Modeling and Monitoring Submerged Prehistoric Sites during Offshore Sand Dredging and Implications for the Study of Early Holocene Coastal Occupation of Southern California

Beach sand dredging projects off the coast of San Diego County in Southern California provide data for improved understanding of the strategraphic setting for early Holocene sediments and the potential for offshore presence of buried archaeological materials.  Geophysical data, core sediments, and analysis of recovered fossils allowed models to be developed for six offshore borrow sites within drown river valleys.  These site-specific models were tested during dredging operations, and the dredge spoil was monitored for archaeological materials.  Two of the borrow sites yielded stone bowls consistent with those found in previous offshore archaeological investigations in this region.  These artifacts, however, were determined to come from nearshore and lagoonal sediments, not appropriate for occupation, raising questions about both the function of stone bowls and the process that resulted in their deposition.  This project illustrates the potential for commercial development projects to yield information on offshore archaeological resources, as well as the challenges.

Methods
San Diego Association of Governments, Beach Replenishment Project

1999 Field Methods:

During January 1999, Sea Surveyor, Inc. conducted marine geophysical surveys and vibracore investigations along the San Diego coastline. The geophysical surveys of borrow sites were conducted using the 48' survey boat "WESTWIND". A differential GPS antenna was installed on the mast of the vessel, directly over the transducer well. The subbottom profiling included two acoustic sources: a Datasonics chirp II sonar, operating a frequency of 3-16 kHz, and a Applied Acoustic Engineering Geopulse system operating at 800 -2000 Hz.  The Geopulse acoustic source and hydrophones were towed alongside the vessel and 17' behind the GPS navigation antenna.   The seismic reflection data from both systems were displayed on an EPC Model 1086 thermal graphic recorder. No additional data processing was conducted on the graphic recorder output.

The 165' vessel "AMERICAN PATRIOT" and 20' ALPINE vibratory corer were used to collect sediment core samples at 125 locations in 10 proposed sand borrow sites during 18-24 January 1999. The ALPINE Vibracorer uses 4”-diameter steel barrels for collecting the core samples, and 3.5” diameter cores in clear cellulose acetate butyrate (CAB) liners, which can be split laterally or longitudinally for inspection and geotechnical logging and subsampling. After the vibracore collected a sediment sample, the sediment cores were extracted from the vibracore barrel, and the liner was cut at 5' intervals, capped, taped, and labeled. The collected cores were then shuttled to Oceanside Harbor, where geotechnical personnel split, inspected, described, logged, and subsampled the core samples for laboratory analyses. Subsamples from the sediment cores were transported to MEC Analytical Systems' laboratory in Carlsbad, California for grain-size and chemical analyses.

After the sediment cores were split longitudinally, logged, and photographed, the geotechnical personnel collected one or more representative subsamples from the top 1-3 layers of the core. The sediment subsamples were sealed within transparent ziplock bags, labeled, and transported to MEC Analytical Systems' Carlsbad laboratory for grain-size analyses. A total of 235 sediment samples were analyzed for grain-size. In the laboratory, approximately 40 grams of sediment from each subsample was weighed into a coors dish and placed into an oven to dry overnight. Once dry, the sample weight was determined by first weighing the sample in the coors dish, followed by weighing the dish after the sediment sample had been placed in the sieves. The sieves were stacked on top of one another with the sieve having the largest screen mesh diameter above sieves having progressively smaller screen mesh diameters. The sample was shaken for 10-minutes to sieve the sediment, which left the coarsest material on the upper screens and allowed the finer particles to fall through to the bottom. Once shaken, the contents of each sieve were weighed and the results were entered onto a data sheet. The data sheet was then entered into a computer spreadsheet, and the percentage (by weight) of gravels, sands, and silts were calculated.

