonance includes applying a static magnetic field to the flowing fluid; acquiring a suite of nuclear magnetic resonance measurements on the flowing fluid using a pulse sequence comprising a spoiling pulse, a wait time, and an acquisition pulse sequence, wherein the suite of nuclear magnetic measurements have different values for the wait time; and fitting the suite of nuclear magnetic resonance measurements to a forward model for responses of the flowing fluid to derive a parameter selected from a flow speed, longitudinal relaxation times of the flowing fluid, and a combination thereof.

Abstract: A system and method for assessing and potentially correcting for non-translational motion in a resonance measurement apparatus is provided. The design measures the response of an article, such as an HGA assembly including a read/write head, as well as the excitation of a shaking device, such as a head resonance tester, and computes a correction factor using either two or three point measurement. The correction factor may be evaluated by subjecting the arrangement to further vibration at varying frequencies. Measurement of the shaking device may be accomplished using an accelerometer or by optical measurement using a light beam.

Abstract: A method is disclosed for downhole determination of the resistivity of fluids from formations surrounding an earth borehole, including the following steps: providing a formation testing logging device that includes a coil and a flow line passing axially through the coil; determining a reference quality factor of the coil; positioning the formation testing device in the borehole, and causing formation fluid to flow through the flow line; measuring the quality factor of the coil with formation fluid in the flow line; and determining the resistivity of the formation fluid in the flow line as a function of the measured quality factor and the reference quality factor.

Abstract: The invention is related methods for nuclear magnetic resonance measurements; the methods include the steps of applying a first static magnetic field to polarize the sample; radiating a radio frequency pulse onto the sample. Subsequently, applying a second magnetic field such that when combined with the first static magnetic field; a third static magnetic field is produced. Finally, during the application of the second static magnetic field or after removing it, the NMR signal emitted by the sample is measured. Finally, the methods can be used for making nuclear magnetic resonance measurements in wells.

Abstract: The present invention provides a method for obtaining a multi-dimensional proton density distribution from a system of nuclear spins. A plurality of nuclear magnetic resonance (NMR) data is acquired from a fluid containing porous medium having a system of nuclear spins. A multi-dimensional inversion is performed on the plurality of nuclear magnetic resonance data using an inversion algorithm to solve a mathematical problem employing a single composite kernel to arrive at a multi-dimensional proton density distribution. Ideally, the mathematical problem can be cast in the form of a Fredholm integral of the first kind wherein a two or more kernels can be reduced to a single composite kernel for ease of solution. Preferably, a series of conventional CPMG pulse sequences, using a conventional NMR tool, can be used to excite the system of nuclear spins.

Abstract: This invention is a method to measure fluid flow properties of a porous medium, including, but not limited to, the fluid flow permeability. In a preferred embodiment, the measurements are made down hole in drill wells exploring for hydrocarbons or acquifers. The measurement involves two types of instruments. One instrument creates a pressure wave in the porous medium, which generates motion of the fluid in the pore space. The second instrument measures the fluid motion in the pore space using Nuclear Magnetic Resonance (NMR) methods. Any type of instrument that can generate a pressure gradient is suitable, including instruments that are remote from the NMR instrument. Magnetic field gradients can be used to localize the NMR signal to a specific region within the porous medium. The magnetic field gradient also provides the method by which the fluid motion is encoded on to the NMR signal.

Abstract: A pad which has a magnetic permeability greater than that of free space at the frequency of nuclear magnetic resonance but which is non-magnetic in steady magnetic fields is disposed between an RF coil and the surface of a patient. The pad is surmounted by a block having similar properties. Two further pads extend from the ends of the block to the surface of the patient. The pads and block guide magnetic flux, which is indicative of magnetic resonant active nuclei in the patient, towards an RF coil which is spaced apart from the surface of the patient/object under examination.

Abstract: An method and system are disclosed for NMR echo-train data acquisition and processing for enhanced vertical resolution for a given signal to noise ratio. In one aspect, the method is based on providing an estimate of non-formation signal components and removing the estimate from the NMR signals. Computation of the estimate is done from the data itself or using a direct measurement of non-formation signals. In another aspect, the functions of reducing coherent noise components is separated from the function of reducing the random noise components, to enhance the resolution of the NMR pulse echo data for a given signal to noise ratio (SNR) of the data. Combination processing is disclosed, which enables efficient filtering of the input NMR data for both relatively high and relatively low SNR of the formation data.

Abstract: A method and apparatus for performing NMR measurements suppressing contribution to NMR signals from within the borehole. Within the region of examination, the RF magnetic field has a spatially varying intensity. NMR signals (free induction decay or spin echo signals) are inverted to give spin density as a function of field intensity. This inversion is then mapped to spatial positions using the known RF field variation. The effect of signals arising from within the borehole can be suppressed. It is also possible to obtain an azimuthal image of the spin density.

Abstract: The invention is a method of reducing the effects of non-formation signals in an NMR logging echo signal obtained within a borehole in an earth formation. The method obtains a non-formation signal by the application of at least an excitation pulse, and preferably also at least one refocusing pulse. The obtained signal is used to numerically construct a synthetic ringing signal sequence. The constructed signal can then be subtracted from an NMR echo signal to reduce the effects of ringing.