Abstract:
A method is provided for analyzing received seismic signals which are received from a plurality of seismic sensors in response to operation of an acoustic source during a marine seismic survey. The seismic sensors are located at different offsets from the acoustic source. The method comprises the steps of selecting a time window within a particular seismic signal which frames a relatively well-defined event represented in the signal. The particular seismic signal is received from a particular seismic sensor. The method further comprises determining a receiver ghost notch frequency from an amplitude/frequency spectrum of the seismic signal in the window and deriving from the receiver ghost notch frequency an estimate of a height of a water column above the particular seismic sensor which generated said particular seismic signal.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method of analysing seismic signals and in particular to a method of analysing seismic signals adapted for use in connection with marine seismic data acquisition activities that provides for improved determination of local wave heights and acoustic sensor depths and allows “noise” in seismic data associated with changes in local wave heights and seismic sensor depths to be reduced during subsequent data processing. 
     Seismic data is collected to remotely sense subsurface geologic conditions, particularly in connection with the exploration for and production of hydrocarbons, such as oil and natural gas. To gather seismic data in a marine environment, acoustic sources, such as airguns, are used to produce an acoustic signal that is transmitted through the seawater and into the subsurface geologic formations. Changes in acoustic impedance at the sea bottom and between different geologic layers cause a portion of the acoustic energy to be reflected and returned toward the sea surface. These reflected signals are received by acoustic sensors and are processed to create images of the subsurface geology. 
     In a marine environment, these acoustic sensors (also called seismic sensors, often pressure sensors known as hydrophones) are typically contained within long tube-shaped streamers and are towed behind a seismic survey vessel. The streamers are often filled with kerosene or other buoyant materials that allow the sections of the streamers to achieve approximately neutral buoyancy. The streamers often have one or more internal stress members (such as steel cables) that provide substantial tensile strength and inhibit stretching of the streamer sections, while simultaneously allowing the streamer to be relatively flexible and able to be wound around a drum of a reasonable diameter on the seismic survey vessel. The depth (or “elevation”) a streamer is towed at is typically regulated by a deflector located at the end of the streamer nearest the seismic survey vessel (see, for instance, our U.S. Pat. No. 5,357,892) and by control devices called birds that are typically placed at regular intervals along the streamer&#39;s length (see, for instance, our published PCT International Application No. WO 98/28636). 
     The depths of the hydrophones in the streamer are typically monitored on the seismic survey vessel by depth sensors attached to the birds. Because the birds are widely spaced along the streamer (such as every 300 meters), compared to the significantly closer hydrophone spacing (such as a group of hydrophones every 12.5 meters), the depth of a particular acoustic sensor or a group of acoustic sensors must typically be approximated by interpolating from the depth values of the birds on either side of the sensor or sensor group. 
     This type of relatively crude depth determination system makes it difficult for a seismic survey vessel crew to determine when certain types of problems are occurring within the streamers. For instance, streamer sections are typically “balanced” until they are approximately neutrally buoyant. Due to temperature changes on the seismic survey vessel and in the sea water, balancing problems (excessive positive or negative buoyancy) sometimes occur. If the depth of each of the hydrophone in each section could be monitored, however, it may be possible to determine which sections are experiencing balancing problems and to correct these problems before they impact the quality of the seismic data acquired or cause towing problems. 
     Depth sensors on the birds typically sense the local ambient water pressure and convert this pressure reading into a depth value. The water pressure measured at the bird, however, incorporates two types of transient conditions that are constantly changing as the streamer is towed. The first transient condition is the local wave height, the local sea level immediately above the sensor minus the mean sea level. Changes in the local wave height are also referred to as waves. The second transient condition is the actual streamer elevation (or depth) measured with respect to mean sea level. Changes in the actual streamer elevation are typically due to forces such as positive or negative buoyancy in the streamer sections, wave-induced forces, currents, the deflector, the birds, etc. The water pressure at the bird is influenced by both of these transient conditions. To eliminate wave effects, the measured water pressure values are typically averaged or filtered over an extended period of time (such as between 10 and 100 seconds). While this averaging or filtering produces more accurate “average” depth values for the birds, it eliminates any possibility of using the measured depth values to compensate for transient conditions having a cycle period less than half the averaging period or filter length, such as waves. 