More information on the 1999 field operations can be found at: https://www.sandag.org/index.asp?publicationid=590&fuseaction=publications.detail

2008 Field Methods:

The seismic reflection survey was initiated on October 6 and 7, 2008 using Fugro Inc. geophysical systems on the vessel "Julie Ann", a 24-foot aluminum hull vessel. The Starfix Seis navigation system was interfaced to a Trimble 12-channel GPS receiver with an integrated Starfix Differential receiver. The Trimble unit receives ranging information from the same satellites as the Starfix differential reference stations. These corrections are applied to the DGPS receiver's satellite data to produce an accurate (±5 feet) position of the vessel in real-time and a post processed accuracy of less than 2.5 feet. The differentially-corrected position from the Trimble receiver is then passed to the navigation computer.

The seismic reflection system used a mechanical "boomer" energy source and a multichannel, Geo-Eel hydrophone array. An Applied Acoustics Engineering AA300J portable seismic energy source was used to power the AA300 boomer plate towed from a catamaran configuration. The boomer plate is an electro- mechanical transducer made of an insulated metal plate and a rubber diaphragm adjacent to a flat wound electrical coil. A short duration high-energy pulse is discharged from the AA300J energy source into the coil and the resulting magnetic field repels the metal plate in the transducer. The plate motion is transferred to the water by the rubber diaphragm, generating a broad-band acoustic pulse that does not have strong cavitations or ringing. The Applied Acoustics Model AA300 sub-bottom tow fish was deployed and towed from the starboard side stern of the vessel. Sufficient tow cable was deployed such that the tow fish was clear of and beneath the vessel's wake. The system was triggered at an 8 to 10 Hz pulse rate and swept frequency range between 2 to 10 kHz. The recorded record length was dependent on water depth. Navigation fix marks were sent to the systems' printer every 100 meters down the survey line. All navigation information and sub-bottom data were time tagged and logged to a hard drive.

The reflected acoustic pulse, generated by the boomer source, was received by a multichannel Geo-Eel hydrophone array. The Geo-Eel includes hydrophones enclosed in a silica gel at specified interval. All track lines used a 24-channel (location dependent) hydrophone array. The hydrophone array included 5.1 foot (1.56 meter) group spacing of the channels. The raw data recorded by the hydrophone channels was logged to a Geometrics CNT recording system in a SEG-Y format for later post-processing to an accuracy of within 2.5% of penetration depth.

Seismic data processing was conducted by Mark Legg (Legg Geophysical), and the following description of the processing steps applied to the 2008 data was provided by him.

"Data processing of the multichannel seismic reflection profiles acquired offshore San Diego County, followed the basic premise of the wavelet processing method used in the petroleum exploration industry.  A single-plate boomer source type was used combined with a 24-channel hydrophone streamer configuration, with the objective high resolution at shallow sub-bottom depths (<100 m).  The boomer produced a source energy level of about 500 Joules over a broad bandwidth ranging from about 100 Hz to more than 1,000 Hz.  The 24-channel mini-streamer had a group interval of 1.56 m and a near trace to source offset of 3.125 m.  The short offset and group interval of the mini-streamer provides good imaging of the seafloor and shallow sub-bottom, while the far offset is sufficient to provide some water bottom multiple suppression in shallow water areas.  The short hydrophone group intervals of the streamer are very important for avoiding spatial aliasing of these digitally sampled data, thereby improving the overall signal-to-noise ratio of the seismic data.

Basic seismic data processing consists of filtering in both time and space, deconvolution to provide a sharper and more consistent seismic wavelet for interpretation, correction of normal moveout due to varying subsurface velocity structure and source-to-hydrophone offset, stacking of data traces to increase the signal-to-noise ratio, and migration to put the reflecting horizons back into their proper lateral positions.  A more detailed discussion of these steps, and the specific parameters for the processing flows for mini-streamer data follows.

