Weak reflection enhancement and picking from ultrasonic pitch-catch measurements in a cased-hole

ABSTRACT Ultrasonic logging is a widely used technology to assess the cement bond condition in a cased-hole. Advanced ultrasonic pitch-catch measurements can capture reflections at the cement-formation interface (in short, the third interface echo [TIE]), which enables the evaluation of the bond condition, casing eccentering, and the velocity of the material in the cement annulus. However, the usage is limited by the TIE arrival picking due to the low amplitude that results from the eccentered tool and casing, overlapping with the tool-casing multiple reflections, as well as interference from the primary zero-order antisymmetric Lamb wave (A0). We develop a two-step method to enhance and accurately pick the TIE arrivals. We first use mathematical morphological filtering with a neural network framework to mitigate the influence of A0. Then, we use the local window multichannel singular-spectrum analysis to suppress the effect of tool-casing multiples on the TIE. In the second step, we develop a sinusoidal search method with a path postcorrection algorithm to track the TIE arrivals from the enhanced TIE waveforms automatically. Synthetic and field data results show that our TIE processing can effectively eliminate the A0 mode and suppress tool-casing multiples. In addition, the picked TIE arrivals are consistent with actual TIE curves.