     Two types of “noise” are introduced into the data by the fluctuations in the streamer depth and the local wave height. A first type of noise is caused by ghost effects. Acoustic reflections from the sea surface above an acoustic sensor or an acoustic source will cause cancellation of the received acoustic signals at frequencies that are related to the depth of the sensor or source (i.e. the “ghost” effect). Ghosts are notches in the frequency spectrum that occur at frequencies F=n/T g , where n is an integer (0,1,2, . . . ) and the ghost period T g  is equal to twice the receiver (or source) depth H (distance to the sea surface) divided by the seawater acoustic transmission velocity. The depth H (and therefore the ghost notch frequency F) needs to be corrected for the angle of incidence (as will be discussed in more detail below). There are two ghosts, one introduced on the source side and one introduced on the receiver side. Variations in the ghost notch frequency occur when the depth of the receiver or source varies. These variations can be due to a change in the absolute elevation of the streamer or the source or due to changes in the wave height above the streamer or the source. 
     To compensate for this ghost effect, seismic sensors are typically towed at a depth where the first non-zero ghost notch frequency is outside the seismic spectrum (between approximately 5 Hz and approximately 80 Hz) where the vast majority of information regarding the geologic subsurface of interest is obtained during a seismic survey. A deconvolution procedure can be used to compensate for the frequency-dependent attenuation of the received seismic signals caused by the ghost effect (i.e. “de-ghosting” the data). In conventional seismic data processing procedures, however, this deconvolution procedure will assume that the seismic sensors are placed a constant distance beneath the sea surface. Any deviation in the position of the sensor from this assumed position will cause the de-ghosting procedure to operate to some degree improperly; certain frequencies will be over amplified and certain frequencies will remain under amplified. In that the depth values are averaged or otherwise filtered over an extended period of time to remove wave effects on the depth values, the depth values provided by conventional seismic data acquisition equipment cannot be used to provide customised or individualised de-ghosting of the seismic data to account for the actual (and changing) depth values of the sensors when they were receiving the seismic data of interest. 
     A second type of noise is due to changes in the absolute elevation of the streamer which causes unintended shifts in the arrival times of the acoustic signals received from the underlying seismic reflectors. As the vast majority of seismic data analysis involves combining together numerous seismic traces imaging the same subsurface position, these time shifts will cause a blurring of the seismic image of the reflectors. 
     While these two types of deviations do not introduce “noise” in its conventional sense (i.e. unwanted signals that interfere with or mask the desired signals), it will be readily understood that they inhibit proper seismic imaging of the subsurface and therefore constitute noise in its more general sense. For some types of seismic imaging, such as analysing time-lapsed images of producing hydrocarbon reservoirs, these effects may be sufficient to mask any change in the seismic response that could be expected to result from the withdrawal of reservoir fluids. A study conducted on behalf of the Applicant has concluded that if conventional seismic data processing schemes are utilised, rough sea effects from only a 2 meter significant wave height (SWH) sea can mask any changes in seismic response that could be expected to be associated with hydrocarbon production, at least for certain reservoir types. 
     In conventional marine seismic surveying, the only attempts made to compensate for changes in local sea height involve compensating for changes in mean sea level due to tidal effects. No attempt is made to correct the seismic data for wave effects or short cycle-time variations in the streamer depth values. While it is well known that the quality of seismic data will be seriously degraded if the seismic data is acquired during rough sea periods, no attempt is normally made to compensate for these type of transient conditions. When a seismic survey vessel crew or their client&#39;s onboard observer decides that the sea conditions are too rough or fail to meet the agreed upon contractual specifications, acquisition of seismic data by the seismic survey vessel is simply stopped. The client is simply forced to live with the fact that seismic data acquired during rougher sea conditions is noisier (i.e. of lower quality) than seismic data acquired during calmer sea conditions. 
     Seismic data acquisition contractors have a significant incentive to acquire seismic data under “questionable” weather conditions because they are not typically compensated for downtime resulting from bad weather and the amount of time spent down for bad weather can easily range between 10% and 50% of the entire mobilisation period. Some seismic data acquisition contractors are particularly aggressive about continuing seismic data acquisition activities in bad weather. This is particularly true when the seismic survey vessel is acquiring multi-client data. Multi-client data is typically acquired “on-spec” with the seismic contractor paying for the cost of the acquisition activities and then attempting to recoup these costs and make a profit by licensing access to the acquired seismic data. Some contractors apparently believe that the effects of bad weather can be removed (or at least masked) during subsequent data processing or that the clients may not realise how noisy the data actually is. This situation has been further complicated in the past because clients have lacked a method for independently determining what the sea state was when the seismic data was acquired. 
