48 refractor and short shot data may therefore be

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48 refractor, and short-shot data may therefore be needed before long-shot offsets can be decided. Distances to long shots need be measured accurately only if continuous coverage is being obtained and the long-shot to one spread is to be in the same place as a short or centre shot to another. If explosives are being used, it may be worthwhile using a very long offset if this will allow firing in water (see Section 1.2.1). 3.1.5 Centre shots The information provided by a conventional four-shot pattern may be supplemented by a centre shot. Centre shots are especially useful if there are considerable differences in interpretation at opposite ends of the spread, and especially if these seem to imply different numbers of refractors. They may make it possible to obtain a more reliable estimate of the velocity along an intermediate refractor or to monitor the thinning of an intermediate layer that is hidden, at one end of the spread, by refractions from greater depths. An additional reliable depth estimate is obtained that does not depend on assumptions about the ways in which the thicknesses of the various layers vary along the spread, and there will be extra data on the direct wave velocity. Centre shots are used less than they deserve. The extra effort is generally trivial compared to the work done in laying out the spread, and the additional and possibly vital information is cheaply obtained. 3.1.6 Annotation of field records Hard-copy records can be (and should be) produced in the field from most of the seismographs now used for shallow refraction surveys. The several dozen records produced in a day’s work that includes repeats, checks and tests as well as the completion of a number of different spreads must be carefully annotated if confusion is to be avoided. Annotations should obviously include the date and the name of the observer-in-charge, along with the survey location and spread number. Orientation should be noted, and the position of Geophone 1 should be defined. Unless the geophone spacing is absolutely uniform, a sketch showing shot and geophone locations should be added. If the interval between timing lines on the records can be varied and/or variable time offsets can be applied, these settings must also be noted. In many cases this is now done automatically. Other items are optional. Amplifier gains and filter settings are not often recorded but such information may be useful. The number of shots or impacts combined in a single record can also be important with enhancement instruments. And, of course, features such as the use of S-wave geophones at some points or peculiarities in the locations of some of the geophones should always be noted. Many of the items listed above can be printed directly on to the hard-copy
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49 record, provided they are first entered into the machine. This is often a more tedious, and more error-prone, process than simply writing the information on each record by hand.
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