Initially, the raw segy data must be loaded into the seismic data processing workstation.  With the simple and regular geometry of continuous marine seismic reflection profiling, where the offset from source to receiver positions is constant along the profile, these offset values are loaded into the trace headers during the data loading phase.  In order to retrieve the original field record numbers after data are stacked, a simple definition of shot points or station numbering scheme is applied with the first shot point defined as #101, and subsequent shot points numbered sequentially based on the particular recording geometry.  For this project, the shot point interval was nominally 0.78 m, which is equal to the common-mid-point spacing and equal to one-half the hydrophone group interval (1.56 m).  This geometry provides subsurface coverage at 2400 percent, i.e., nominal fold equal to 24. The station numbering scheme is designed to match the CMP numbering scheme, so the working shot point numbers during processing will have an increment of one (1) for this geometry.

The first step used in the processing flow, after loading the segy data into the processing workstation, is to filter and scale the data.  Preliminary review of data traces to edit or “kill” bad traces is performed.  This was particularly important for data acquired during the more severe sea state, with wind waves and short period swell that creates frequent “noise bursts” in the streamer.  An anti-alias filter is used as a band-pass filter to avoid aliasing in the time domain above the Nyquist frequency (2,000 Hz for 0.25-ms sample rate) and to remove low-frequency streamer noise, like bulge waves and water wave motion.  An Ormsby filter (trapezoid band-pass shape) was used with parameters of 10/15-1500/1800 Hz for the 24-channel mini-streamer data.  Scaling was then applied in two parts: first to remove the geometrical spreading attenuation with a time varying exponential function with an exponent of 0.3, and second to equalize the average amplitudes of each trace in the data set, using an RMS scaling factor for a window of data with reasonable signal-to-noise ratio, i.e., below the direct water wave and strong water bottom reflection and above areas where data consist mostly of background noise.

Spiking deconvolution is applied to shrink the original source wavelet down to an “ideal” zero-phase wavelet that is consistent from trace-to-trace and record to record.  With infinite bandwidth, this ideal trace would be a delta function, or spike at the appropriate arrival time.  For real band-limited data, a Ricker or similar symmetrical wavelet with minimal side-lobes is desired.  For these high-resolution data, spiking deconvolution used an operator length of 20-ms, and was designed using a window below the water bottom reflection where good signal-to-noise ratio is observed.  Another Ormsby filter is applied after deconvolution to eliminate high-frequency noise, using the parameters of 10/15-1500/1800 Hz to maintain broad bandwidth and eliminate some low frequency streamer noise.

After the first trace processing and editing steps, frequency-wavenumber (FK) analysis is done on select shot records to design FK filters to attack spatial aliasing.  Aliasing due to inadequate sampling in the spatial domain is often overlooked and may result in data artifacts from aliased high-frequency events that may appear as real reflection events.  For marine data, where the velocity of sound in water is about 1500 m/s, we can predict the frequencies where coherent noise traveling through the water past the streamer may become aliased: for the 1.56-mgi streamer, the spatial Nyquist frequency is 480 Hz.  These frequencies are lower than much of the source energy, and so array forming in the streamer must be accomplished to attenuate noise traveling horizontally in the water column.  Direct source to streamer wave propagation produces this coherent noise energy as does propellor noise in the water, from the shooting vessel as well as from other boats passing through the area.  An FK filter is well-suited to this, as it can preserve more of the vertically-incident high-frequency reflection signal.  The FK filters were designed to attenuate the low-velocity energy, assumed to be noise in the water column, and preserve high-velocity energy from subsurface reflections.  For this project, the FK filtering allowed us to expand the frequency range by attenuating the high-frequency source generated noise that travels horizontally along the streamer.  An Ormsby frequency domain filter follows the spatial (FK) filter to remove the higher frequency aliased noise; parameters used were 25/50 to 640/960 Hz.

From previous experience with the 24-channel mini-streamer, we observed that water-saturated sediment velocities in the shallow sub-bottom is very close to that of the water column, and sometimes slower due to gas content. For the mini-streamer data, the constant velocity brute stacks using the acoustic velocity of water at 5000 ft/sec were almost indistinguishable from the stacks made using the stacking velocity derived from longer offset 48-channel streamer data.  Therefore, all data were stacked using a normal moveout correction based on the constant 5000 ft/sec acoustic velocity.