     It is therefore an object of the present invention to provide for an improved method of analysing seismic signals. 
     An advantage of the described embodiment of the present method is that it allows local wave heights and acoustic sensor elevations to be determined in connection with marine seismic data acquisition activities. 
     A further advantage of the described embodiment of the present invention is that it provides an objective method for determining the local wave heights directly from seismic data. 
     Another advantage of the described embodiment of the present invention is that the elevations of individual acoustic sensors or arrays of acoustic sensors may be determined in the absence of conventional water-pressure-based depth sensors. 
     An additional advantage of the described embodiment of the present invention is that “noise” introduced into the seismic data by changes in local wave heights and/or changes in seismic sensor elevations may be attenuated during subsequent data processing. 
     SUMMARY OF THE INVENTION 
     The present invention involves a method of analysing seismic signals acquired by a plurality of submerged seismic sensors in response to operation of an acoustic source during a marine seismic survey, the method comprising, for each of at least some of the signals, the steps of: selecting a time window within the signal which frames a relatively well-defined event represented in the signal; determining the receiver ghost notch frequency from the amplitude/frequency spectrum of the signal in said window; and deriving from said receiver ghost notch frequency an estimate of the height of the water column above the sensor which produced the signal. The method may further include the steps of: identifying changes in arrival times from seismic signals received by a plurality of submerged acoustic sensors located at different offsets from an acoustic source; determining time differences between the identified changes in arrival times and expected changes in arrival times associated with an assumed acoustic sensor depth profile; and converting the time differences into depth differences between the assumed acoustic sensor depth profile and the actual depth profile of said acoustic sensors. This method provides for improved determination of local wave heights and acoustic sensor elevations and allows “noise” in seismic data associated with changes in local wave heights and seismic sensor elevations to be attenuated during subsequent data processing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a seismic survey vessel conducting a marine seismic survey; 
     FIG. 2 is an enlarged view of a portion of the seismic streamer and the sea surface from FIG. 1; 
     FIG. 3 is a panel of seismic traces received from a single acoustic pulse; 
     FIG. 4 is a windowed portion of seismic traces from FIG. 3 after the sea bottom reflections have been aligned at approximately the same arrival time; 
     FIG. 5 is a plot showing amplitude versus frequency spectrums of the windowed seismic traces from FIG. 4; 
     FIG. 6 is a chart plotting waterbottom reflection arrival times versus offset distances and a travel time moveout curve; and 
     FIG. 7 is a plot showing seismic sensor depths and local sea heights determined in accordance with the present invention for a group of seismic traces shown in FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a schematic view of a seismic survey vessel and associated equipment conducting a marine seismic survey. A conventional marine seismic survey system  10  comprises a seismic survey vessel  12 , an acoustic source  14 , one or more streamers  16  (each containing a plurality of hydrophones  18 , a type of acoustic sensor), and recording equipment  20 . When acquiring seismic data using the seismic survey system  10 , the acoustic source  14  (typically one or more airguns) produces an acoustic pulse which is transmitted through the seawater  22  and is partially reflected by both the sea bottom  24  and by the interfaces  26  between various geologic layers where the acoustic impedances of the layers change. 
     When acquiring seismic data using this type of seismic survey system  10 , the most prominent reflection event recorded will typically be the direct waterbottom arrival  28 . The direct waterbottom arrival  28  can also be thought of as a first portion  28  of the acoustic pulse produced by acoustic source  14  that reflects directly off the sea bottom  24  and is received directly by the hydrophone  18 . A second portion  30  of the acoustic pulse reflects off the sea bottom  24 , reflects off the sea surface  32  above the hydrophone  18 , and is then received by the hydrophone. This second portion  30  (known as the receiver ghost signal) is out of phase with respect to the first portion  28  and will result in destructive interference of the received acoustic signal at the receiver ghost notch frequencies, as discussed above. A third portion  34  of the acoustic pulse reflects off the sea surface  32  immediately above the acoustic source  14  before reflecting off the sea bottom  24  and being received by the hydrophone  18 . This third portion  34  will similarly be out of phase with respect to the first portion  28  and will result in the destructive interference of the received acoustic signal at the source ghost notch frequencies. While the first portion  28 , second portion  30 , and third portion  34  are shown as raypaths, this is merely for convenience and it should be understood that acoustic signals spread in a generally spherical manner and that similar raypaths would exist between the acoustic source  14  and each of the hydrophones  18 . The seismic survey vessel  12  is also typically be equipped with a depth sounder  36  that allows the water depth beneath the vessel to be monitored. 
