Obtaining multilayer reciprocal times through phantoming

Geophysics ◽  
1986 ◽  
Vol 51 (1) ◽  
pp. 45-49 ◽  
Author(s):  
Robert W. Lankston ◽  
Marian M. Lankston
Keyword(s):  
Critical Parameter ◽  
Seismic Refraction ◽  
Special Equipment ◽  
Seismic Line ◽  
Arrival Times ◽  
Seismic Refraction Data ◽  
Reciprocal Method ◽  
Refraction Data

A critical parameter in interpreting seismic refraction data with the generalized reciprocal method (GRM) is the reciprocal time, which must be available for each layer from which refracted rays return to the surface. The reciprocal time can be measured in the field, but this requires special equipment or procedures. Shooting to obtain the reciprocal time from each layer along a long seismic line may be operationally impractical. However, the method of phantoming arrivals overcame the problems. In phantoming, a reciprocal time is actually measured along any length of the seismic refraction line for any refractor and that value can be used as the reciprocal time in GRM processing if the first‐break arrival times are phantomed properly. Realizing that the reciprocal time may be extracted from overlapping normal forward and reverse shots and phantoming the data accordingly will save much field time and expense. An example shows the results of using a reciprocal time measured across one spread for simultaneously processing and interpreting collinear, overlapping spreads.

An Introduction to the generalized reciprocal method of seismic refraction interpretation

Geophysics ◽  
1981 ◽  
Vol 46 (11) ◽  
pp. 1508-1518 ◽  
Author(s):  
Derecke Palmer
Keyword(s):  
Average Velocity ◽  
Seismic Refraction ◽  
Velocity Analysis ◽  
Velocity Gradients ◽  
Variable Distance ◽  
Velocity Changes ◽  
Seismic Refraction Data ◽  
Reciprocal Method ◽  
Refraction Data ◽  
Overlying Strata

The generalized reciprocal method (GRM) is a technique for delineating undulating refractors at any depth from in‐line seismic refraction data consisting of forward and reverse traveltimes. The traveltimes at two geophones, separated by a variable distance XY, are used in refractor velocity analysis and time‐depth calculations. At the optimum XY spacing, the upward traveling segments of the rays to each geophone emerge from near the same point on the refractor. This results in the refractor velocity analysis being the simplest and the time‐depths showing the most detail. In contrast, the conventional reciprocal method which has XY equal to zero is especially prone to produce numerous fictitious refractor velocity changes, as well as producing gross smoothing of irregular refractor topography. The depth conversion factor is relatively insensitive to dip angles up to about 20 degrees, because both forward and reverse data are used. As a result, depth calculations to an undulating refractor are particularly convenient even when the overlying strata have velocity gradients. The GRM provides a means of recognizing and accommodating undetected layers, provided an optimum XY value can be recovered from the traveltime data, the refractor velocity analysis, and/or the time‐depths. The presence of undetected layers can be inferred when the observed optimum XY value differs from the XY value calculated from the computed depth section. The undetected layers can be accommodated by using an average velocity based on the optimum XY value. This average velocity permits accurate depth calculations with commonly encountered velocity contrasts.

scholarly journals A semi-automatic approach to identify first arrival time: the Cross-Correlation Technique

2015 ◽  
Vol 18 (2) ◽  
pp. 107-113 ◽  
Author(s):  
Mustafa Senkaya ◽  
Hakan Karslı
Keyword(s):  
Cross Correlation ◽  
Seismic Refraction ◽  
Correlation Technique ◽  
Arrival Times ◽  
Refraction Tomography ◽  
First Arrival ◽  
The Cross ◽  
Seismic Refraction Data ◽  
Cross Correlation Technique ◽  
Refraction Data