The deconvolved and filtered traces were sorted into the Common Mid-Point (CMP) order, spatially filtered (FK filter), frequency filtered (Ormsby), normal-moveout corrected, and stacked to produce a CMP stack record section.  A static shift to correct for elevation (depth) of streamer and source, as well as the tidal elevation was also applied to the segy output data for the stack, and prior to migration for both plot and segy output.  [Note: the pdf plots of the stack data do not include the tidal or streamer and source elevation static correction.] The normal-moveout correction included a 75% stretch mute, to remove near surface data from the far offset hydrophone traces that get overly stretched in shallow water.  This stretch mute helps to maintain a better image of the water bottom and very shallow subsurface, which is stretched and smeared by the longer offset data.  Thus, stacking includes only the near offset traces for the water bottom and very shallow subsurface, but all traces for the deeper data.

For noisy data, acquired during the inclement weather, a 90th percentile “Alpha Trim” stack was applied to reduce noise bursts from water waves and cable jerks.  The Alpha Trim stack sums the trace values that fall within the 90th percentile of the median, thereby ignoring data spikes and outliers.  In general, the full stack with 1/N weighting when applied produced similar results, after trace editing had removed the most severe noise spikes in the raw data.  For the mini-streamer, nominal fold used in the stack is 2400%, i.e., 24 traces per CMP gather were summed and output at the CMP trace spacing of 0.78 m.  The high fold helps to minimize the effects of remaining noise spikes in the data.  With the 90% Alpha Trim stack, the actual fold varies for each trace, but is generally around 21-22.  Navigation and tidal static corrections were written to the trace headers of the stacked data, which then were filtered and output in segy format for loading into the interpretation workstation.  Ormsby filter parameters used after stack were: 25/50-640/960 Hz for the mini-streamer data.

Stacked data contain hyperbolic reflections and diffractions that need to be collapsed into proper spatial locations to further sharpen image of subsurface reflection horizons and faults.  We used a simple frequency-wavenumber migration, which works for all dip angles.  Because these high-resolution data involve shallow, mostly water-saturated subsurface sediments, a constant velocity of 4800 ft/sec was used for the migration.  Migration velocities are generally lower than stacking velocities because the latter are affected by horizon dip, and migration depends on the actual velocity of the geologic layers, i.e., interval velocities.  Data were filtered after migration and saved as segy data for loading into the interpretation workstation.  Ormsby filter parameters were: 90/120-640/960 Hz for the mini-streamer data.  The high-cut for these filters is designed to avoid spatial aliasing and remove high-frequency noise from the image.  The low-cut for these filters is designed to provide a higher resolution image for the mini-streamer and to maintain at least two octaves bandwidth.  A “top mute” was applied to the stack data prior to migration, to eliminate stack noise in the water column, which could “wrap around” to the bottom of the migrated traces during the FK migration.  These migrated noise spikes would appear as migration “smiles” that can obscure the primary reflection signal and complicate interpretation of the data. In addition to the segy output files, pdf plots of both stack and migrated data were prepared at a constant vertical exaggeration of about 10:1, for a seismic velocity of 1500 m/sec."

The M/V Supplier, a 63-foot steel work boat built as a military landing craft, was used as the platform for Vibracore operations between October 29 and November 8, 2008. A model 271 B Alpine Pneumatic Vibracore, configured to take cores 20 feet in length, was used, the same as the unit used for the 1999 investigations.

When the vibracore reached penetrations of up to about 18 feet, the unit was then retrieved to the support vessel, where the sediment contained in the plastic liner was extracted from the core barrel. While on board ship, the plastic liner was cut into 5-foot long sections for ease of handling, and to allow preliminary geologic logging of the collected sediment. More detailed geologic logging (requiring cutting the plastic liner along its length, then splitting the sediment core) was done onshore in a geotechnical laboratory.  Geotechnical logging of the recovered material included descriptions of the recovered sediment using the Unified Soil Classification System, and a Munsell soil color chart. Color digital photographs were taken of the split sediment samples. The color descriptions and photographs were of the sediment in a wet condition.