     FIG. 2 shows an enlarged portion of the seismic streamer  16  and the sea surface  32  from FIG. 1 at a particular instant in time. The sea surface  32  is overlain by a dashed mean sea level  38  reference line. The mean sea level  38  is typically compensated for tidal effects and will ideally represent the sea level that would exist if the seas in the area were completely calm. Sea height variation  40  quantifies the distance the sea surface  32 , directly above hydrophone  18 , is above or below this mean sea level  38 . Similarly, the streamer  16  and the hydrophones  18  are overlain by a dashed desired streamer depth  42  reference line. This desired streamer depth  42  typically represents the depth below the mean sea level  38  that the streamer  16  is intended to be towed at. Even if the deflector and birds are functioning properly, however it is understood that the streamer  16  will undulate to a certain degree and that the hydrophones  18  within the streamer will not be precisely located at the desired streamer depth  42 . Sensor depth variation  44  quantifies the distance the hydrophone  18  is above or below the desired streamer depth  42 . The degree of the sea height variation  40  and sensor depth variation  44  has been exaggerated in FIG. 2 for illustrative purposes. Also shown in FIG. 2 is an offset angle  46 , also referred to as theta, that represents the angle between the direct waterbottom arrival  28  raypath and a vertical raypath. This offset angle  46  is used later to describe calculations made in the preferred embodiment of the present invention. 
     FIG. 3 is a seismic trace panel  48  showing a number of seismic traces that have been received by the hydrophones  18  on a single streamer  16  from a single acoustic pulse produced by an acoustic source  14 . Those of ordinary skill in the art will understand that these seismic traces reflect changes in the water pressure amplitude measured by each of the hydrophones  18  over time. The first and most visible event in this seismic trace panel  48  is the direct waterbottom arrival  50 . In this case, the time required for an acoustic pulse produced by the acoustic source  14  to reach the sea bottom  24  and be reflected to the hydrophone  18  nearest the seismic survey vessel  12  is approximately 4.2 seconds (the water depth is about 3100 meters in this example). The time required for the direct waterbottom arrival portion of this same pulse to reach the hydrophone  18  farthest from the seismic survey vessel  12  is just over 5 seconds. It was noted that the sea state was about 2 meters significant wave height and the angle of incidence of the direct waterbottom arrival raypath at the far offsets was about 35 degrees when this particular seismic trace panel  48  was acquired. 
     FIG. 4 shows windowed portions  52  of the seismic traces from FIG. 3 after the direct waterbottom arrivals  50  have been moveout corrected so they align at approximately the same time and an identical  32  millisecond portion of each trace (including the direct waterbottom arrival  50 ) has been selected. No “stretching” of the windowed portion of the trace from FIG. 3 has taken place. The trace has not been normal moveout corrected to attempt to create an equivalent zero offset trace. The time window has merely been time shifted so the direct waterbottom arrival  50  appears in approximately the same place in the window for each of the traces. 
     It should be understood that the local sea height directly above the hydrophone  18  is constantly changing as the seismic data is being acquired. The time variant nature of this problem smears the ghost notch that would be obtained for the entire seismic trace. To address this, a short time window (preferably less than approximately 250 milliseconds) is selected to provide a “snap-shot” of the water column height in the vicinity of the acoustic sensor over a relatively short period of time. In the preferred embodiment, this is the period of time immediately before and immediately after the direct waterbottom arrival  50  is received by the hydrophone  18 . The use of these short time windows, however, can cause traditional wavelet extraction methods (which could be used for determining ghost notch frequencies) to fail, due (for example) to the failure of the stationary wavelet assumption. Amplitude versus frequency spectra are preferably used instead to determine the receiver ghost notch frequencies for each of the selected time windows. 
