Active and Passive Near-Field Hydrophones Used in Ultrashallow Waters

Abstract

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 207728, “Using Active and Passive Near-Field Hydrophones To Image the Near-Surface in Ultrashallow Waters Offshore Abu Dhabi,” by Oleg Khakimov, Yaser Gholami, and Bertrand Tertrais, CGG, et al. The paper has not been peer reviewed. _ Seismic surveys can leave the near-surface undersampled. This is particularly a challenge offshore Abu Dhabi, where a complex near-surface contaminates the seismic image. To mitigate these effects, the authors use near-field hydrophone (NFH) data. NFHs are positioned approximately a meter above each airgun and are designed to record near-field pressure. The authors propose combining active and passive NFH measurements, using active NFHs for the very shallow section and passive NFHs for the deeper section. The authors have applied this technique to a recent node survey acquired offshore Abu Dhabi. Introduction Ocean-bottom-node (OBN) acquisition in shallow-water environments has many benefits. One technique involves imaging using NFH data. The main purpose of an NFH is to monitor proper functioning of the aiguns during acquisition. They also are used often to compute the far-field signature using the concept of notional sources. But the NFHs also record reflections from the near surface and can be used to construct an image. Two types of NFH are recorded: passive, when the recording hydrophones are on top of the array that is currently not firing; and active, when the hydrophones are on top of the firing arrays. Usually, passive NFHs are the preferred choice for imaging because of less-energetic direct arrival contamination; however, in extremely shallow environments, the distance between sources and receivers for the passive NFHs is too large to capture reflections from the water bottom and reflectors immediately below. In such situations, active NFHs have the advantage of recording zero-offset data. The sampling rate of the NFHs in this study was 1 ms, which allowed imaging of frequencies above 250 Hz. Careful processing is required for active NFH data because it is strongly contaminated by the near-field signature. To extend the maximum depth of the image, active NFHs were combined with passive. The complete paper presents work performed on NFH data acquired during OBN acquisition in Abu Dhabi shallow waters. The data examples come from a priority area where many tests were performed to select an optimal work flow. The total size of the study area was approximately 790 km2. Acquisition Parameters Data were acquired using five vessels. OBN acquisition involved blended acquisition, with four vessels operating at the same time with a minimum distance separation of 6 km. Vessels moved at an average speed of 9 km/h, which provided approximately 5-second time separation between shots. Two of the vessels had dual-source arrays, and the other two had only single arrays. In the shallower areas, only smaller vessels with single arrays were shooting. This meant that passive NFH data acquisition was not applicable everywhere.