More information on the 2008 field operations can be found at: https://www.sandag.org/uploads/projectid/projectid_358_12897.pdf

美国加利福尼亚州南部圣迭戈县沿岸的海滩采砂项目，为深化我们对全新世早期沉积物的地层（stratigraphic）背景以及近海埋藏考古遗存的潜在分布情况的认知提供了数据支撑。通过地球物理数据、沉积物岩芯以及出土化石的分析，研究人员得以在淹没河谷内的6处近海采砂场构建相关模型。这些针对特定场地的模型在采砂作业期间得到了验证，同时研究人员对采砂废弃物中的考古遗存开展了监测。其中两处采砂场出土了与该区域以往近海考古调查中发现的石碗形制一致的器物。但经鉴定，这些遗物源自近岸与泻湖相沉积物，并非人类居住遗址的遗存，这引发了关于石碗功能及其埋藏过程的诸多疑问。本项目既展现了商业开发项目为近海考古资源研究提供信息的潜力，也凸显了其中面临的挑战。

研究方法
圣迭戈政府联合会（San Diego Association of Governments）海滩补沙项目
1999年野外作业方法：
1999年1月，海洋测量公司（Sea Surveyor, Inc.）沿圣迭戈海岸线开展了海洋地球物理勘探与振动取芯调查。采砂场的地球物理勘探作业由长48英尺的调查船“西风号（WESTWIND）”执行。差分GPS（Differential GPS）天线安装于船体桅杆上，直接位于换能器舱上方。海底剖面探测采用两套声源系统：一款工作频率为3-16 kHz的Datasonics chirp II声纳，以及一款工作频率为800-2000 Hz的应用声学工程（Applied Acoustic Engineering）Geopulse系统。Geopulse声源与水听器拖挂于船体旁侧，位于GPS导航天线后方17英尺处。两套系统采集的地震反射数据均显示于EPC Model 1086热成像记录仪上，未对记录仪输出的原始数据进行额外处理。

1999年1月18日至24日，研究团队使用长165英尺的“美利坚爱国者号（AMERICAN PATRIOT）”作业船与20英尺ALPINE振动取芯器，在10处拟建采砂场的125个点位采集沉积物岩芯样品。ALPINE振动取芯器采用直径4英寸的钢制岩芯筒采集样品，其透明醋酸丁酸纤维素（cellulose acetate butyrate, CAB）衬管可容纳直径3.5英寸的岩芯，该衬管可横向或纵向剖开，以便进行岩芯检查、岩土测井与分样。完成沉积物样品采集后，研究人员将岩芯从取芯筒中取出，将衬管按5英尺间隔截断、加盖、封装并标记。采集的岩芯随后被转运至欧申赛德港，由岩土技术人员对岩芯进行剖开、检查、描述、测井与分样，以供实验室分析。沉积物岩芯的分样被送往位于加利福尼亚州卡尔斯巴德的MEC分析系统（MEC Analytical Systems）实验室，开展粒度与化学分析。

在对沉积物岩芯进行纵向剖开、测井与拍照后，岩土技术人员从岩芯顶部1-3层采集一份或多份具有代表性的分样。沉积物分样被密封于透明自封袋中，标记后送往MEC分析系统卡尔斯巴德实验室开展粒度分析。本次共计分析了235份沉积物样品的粒度参数。实验室中，研究人员将每份分样约40克沉积物称入库尔斯瓷盘（coors dish），置于烘箱中过夜烘干。烘干后，先称量装有沉积物的瓷盘重量，再将样品倒入套叠的筛具中，再次称量空瓷盘的重量，以计算沉积物的净重。筛具按筛孔直径从大到小自上而下堆叠，将样品置于筛具上振荡10分钟以完成筛分，最粗的颗粒将留存于上层筛网，细颗粒则沉降至最下层。振荡结束后，研究人员称量每个筛网内的留存物重量，并将结果记录至数据表。随后将数据表录入计算机电子表格，计算砾石、砂与粉砂的重量占比。

