The Mini Spread

The modern trend of longer cables and less cable noise has brought about the undesireable result of long offset distances between the acoustic energy source and the nearest hydrophone group. This paper presents our ideas on the MINI-SPREAD for recording the desired short offset data to define the sea floor and very shallow reflections while simultaneously recording the main streamer data which define deep geological zones of interest. We shall also illustrate the contribution of the short offset traces to the final stack data and its effects on geologic interpretation. The viscous drag on a long streamer being towed at normal recording speeds is such that it is difficult to obtain a short offset distance between source and the near hydrophone group. In typical marine operations (due to the vertical component of the drag force) an offset distance of 1,025 feet or greater is required to allow the near hydrophone group to reach the desired streamer depth of at least 40 feet. This offset is frequently extended further in order to isolate the ship's noise and effects of wave action on the lead-in. However, if the sea floor reflection is to be recorded at a near-normal incident angle, it is desirable that the offset from source to the near hydrophone group is not greater than the water depth. One solution to the problem posed by these two differing requirements is a two-cable operation. For example, a MINI-SPREAD may be used with a relatively short offset, and a main streamer with a relatively long offset, for simultaneous recording of the whole range of source detector distances. We wish to stress that nothing in this paper is to be taken as derogatory to long spreads. Where the geology allows them, we are all infavor of long spreads; what we are against is long offsets. The first slide illustrates the side view of a normal cable configuration on our "M/V American Delta. The normal configuration used incorporates both the MINI-SPREAD and the main streamer for a total of 48 channels. Note that Traces 48 through 43 are the MINI-SPREAD consisting of groups of hydrophones spaced equidistant over Sam. The cable section "s" preceeding Trace 48 is a Sam stretchor decoupling section. The length of lead-in to the boat may be varied according to offset distance requirements, and preferably should be less than water depth. The MINI-SPREAD is wound onto a relatively small reel separate from the main streamer, as illustrated in this slide. In our normal configuration, the main streamer consists of Traces 42 through 1. Consistent with the industry, our trace-numbering convention always considers Trace 1 as the far trace. Preceeding Group 42 (the nearest main streamer trace to the boat) are three stretch sections of Sam each and the lead-in attached to the main streamer reel. Also shown on the diagram are the cable depth transducer locations normally used. These are located at each end of the MINI-SPREAD and at several locations on the main streamer including the front and back.