     FIG. 5 is a plot showing amplitude versus frequency spectra for a number of the windowed portions of the seismic traces from FIG. 4 (traces having offsets of 0 to 800 meters). FIG. 5 shows how the source notch frequencies  58  are virtually constant for each of the traces (because the seismic traces came from a single acoustic source pulse so the source notch frequencies will only vary by the cosine of the arrival incidence angle) while the receiver notch frequencies  56  vary significantly because the water column height above each of the hydrophones  18  in the streamer  16  is different, primarily due to wave effects. 
     If the source depth is close to the receiver depth, the two ghost notches will be close in frequency and it may be difficult to distinguish between them and to make accurate picks. This is most simply addressed by placing the source and receivers at different depths. The required depth separation is not simple to define as it depends on the notch variation, which is principally determined by the wave heights. If the source and receiver are separated in depth by at least half of the maximum peak-to-trough wave height (the significant wave height), then it is unlikely that the notches would ever be coincident. 
     Other methods for distinguishing between the source and receiver notches can also be used. The source notch frequency  58  typically varies less than the receiver notch frequency  56  because the airgun suspension system “rides” on the waves, keeping the source ghost period more consistent. Also, by working in the common-shot domain, the source notch frequency  58  will be constant (although it varies with the cosine of the incidence angle) for all receivers associated with the same shot. The apparent ghost notch period decreases with offset by the cosine of the angle of incidence from the vertical. This moveout curve needs to be removed from the notch picks before inversion to local sea surface height either through computation using the offsets and water depth, or by fitting a best-fit smooth curve through the picks (such as a cosine type curve or a second or third order polynomial curve). The offset will also affect the lateral position of the inverted sea surface points since the reflection points will no longer be vertically above the receivers. As discussed below, this lateral shift with offset is easily corrected. 
     As discussed above, inversion of the receiver ghost frequency to calculate the height of the water column above the acoustic sensor is quite straightforward. If the first non-zero receiver ghost notch frequency is selected, the height of the water column can be estimated as follows: 
     
       
           WCH =( ATV )/(2 *RGNF *COS( THETA )) 
       
     
     where: 
     WCH=Water Column Height 
     ATV=Acoustic Transmission Velocity 
     RGNF=First Non-Zero Receiver Ghost Notch Frequency 
     THETA=Arrival Incidence Angle 
     If, for instance, a hydrophone  18  having a 400 meter offset has an arrival incidence angle of 2 degrees (THETA=2 degrees, cosine of THETA 0.9994), the first non-zero receiver ghost notch frequency is found at 160 Hz (RGNF=160), and the acoustic transmission velocity of seawater is 1460 meters per second (ATV=1460), then the water column height above the hydrophone may be estimated by the following calculation: 
     
       
           WCH =(1460) I (2*160*0.9994)=4.6 meters 
       
     
     The approximate depth of the source and receiver, the water depth, and the offset distance will typically be known, allowing the THETA value to be calculated quite accurately. 
     While the above formula provides an estimate of the water column height above the hydrophone  18 . it more precisely estimates the water column height above a horizontal plane passing through the hydrophone for a point that is WCH*TAN(THETA) closer to the source than the hydrophone, because this is the lateral position at which the second portion  30  of the acoustic pulse is assumed to have reflected off the sea surface  32 . 
     The above method allows estimates of the water column height in the vicinity of a particular acoustic sensor to be determined from the determined receiver ghost notch frequency  56 . If these calculations are made for a number of different hydrophones  18  in a streamer  16 , and the assumption is made that the hydrophones  18  are horizontal, the local sea state may also be determined. A refinement of this method allows deviations between the actual positions of the hydrophones  18  and this horizontal alignment assumption to be determined as well. 
     If changes in arrival times can be identified for a group of acoustic sensors located at different offsets from an acoustic source, and if time differences can be determined between these identified time changes and the time changes that would be expected if the acoustic sensors were positioned at a particular depth profile, then it is possible to convert these determined time differences into depth differences between the assumed sensor profile and the actual depth profile. 
     FIG. 6 is a cross-plot  60  showing an expected waterbottom reflection travel time moveout curve  62  and actual waterbottom reflection arrival time picks  64 . The extent of the deviations between the arrival time picks  64  and the travel time moveout curve  62  have been greatly exaggerated in FIG. 6 to more clearly describe the process. The travel time moveout curve  62  associates expected arrival times for the direct waterbottom reflection  28  with offset distances (the distances between the acoustic source  14  and the hydrophones  18 ). The expected arrival times correspond to the travel times that would be expected if each of the sensors  18  were located precisely at the desired streamer depth  42 . 