1999年野外作业的更多详情可查阅：https://www.sandag.org/index.asp?publicationid=590&fuseaction=publications.detail

2008年野外作业方法：
2008年10月6日至7日，研究团队在长24英尺的铝壳船体“朱莉安号（Julie Ann）”上使用富格罗公司（Fugro Inc.）的地球物理系统开展地震反射勘探。Starfix Seis导航系统与配备集成Starfix差分接收机的天宝（Trimble）12通道GPS接收机相连。天宝接收机与Starfix差分基准站接收同一卫星组的测距信息，将差分修正值应用于DGPS（差分全球定位系统）的卫星数据后，可实时获得精度达±5英尺的船体位置，后处理精度则优于2.5英尺。经差分修正的天宝接收机位置数据将被传输至导航计算机。

本次地震反射系统采用机械式“电火花震源（boomer）”作为能量源，以及多通道Geo-Eel水听器阵列。研究团队使用应用声学工程AA300J便携式地震能量源为双体船拖挂的AA300电火花震源板供电。电火花震源板是一种机电换能器，由绝缘金属板与紧邻扁平缠绕电线圈的橡胶膜组成。AA300J能量源将短时高能脉冲释放至线圈中，产生的磁场排斥换能器内的金属板，金属板的运动通过橡胶膜传递至水体，产生宽带声学脉冲，该脉冲不会产生强烈的空化现象与振铃效应。应用声学AA300型海底拖曳式换能器（sub-bottom tow fish）被部署于船体右舷船尾进行拖挂，拖缆长度充足，确保换能器处于船体尾流之外且位于尾流下方。系统以8-10 Hz的脉冲频率触发，扫描频率范围为2-10 kHz，记录时长取决于水深。勘探航线上每行进100米，系统打印机将输出一次导航定位标记。所有导航信息与海底探测数据均被打上时间戳，并记录至硬盘中。

电火花震源产生的反射声脉冲由多通道Geo-Eel水听器阵列接收。Geo-Eel水听器阵列的水听器按指定间距封装于硅胶中。所有勘探测线均采用24通道（依点位调整）水听器阵列，其通道组间距为5.1英尺（1.56米）。水听器通道采集的原始数据以SEG-Y格式记录于Geometrics CNT记录系统中，用于后续后处理，处理后精度可达穿透深度的2.5%以内。

地震数据处理由马克·莱格（Legg Geophysical公司）完成，以下关于2008年数据处理步骤的说明均出自他之手。
"针对圣迭戈县近海采集的多道地震反射剖面的数据处理，遵循了石油勘探行业通用的子波（wavelet）处理方法基本原理。本次处理采用单板电火花震源与24道拖缆式水听器阵列配置，目标是在浅海底深度（<100米）范围内实现高分辨率成像。电火花震源的源能量约为500焦耳，工作带宽覆盖约100 Hz至1000 Hz以上。该24道微型拖缆的组间距为1.56米，近道与震源的偏移距为3.125米。微型拖缆的短偏移距与小组间距可实现海底与浅海底的良好成像，而远偏移距则可在浅水区一定程度上压制海底多次波。拖缆的短水听器组间距对避免数字采样数据的空间假频（spatial aliasing）至关重要，从而提升了地震数据的整体信噪比。

基础地震数据处理包括时域与空域滤波、反褶积（以获得更清晰、更一致的地震子波用于解释）、针对地下速度结构与震源-水听器偏移距变化导致的正常时差（normal moveout）校正、道叠加以提升信噪比，以及偏移（migration）处理以将反射层位恢复至正确的横向位置。下文将详细阐述这些处理步骤，以及微型拖缆数据处理流程的具体参数。

