Field Data: are they reciprocal ?

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Field Data: are they reciprocal ?

The Pembrook data (traces shown in the next Figures) has split spread geometry and thus is ideal for comparing reciprocal trace pairs. During the acquisition no particular effort has been made to ensure that the employment of sources and geophone in the field was perfectly reciprocal. In that respect this dataset presents a typical acquisition for multi-component data. Surface impact sources were used for acquiring this data set. Such sources are relatively weak compared to explosive sources. Consequently each source is not only activated once, but multiple times and the resulting traces stacked to create the final field trace. Furthermore each source is activated at an angle with respect to the vertical, so that horizontal and vertical components can be created by weighted subtraction or addition.

Figures 8 through 13 show reciprocal trace pairs at two different offsets (near and far). None of the trace pairs is perfectly reciprocal, but in general the trend is the same. Most noticable are time shifts and amplitude differences for each trace pair. The differences persist over the whole length of the trace with good fits at various times. Such variations in the reciprocal match can be explained by noise sources that are not part of the reciprocal experiment, such as drill, pumping or surface noise.

Figure 8: [ IMAGE ]

Figure 9: [ IMAGE ]

Figure 10: [ IMAGE ]

Figure 11: [ IMAGE ]

Figure 12: [ IMAGE ]

Figure 13: [ IMAGE ]

These questions then remain:

• Are the differences in the reciprocal trace pairs significant ?
• To what mechanism do we attribute them ?
• Can we estimate source and receiver properties ?

The next section tries to give a reason as to why we should atrribute them to differences in the source behaviour.