     The arrival time picks  64  represent the actual direct waterbottom arrival time for the hydrophone  18  located at that particular offset and these time picks are made from the seismic traces shown in seismic trace panel  48 . While it would be possible to use other techniques to identify these reflector arrival times (or equivalently changes in reflector arrival times), such as by using correlation techniques to determine the arrival time shift between adjacent traces or windowed portions of adjacent traces, Applicants have found that good results can be obtained by precisely picking the wavelet onset time for the direct waterbottom arrival reflection. Preferably, the arrival time picks  64  should be made to the nearest 0.1 millisecond, which corresponds to an accuracy of approximately 15 centimeters. If the seismic signals are recorded at a sampling interval greater than 0.1 millisecond, it may be necessary to interpolate the recorded values to properly select the event arrival time with sufficient accuracy. Any swell noise in the received seismic signals should be removed, preferably with a minimum-phase low-cut filter. It is also preferable to pick as near to the wavelet onset as possible (first arrival, first peak), so that the varying ghost arrivals do not interfere with the time picks. If correlation techniques were used to determine the change in event arrival times, it would be preferable to severely limit the portions of the traces that are correlated, to avoid the source and/or receiver ghosts from interfering with the accurate determination of the event arrival time differences. 
     The travel time moveout curve  62  maps expected changes in arrival times with changing offsets for a given acoustic sensor depth profile. In this described embodiment, as will be the case for virtually all towed streamer-related applications of this method, the assumed acoustic sensor depth profile is horizontal, i.e. parallel to the desired streamer depth  42 . Estimations or assumptions regarding the source/receiver offset distances, the source depth, the average receiver depth, the reflection point depths, and/or the acoustic transmission velocity (or velocities) can be used to constrain the travel time moveout curve  62 . 
     In the most simplistic model, the streamer  16  is assumed to be horizontal, the sea bottom  24  is assumed to be horizontal, the seawater  22  is assumed to have a single acoustic transmission velocity, and therefore the expected waterbottom travel time moveout curve  62  is expected to be a simple hyperbola (i.e. the direct waterbottom arrival will show what geophysicists refer to as “normal moveout” or “hyperbolic moveout”). A hyperbola that best fits the actual waterbottom reflection arrival time picks  56  can be then be selected and used to determine the time differences between the expected reflector arrival times and the actual reflector arrival time (or, equivalently, between the expected changes and the actual changes in the reflector arrival time). In practice, a third order polynomial curve has typically been fitted to the arrival time picks  56  to produce the travel time moveout curve  62 . The third polynomial coefficient allows some degree of variation in the simplistic assumptions identified above. 
     More sophisticated assumptions can also be made regarding the appropriate travel time moveout curve  62 . The depth measurements from the depth sounder  36  can be used, for instance, to produce a profile of the sea bottom surface. This sea bottom profile and the designed layout of the acoustic source  14  and the hydrophones  18  can be used to produce an expected travel time moveout curve  62 . This model-based calculated travel time moveout curve  62  can further be fitted to the sea bottom arrival picks  62 . Physical parameters that effect the expected travel time moveout values, such as slowly changing sea bottom structures, variations in water velocity with depth, and directivity effects of the wavelet, may similarly be modelled to determine an appropriate travel time moveout curve  62 . It is also possible to identify these parameters by looking for correlations across the results obtained from different acoustic pulses. 
