Pulse sequence and method for suppression of magneto-acoustic artifacts in NMR data
AssigneeNumar Corporation
InventorManfred G Prammer
Life of this patent
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Abstract
Novel pulse sequence and data acquisition method are disclosed which eliminate the effects of spurious NMR signals caused by mechanical resonances within the measurement apparatus. The proposed method alleviates interference problems typically arising from strong “excitation” pulses in a sequence, and enables the use of the corresponding data points to increase the resolution of the measurement. The method is based on changing the measurement frequency between pulse sequences and averaging out data points obtained from the different sequences in a way that effectuates cancellation of the spurious signals. The novel cycle of ulse sequences and a data acquisition method can be used, or example, with any existing NMR logging instruments.
Claims20
An NMR method for measuring attributes of a material, comprising the steps of:
- a) applying at least one first pulse-echo sequence having an associated measurement frequency F1;
- b) applying at least one second pulse-echo sequence having an associated measurement frequency F2 different from F1, the difference being a function of the time delay between excitation pulse and data acquisition for said first and second sequences;
- c) measuring NMR signals corresponding to said at least one first pulse-echo sequence and said at least one second pulse-echo sequence, said NMR signals representing spin-echo relaxation in the material, at least some of said measured NMR signals being corrupted by spurious signals;
- d) combining measured NMR signals from said at least one first pulse-echo sequence and from said at least one second pulse-echo sequence to reduce the effect of said spurious signals;
- and
- e) determining properties of the material on the basis of the combination of measured signals.
A method for NMR borehole logging comprising the steps of:
- a) providing at least one first pulse-echo sequence associated with a first measurement frequency F1;
- b) providing at least one second pulse-echo sequence associated with a second measurement frequency F2 different from F1, the difference being a function of the time delay between excitation pulse and data acquisition for said first and second sequences;
- c) measuring NMR signals corresponding to said at least one first pulse-echo sequence and said at least one second pulse-echo sequence, said NMR signals representing spin-echo relaxation of a geologic formation in the borehole, at least some of said measured NMR signals being corrupted by spurious signals;
- d) combining measured NMR signals from said at least one first pulse-echo sequence and from said at least one second pulse-echo sequence to reduce the effect of said spurious signals;
- and
- e) determining properties of the geologic formation in the borehole on the basis of the combination of measured signals.
A method for NMR borehole logging comprising the steps of:
- a) providing at least one first pulse-echo sequence associated with a first measurement frequency F1;
- b) providing at least one second pulse-echo sequence associated with a second measurement frequency F2 different from F1, wherein said at least one first pulse-echo sequence and said at least one second pulse-echo sequence have an associated pulse echo spacing τ such that:
- [LeftBracketingBar] F 1 - F 2 [RightBracketingBar] ≅ ( n + 1 2 ) 1 2 τ ;
- in which n is any integer or zero;
- c) receiving NMR signals in response to said at least one first and said at least one second pulse-echo sequences;
- and
- d) determining properties of a geologic formation in the borehole on the basis of the received NMR signals.
A method for NMR borehole logging, comprising:
- (a) producing a static magnetic field into the volume of the formation substantially surrounding a borehole;
- (b) producing a cycle of CPMG pulse sequences according to the expression:
- (1)F1:
- Ex (-Ry-Ai1)i i≧1
- (2)F2:
- Ex (-Ry-Ai2)i i≧1
- (3)F1:
- Ex (-R−y-Ai3)i i≧1
- (4)F2:
- Ex (-R−y-Ai4)i i≧1
- where the first (1) and third (3) pulse sequences use measurement frequency F1;
- the second (2) and fourth (4) pulse sequences use measurement frequency F2;
- Ex is an excitation pulse with R.F. phase of 0°;
- R±y designates a refocusing pulse with R.F. phase equal to ±90°;
- Aij designates an acquisition window for the ith echo in the jth pulse sequence (j=1, 2, 3, 4);
- and
- —is a constant delay time τ;
- (c) combining data acquired from the four pulse sequences in (b) as to cancel excitation and refocusing interference signals;
- and
- (d) determining properties of the formation substantially surrounding the borehole based on the combined data.
Dependencies are inferred from the claim text.