<p class="MsoNormal" style="line-height: 200%;">The high-quality interpretation of seismic refraction data depends on the accurate and reliable identification of the first arrival times. First arrivals can be identified on a graphic or image by conventional picking, but this process depends on external factors, such as the scale and quality of the imaging data, amplitude ratio, sensitivity of the picking cursor and user experience. Under these considerations, identifying first arrivals in noisy data becomes more complex and unstable. In this study, the Cross-Correlation Technique (CCT), which is widely used in the process of analyzing reflection data, has been used to pick the first arrival times in noisy or noiseless seismic refraction data by a semi-automatic process. The CCT has reduced the dependence on user and decreased incorrect picking caused by environmental noise, displaying characteristics and scaling factors. The CCT has been tested with synthetic models with different noise contents and various field data. The Chi-square error criterion was used to assess the performance of the pickings. In addition, effects of small-time differences between the conventional picking process and the CCT have been demonstrated on a refraction tomography velocity section. Therefore, we believe that our proposed method is a useful contribution to the existing methods of first arrival picking.</p><p class="MsoNormal" style="line-height: 200%;"> </p><p class="MsoNormal" style="line-height: 200%;"><strong>Resumen</strong></p><p class="MsoNormal" style="line-height: 200%;">La buena interpretación de datos estadísticos de refracción sísmica depende de la identificación acertada y confiable de los tiempos de llegada. Los primeros tiempos de llegada se pueden identificar en un gráfico o imagen por picado convencional, pero este proceso depende de factores externos como la escala y la calidad de información de la imagen, el índice de amplitud, la sensibilidad del cursor de recolección y la experiencia del usuario. Bajo estas consideraciones, la identificación de los tiempos de llegada bajo información ruidosa se vuelve más compleja e inestable. En este estudio, la técnica de Correlación Cruzada (CCT, en inglés), que es ampliamente trabajada en el proceso de análisis de datos de reflexión, se utilizó para seleccionar los primeros tiempos de llegada en información sísmica ruidosa o no ruidosa con un proceso semiautomático. La CCT redujo la dependencia en el usuario y bajó el nivel de selección incorrecta causada por el ruido ambiental al desplegar características y factores de escala. La CCT se ha probado en modelos sintéticos con diferentes contenidos de ruidos y diversa información de campo. El error de la norma Chi-cuadrado se utilizó para evaluar el desempeño de las selecciones. En adición, los efectos de las pequeñas diferencias de tiempo entre el proceso convencional de selección y la CCT se han demostrado en una tomografía reflexiva de velocidad. Además, se estima que el método propuesto es una contribución útil a los métodos existentes de la recolección de los primeros tiempos de llegada.</p>

Controls of traveltime data and problems of the generalized reciprocal method

Geophysics ◽  
2003 ◽  
Vol 68 (5) ◽  
pp. 1626-1632 ◽  
Author(s):  
Tak Ming Leung
Keyword(s):  
Seismic Refraction ◽  
Seismic Model ◽  
Inversion Process ◽  
Offset Distance ◽  
Ray Trace ◽  
Seismic Refraction Data ◽  
High Degree ◽  
Process Controls ◽  
Reciprocal Method ◽  
Refraction Data

Traveltime data required for 2D seismic refraction surveys are 2D first arrivals. To obtain a high degree of consistency between traveltime data and the seismic model, it is important to verify that traveltime data are appropriate for interpretation or an inversion process. Controls or checkpoints presented here inspect compatibility among traveltime data. Similar to the ray‐trace check on the consistency of interpretation, these controls provide an objective means of quality assessment of seismic refraction data. The theoretical aspects of the generalized reciprocal method (GRM) are studied because concerns have been raised regarding the accuracy of some interpretations using this method. The problem of the GRM is that the optimum XY value, which is the most important parameter in the method, is assumed to be twice the offset distance. Consequently, based on this unproven assumption, the efficacy of the optimum XY value is somewhat exaggerated.

scholarly journals Analisis Data Seismik Refraksi dengan Metode Generalized-Reciprocal

2012 ◽  
Vol 3 (1) ◽  
pp. 162
Author(s):  
Ashadi Salim
Keyword(s):  
Wave Velocity ◽  
Seismic Wave ◽  
Approximate Calculation ◽  
Seismic Refraction ◽  
Velocity Analysis ◽  
Seismic Wave Velocity ◽  
Seismic Refraction Data ◽  
Reciprocal Method ◽  
Refraction Data

The analysis of seismic refraction data by the generalized reciprocal method can be used for delineating undulating refractors. The forward and reverse times of arrival at different geophones with XY distance along a refraction profile, are used for calculating time depth. The seismic wave velocity in refractor may be obtained from velocity analysis function, and the depth of refractor under each geophone is obtained from time-depths function. This method has been applied at one line of seismic refraction measurement that was 440 m long with 45 geophone positions. The measurement obtained 20 m as the optimum XY-value and 2250 m/s as the velocity of seismic wave in refractor, and the undulating refractor topography with the depths varies 10.4 – 22.1 m. The optimum XY-value was obtained from approximate calculation derived from the observation, that was indicated the absent of undetected layer.

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