     The distance the hydrophone  18  is above or below the desired streamer depth  42  can be calculated using the following formula: 
     
       
           AASE=EASE +(( ARAT−ERAT )* ATV )/COS( THETA ) 
       
     
     where: 
     AASE=Actual Acoustic Sensor Elevation 
     EASE=Expected Acoustic Sensor Elevation 
     ARAT=Actual Reflection Arrival Time 
     ERAT=Expected Reflection Arrival Time 
     ATV=Acoustic Transmission Velocity 
     THETA=Arrival Incidence Angle 
     If, for instance, the hydrophone  18  at 2000 meter offset has an expected elevation of −5 meters (EASE −5), the expected waterbottom reflection arrival time from the travel time moveout curve  54  for 2000 meter offset is 4.4050 seconds (ERAT 4.4050), the actual waterbottom reflection arrival time identified from the seismic trace acquired by the hydrophone is 4.4046 seconds (ARAT 4.4046), the acoustic transmission velocity of seawater is 1460 meters per second (ATV=1460), and the arrival incidence angle is 20 degrees (THETA=20 degrees, and the cosine of THETA=0.9397), then the actual acoustic sensor elevation may be calculated as follows: 
       AASE =−5+((4.4046−4.4050)*1460)/0.9397=−5.6 meters 
     While any seismic event at a different depth than the source and receivers could be used in the method (such as a dominant seismic reflector in the geologic area of interest), the water-bottom reflection is generally the strongest reflection in the seismic record and it is often possible to accurately isolate this event from other interfering reflections. The sea bottom is also often relatively flat (or at least has a relatively uniform gradient over the distances of interest) and this greatly simplifies the process of selecting an appropriate travel time moveout curve  62 . The actual acoustic sensor elevation may then be used to “fine tune” the estimate of the local sea heights discussed previously by correcting the estimated water column height by the distance the sensor is above or below the desired streamer depth  42 . 
     FIG. 7 is a cross plot  66  showing local sea heights  68  and streamer depths  70  determined in accordance with the present invention for some of the seismic traces shown in FIG.  3 . These traces have offsets of 300 to 725 meters, with the bottom scale representing the relative distance from the sensor nearest the seismic survey vessel. This plot represents a “snap-shot” picture of the sea surface height above the sensors and the seismic sensor depths below mean sea level for a short time period around the time when the direct sea bottom arrival reached each of the hydrophones  18 . The time of the snap-shot values for the larger offsets is therefore a fraction of a second later than the time of the snap-shot values for the smaller offsets. Mean sea level  38  is assumed to be zero depth, and all depths are plotted relative to this level. Because the calculated local sea height  68  values have been adjusted for both the local water column heights and the deviations between the desired streamer depth and the actual streamer depth, mean sea level  38  may be determined by simply averaging the calculated sea surface heights over a relatively large range of offsets. A correction has been made for the lateral shift with offset (the WCH*TAN (THETA) correction mentioned above), although at these small arrival angles (&lt;13 degrees), this makes little difference. The intended streamer depth  42  was 5 meters, with the birds set to control the depth to plus or minus 1 meter, and it can be seen that the actual streamer depths  70  determined in accordance with the inventive method fall within these excursion limits. 
     While the inventive method has been described in connection with point receiver streamer acquisition equipment (i.e. the seismic signals received by each individual hydrophone is recorded) and better resolution may be obtained for this type of data, the method may also be used with conventional group formed data (i.e. the seismic signals received by a group of adjacent hydrophones are added together before being recorded). Under at least some conditions, when using single sensor data, local sea heights and streamer depths can be expected to be determined by the inventive method with a precision greater than plus or minus 50 centimeters. 
     The inventive method may be used to monitor local wave heights and seismic sensor elevations during marine seismic data acquisition activities. This may be useful, for instance, to monitor the sea state to ensure compliance with contractual obligations regarding weather conditions. It may also be useful for monitoring the impact that the sea state is having on in-sea equipment such as the streamers, the deflectors and the birds. The determined sensor depth values can also be used to control sensor positioning devices, such as the deflector and the birds. 
     This method may also be useful in subsequent seismic data processing applications. Seismic data acquired under non-ideal weather conditions may be reprocessed and the local wave heights and seismic sensor elevations may be determined for each seismic sensor (or seismic sensor group). This information can be used to evaluate the sea conditions that were present when the seismic data was acquired and how accurately the hydrophones were maintained at their desired depth. The seismic data can be further reprocessed using this information to better de-ghost the data (i.e. tune the deconvolution operator to account for the actual distance between the seismic sensor and the sea surface rather than the assumed distance between the seismic sensor and the sea surface) and to correct the reflector arrival times for the time shift caused by the seismic sensor being at the actual elevation rather than at the desired elevation. 
     Because the method can be performed using seismic data acquired using conventional seismic data acquisition techniques and equipment, the method can be used to significantly improve the imaging characteristics of seismic data acquired in non-optimal weather conditions. Most seismic surveys acquire a great deal of data over areas that, in the end, do not identify potential hydrocarbon prospects. In those areas that do show significant potential, however, it may be advantageous to reprocess the seismic data using the inventive method to “fine tune” the seismic image of the prospect under investigation. 
