Geophysical studies of mass transport deposits on the slope canyon floor with high-resolution autonomous underwater vehicle (AUV) in the Shenhu area and its implications for sediment transportation
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摘要: 海底峡谷是陆源物质向深海运移的重要通道。对于远离陆地的海底峡谷，通常认为浊流是物质搬运的主要营力。受限于探测精度和复杂作业环境影响，使用常规地球物理资料对深水海底峡谷尤其是对谷底沉积体的形态和结构特征的刻画不够精细。基于水下自主航行器（AUV, Autonomous Underwater Vehicle）采集的高分辨率多波束、旁扫声呐和浅地层剖面资料，对神狐峡谷群中的一条峡谷的谷底表面及部分浅部地层的沉积特征进行了分析。结果表明，峡谷谷底浅部地层并不像它平滑的表面那么简单，而是由大量内部杂乱弱反射、厚度在8.4 m及以下的块体搬运沉积体组成。峡谷中下游块体搬运沉积体大都沿峡谷走向整体呈条带状展布，不是直接来源于相邻的峡谷脊部。研究认为在特定沉积环境下（例如高海平面时期），陆坡限定性峡谷谷底的块体搬运沉积过程的重复进行是峡谷谷底物质输运的重要途径，与浊流共同雕刻了峡谷的地形地貌。基于AUV的地球物理探测技术将是研究海底浅表层沉积过程和保障海底工程施工的重要手段。
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关键词:
- 海底峡谷 /
- 物质输运 /
- 水下自主航行器（AUV） /
- 块体搬运沉积 /
- 神狐峡谷群
Abstract: Submarine canyons are important pathways for terrestrial sediment moving to the deep water far away from the land, and turbidity current is considered as the primary agent. Detailed sedimentary features of the canyon floor are not clear so far due to the low resolution of conventional geophysical data and complicated operating setting in deep water. Here, in this program, high-resolution autonomous underwater vehicle (AUV) based multi-beam bathymetry data, side-scan sonar data and chirp sub-bottom profiles are jointly used to map the floor of a canyon and image its shallow strata in the Shenhu area. The canyon floor looks quite flat. However, the underlying shallow strata are composed of widespread mass transport deposits (MTDs) which are a little less than 8.4 meters with weak and chaotic acoustic reflections. They are distributed along the canyon floor in an elongated shape and show little direct links with adjacent steep canyon ridges at the middle and lower segments of the canyon. Thus, we suggest the repeated mass wasting downslope on the canyon floor is an effective mechanism for sediment transportation in addition to turbidity currents. The AUV based geophysical exploration is a efficient tool for further studies of marine sedimentation and seabed installation under deep water. -
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图 1 研究区位置
a.神狐峡谷群的位置，b.神狐峡谷群海底地形图，c.基于三维地震资料的研究区海底地形图，d.基于AUV采集的多波束的研究区海底地形图。
Figure 1. Locations of the study area
a. The location of the studied slope confined canyons, b. the bathymetric map of canyons, c. the bathymetric map of the studied canyon segment based on 3D seismic reflection data, d. the bathymetric map of the studied canyon segment based on multi-beam data acquired by AUV.
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图 2 峡谷的旁扫声呐图和岩心图
a.研究区所在峡谷的旁扫声呐图，b—e.局部放大的旁扫声呐图，f—g. MTD1上GC-3站位的部分重力柱状岩心样品。虚线多边形指示MTDs的位置。
Figure 2. The side-scan sonar map of the canyon floor and sections of the gravity core acquired from the canyon floor
a. The side-scan sonar map of the studied canyon, b-e. amplified side-scan sonar maps of the study area, f-g. sediment samples from the gravity core GC-3 over MTD1 at 1.25 m and 3.25 m, respectively. The dashed polygons denote locations of MTDs on the canyon floor.
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图 3 研究区3个典型MTDs的地形立体图及穿过MTDs的地形变化曲线
Figure 3. Stereo views of three MTDs and bathymetric curves crossing these MTDs
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图 4 过同一峡谷位置的三维反射地震剖面和AUV浅地层剖面
a.反射地震剖面，b.AUV浅地层剖面。
Figure 4. The seismic profile exttracted from the 3D seismic data and sub-bottom profiles acquired by AUV crossing the same section of the canyon
a. The seismic profile, b. The AUV based sub-bottom profile.
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图 5 过MTD1的AUV浅地层剖面图
a.上部横剖面，b.中部横剖面，c.下部横剖面。
Figure 5. AUV based sub-bottom profiles across MTD1
a. The upper profile perpendicular to the strike, b. the middle profile perpendicular to the strike, c. the lower profile perpendicular to the strike.
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图 6 过MTD2的AUV浅地层剖面图
a.横剖面，b.纵剖面。
Figure 6. AUV based sub-bottom profiles across MTD2
a. The profile perpendicular to the strike, b. the profile parallel to the strike.
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图 7 过MTD3的AUV浅地层剖面图
a.上部横剖面，b.下部横剖面，c.纵剖面。
Figure 7. AUV based sub-bottom profiles across MTD3
a. The upper profile perpendicular to the strike, b. the lower profile perpendicular to the strike, c. the profile parallel to the strike.
表 1 峡谷谷底MTDs的几何参数
Table 1 Geometric parameters of MTDs on the canyon floor
编号 宽度/m 长度/km 长宽比 厚度/m 面积/km2 体积/km3 MTD1 80～500 5.00 10.00～62.50 1.30～8.40 0.90 4.37 MTD2 260～350 0.75 2.14～2.88 0.90～3.20 0.28 0.58 MTD3 70～600 2.00 3.33～28.57 1.20～3.00 1.10 2.31 
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