首先，需将原始SEG-Y格式数据加载至地震数据处理工作站。由于连续海洋地震反射勘探的几何形态简单且规则，沿测线震源至接收点的偏移距恒定，因此在数据加载阶段，这些偏移距值将被录入道头信息。为在数据叠加后恢复原始野外记录编号，我们采用了简单的炮点或测站编号方案：首个炮点编号为#101，后续炮点依采集几何形态依次编号。本项目的炮点间距标称值为0.78米，等于共中心点（Common Mid-Point, CMP）间距，且为水听器组间距（1.56米）的一半。该几何形态实现了2400%的地下覆盖次数，即标称叠加次数为24。测站编号方案与CMP编号方案匹配，因此在本几何形态下，处理过程中的工作炮点编号增量为1。

将SEG-Y数据加载至处理工作站后，处理流程的第一步为数据滤波与增益校正。首先需对数据道进行初步检查，以编辑或“剔除”坏道。这在海况较差的采集数据中尤为重要，当时的风浪与短周期涌浪会在拖缆中产生频繁的“噪声突跳”。我们采用抗假频滤波器作为带通滤波器，以避免奈奎斯特（Nyquist）频率（采样率为0.25 ms时为2000 Hz）以上时域出现假频，并去除拖缆的低频噪声，如膨涨波与水体波动。针对24道微型拖缆数据，我们采用参数为10/15-1500/1800 Hz的奥姆斯比（Ormsby）滤波器（梯形带通形状）。随后进行两步增益校正：首先采用指数为0.3的时变指数函数校正几何扩散衰减，其次采用合理信噪比窗口内数据的均方根（RMS）增益因子，均衡数据集中每个道的平均振幅，该窗口位于直达水波与强海底反射之下，且主要为背景噪声之上的区域。

尖峰反褶积（spiking deconvolution）用于将原始震源子波压缩为“理想”零相位子波，该子波在道道之间、记录与记录之间均保持一致。若带宽无限，该理想道将为冲激函数，即对应到达时间的尖峰。对于实际的带限数据，我们期望获得雷克（Ricker）子波或其他旁瓣最小的对称子波。针对本次高分辨率数据，尖峰反褶积采用20 ms的算子长度，设计窗口选取于海底反射之下信噪比良好的区域。反褶积后，再次应用参数为10/15-1500/1800 Hz的奥姆斯比滤波器，以消除高频噪声，维持宽带宽并去除部分低频拖缆噪声。

完成初始道处理与编辑步骤后，我们对选定的炮记录开展频率-波数（FK）分析，以设计FK滤波器抑制空间假频。空间域采样不足导致的假频常被忽视，其可能产生伪装为真实反射事件的假频高频噪声。对于海洋数据，水体声速约为1500 m/s，我们可预测在水体中沿拖缆传播的相干噪声发生假频的频率：对于1.56米间距的拖缆，空间奈奎斯特频率为480 Hz。该频率低于大部分震源能量，因此需在拖缆中进行阵列合成，以衰减在水体中水平传播的噪声。震源至拖缆的直接波传播与作业船及过往船只的螺旋桨噪声均会产生此类相干噪声。FK滤波器非常适合该场景，因其可保留更多垂直入射的高频反射信号。我们设计的FK滤波器用于衰减假定为水体中噪声的低速能量，保留来自地下反射的高速能量。本项目中，FK滤波通过衰减沿拖缆水平传播的高频震源噪声，拓展了有效频率范围。空间（FK）滤波后，再次采用奥姆斯比频域滤波器去除高频假频噪声，参数为25/50-640/960 Hz。

根据以往24道微型拖缆的作业经验，我们观察到浅海底的饱水沉积物速度与水体非常接近，有时因含气而更低。对于微型拖缆数据，采用5000 ft/s水体声速的常速粗叠加结果，与使用长偏移距48道拖缆数据导出的叠加速度得到的叠加结果几乎无差异。因此，所有数据均采用基于5000 ft/s常速声速的正常时差校正进行叠加。