     While the embodiment of the present invention described above utilises the direct waterbottom arrival (and therefore determines the acoustic sensor elevations and local wave heights at the time the direct waterbottom arrival reaches each hydrophone), the invention is not restricted to the use of this arrival. In particular, in certain circumstances a water salinity interface reflection is produced, and this reflection can be used in place of the direct waterbottom arrival. Additionally, other seismic reflectors and refractors can be used with the inventive method. Although the arrival times for these other seismic reflectors will typically not be as easy to pick and the ghost notches will be less distinct than those associated with the direct waterbottom arrivals, if two or more time-lapsed local wave heights and acoustic sensor elevations can be determined for a particular seismic trace, this opens up the possibility of extrapolating the values to improve the image for a particular geologic target of interest. If we are interested, for instance, in a hydrocarbon reservoir located at a depth of 5,000 meters, and the acoustic sensor elevation and local wave height values for a particular trace determined from the direct sea bottom arrival are −5.6 meters and 1.2 meters respectively, and the acoustic sensor elevation and local wave height values for that trace when determined from a dominant geologic reflector located at 3,000 meters are −5.8 meters and 1.0 meters respectively, it may be desirable to time shift and de-ghost this trace using the extrapolated values of −5.9 meters and 0.9 meters respectively. In this way, it may be possible to more accurately image the geologic target of interest. 
     In some cases, particularly for relatively large offsets, the first recorded seismic event may be a refracted, rather than reflected, arrival. The refracted arrivals travel from the acoustic source to the sea bed, along the sea bed, and then up to the acoustic receiver. These refracted arrivals will demonstrate the same type of receiver ghost notch discussed with respect to the waterbottom arrival  50 , although the arrival angle, theta, will be constant. The travel time moveout curve  62  for refracted arrivals may also, as a first order approximation, be linear, rather than hyperbolic. By making relatively minor changes to the preferred embodiment method described above using waterbottom reflections, these refracted arrivals may also be used to determine local wave heights and seismic sensors elevations. Preferably the seismic reflector or refractor event used with the inventive method will be earlier in the seismic record than other types of arrivals to limit interference caused by “pseudo-ghosts” and other types of acoustic interference caused by alternative transmission modes. 
     While the described embodiment of the present invention utilises a conventional seismic source, such as an airgun, as the acoustic source and utilises a conventional seismic sensor, such as a hydrophone, as the acoustic sensor, the invention is not limited to the use of such devices. It is also possible to use one or more dedicated non-seismic acoustic sources, such as pingers or sparkers. Conventional airguns work well with the inventive method primarily because they typically produce a broadband acoustic signal. If dedicated acoustic sources are used with the method, it may be preferable to place them at the front, middle, and end of the streamers  16  because the method appears to work best when the offset angle  46  is approximately 45 degrees or less. 
     Similarly, it is possible to use dedicated non-seismic acoustic sensors that are intended to receive the direct sea bottom acoustic reflection arrival. Conventional hydrophones work particularly well with the inventive method because they are highly accurate and have good high frequency response. However, it may not be necessary to obtain the streamer depth and local wave height measurements with the same spatial sampling interval as the hydrophones  18  are placed in the streamer  16 . If dedicated sensors and processing equipment are used to determine the local wave height above the sensors and the elevation of the sensors, it may be preferable to space the dedicated sensors at a greater spatial sampling interval than the hydrophones  18  and to sample the reflected portion of the acoustic signal at a sampling interval significantly smaller than the 2 or 3 millisecond sampling interval used in conventional seismic data acquisition (such as every 0.1 millisecond). The seismic signals may similarly not be “seismic data” that is used to analyse the geologic subsurface. It may instead by acquired solely in connection with making the measurements described above. While the preferred embodiment of the invention is described using conventional hydrophones  18  (water pressure sensors), the method will work in a similar manner with geophones (particle velocity sensors), accelerometers, and other types of acoustic sensors. The receiver ghost notch frequency  56  is directly related to the difference in the arrival times of the direct waterbottom arrival  28  and the receiver ghost signal  30 . Alternative embodiments of the inventive method that utilise receiver ghost signal  30  travel time measurements to estimate the height of the water column may also be used. 
     While the described embodiment of the present invention is depicted in connection with conventional towed marine streamer seismic data acquisition, the method is not limited to this type of environment. It may be similarly used in ocean-bottom cable, vertically deployed seismic sensor groups, and other types of seismic data acquisition systems.