将反褶积与滤波后的道按共中心点（CMP）顺序排序，进行空域滤波（FK滤波）、频域滤波（奥姆斯比滤波）、正常时差校正，然后叠加生成CMP叠加剖面。我们还对叠加输出的SEG-Y数据以及偏移前的绘图与SEG-Y输出数据，应用了校正拖缆与震源高程（深度）以及潮汐高程的静态校正。[注：叠加数据的PDF绘图未包含潮汐或拖缆与震源高程静态校正。]正常时差校正包含75%的拉伸静音（stretch mute），用于移除浅水区远偏移距水听器道中因过度拉伸而产生的近地表数据。该拉伸静音有助于保留海底与极浅地下区域的良好成像，这些区域会因长偏移距数据而被拉伸模糊。因此，叠加仅包含近偏移距道用于海底与极浅地下区域的成像，而所有道均可用于深部数据的成像。

对于恶劣天气下采集的噪声数据，我们采用90%分位数的“α截尾（Alpha Trim）”叠加，以抑制水波与缆绳抖动产生的噪声突跳。α截尾叠加仅对落在中位数90%分位数范围内的道值求和，从而忽略数据尖峰与异常值。一般而言，在对原始数据进行道编辑以去除最严重的噪声突跳后，采用1/N加权的全叠加可获得相似结果。对于微型拖缆，叠加的标称覆盖次数为2400%，即每个CMP道集的24道被叠加，输出的CMP道间距为0.78米。高覆盖次数有助于最小化数据中剩余噪声突跳的影响。采用90%α截尾叠加时，每个道的实际覆盖次数有所差异，但通常约为21-22次。导航与潮汐静态校正被写入叠加数据的道头信息，随后对数据进行滤波并以SEG-Y格式输出，用于加载至解释工作站。叠加后采用的奥姆斯比滤波器参数为：25/50-640/960 Hz（针对微型拖缆数据）。

叠加数据包含双曲线反射与绕射，需将其压缩至正确的空间位置，以进一步锐化地下反射层位与断层的成像。我们采用适用于所有倾角的简单频率-波数偏移（FK migration）。由于本次高分辨率数据涉及浅部、多为饱水的地下沉积物，偏移处理采用4800 ft/s的常速。偏移速度通常低于叠加速度，因为叠加速度受层倾角影响，而偏移则依赖于地质层的实际速度，即层间速度。偏移处理后对数据进行滤波，并以SEG-Y格式保存，用于加载至解释工作站。偏移后采用的奥姆斯比滤波器参数为：90/120-640/960 Hz（针对微型拖缆数据）。该滤波器的高频截止用于避免空间假频并去除图像中的高频噪声，低频截止用于提升微型拖缆数据的分辨率，并维持至少两个八度的带宽。我们在偏移处理前对叠加数据应用了“顶部静音”，以消除水体中的叠加噪声，这类噪声可能在FK偏移过程中“环绕”至偏移道的底部，产生偏移“微笑”伪影，掩盖主要反射信号并复杂化数据解释。除SEG-Y输出文件外，我们还以约10:1的垂直放大率制备了叠加与偏移数据的PDF绘图，对应地震速度为1500 m/sec。"

2008年10月29日至11月8日，研究团队使用M/V Supplier号作业船作为振动取芯作业平台，该船为长63英尺的钢制工作船，原为军用登陆艇。本次采用的271 B型阿尔卑斯气动振动取芯器可采集长20英尺的岩芯，与1999年调查使用的设备一致。

当振动取芯器达到约18英尺的穿透深度时，研究人员将其回收至支撑船，从取芯筒中取出包裹沉积物的塑料衬管。在船上，研究人员将塑料衬管按5英尺长度截断，以便于搬运并开展初步的地质编录。更详细的地质编录（需沿衬管长度方向切割，再剖开沉积物岩芯）则在陆上的岩土技术实验室完成。对回收沉积物的岩土编录采用统一土壤分类系统（Unified Soil Classification System）与孟塞尔（Munsell）土壤色卡对采集的沉积物进行描述。研究人员对剖开的沉积物样品拍摄彩色数码照片，颜色描述与照片均为沉积物湿态下的状态。

2008年野外作业的更多详情可查阅：https://www.sandag.org/uploads/projectid/projectid_358_12897.pdf