Acquisition

Acquisition in the field of geophysics is the generation and recording of seismic data. The generation of seismic waves (elastic waves) is carried out by the source. A source can be either natural such as: earthquakes, or controlled (induced) such as dynamite. For exploration purposes, controlled sources are used. An ideal seismic source is a source that is capable of generating a repeatable pulse of known frequency and other desired properties.

Introduction

Acquisition of seismic data is the first phase of the three main phases in the seismic industry, which are:

Seismic Acquisition
Seismic Processing
Seismic Interpretation

The basic principle of seismic data acquisition is that, man-made seismic waves are just sound waves (also called acoustic waves) with frequencies typically ranging from 5 HZ to just over 100 HZ. (The lowest sound frequency audible to human ear is about 30 HZ). As these sound waves leave the seismic source and travel downward int the earth, they encounter changes in the Earth’s geological layering, which cause echoes (or reflections) to travel upward to the surface. Receivers detect the echoes arriving at the surface and convert them into electric signals, which are then goes through the processing phase to produce images of the earth’s shallow structure. The resulted images can then be interpreted to determine what type of rocks they represent and whether those rocks might contain valuable resources^[1].

Seismic acquisition can take place either on the surface of the earth (land acquisition), or seafloor (marine acquisition).

A typical acquisition system consists of the following components:

A source (such as: a vibrator unit, dynamite shot, or an air gun), generates acoustic or elastic vibrations that travel into the Earth, and pass through strata with different seismic responses.
Detectors (also known as receivers, for land acquisition geophones are used as receivers, while for marine acquisitions hydrophones are used), record the seismic responses returning to the surface after passing through strata.
Cables
Recording system

Land Acquisition

Sources

Figure 1: A picture of a vibroseis truck (or "thumper") used by Dawson Geophysical. The plate in the middle is used to transmit energy into the Earth.

Land acquisition for reflection seismology uses an array of sources and receivers. The choices of which sources and receivers to use depend on the goals of the survey along with cost and environmental conditions.

Explosive Sources

Dynamite is a commonly used impulse source for exploration. Dynamite is preferred when the survey area is in harsh terrain that Vibroseis cannot traverse such as marshes, mountains, or environmentally sensitive areas. The dynamite must be buried prior to detonation to increase the amount of energy transmitted into the subsurface and for safety. Since the energy is produced instantly from the detonation, dynamite sources produce a wavelet that is roughly minimum phase. However, dynamite does have its drawbacks. Inconsistencies in the blasts along with variations in the burial depth and the local ground conditions will cause variations in the produced signal. Another impulse source used is modified shotguns called Betsy Guns. Betsy Guns are used for shallower and smaller surveys.

Vibratory Sources

Another commonly used source type for petroleum exploration are vibratory sources. Vibroseis trucks, as shown in figure 1, are used to transmit energy into the earth using a specified range of frequencies over a specified time. The trucks feature a heavy mass that vibrates vertically on a base plate to transfer energy into the subsurface. The range of frequencies (i.e. how fast the mass vibrates) and the length of time that the vibration occurs are unique for each survey. Since the signal inputted into the subsurface is known, it can be mathematically removed in processing to help remove noise and create a trace that resembles the true reflectivity of the survey area. In an effort to improve the post-correlation signal to noise ratio, an array of vibroseis trucks may be used, as the post-correlation signal to noise ratio is S:R = F(LN)^1/2, where F = the weight of the truck(force applied), L = the length of the sweep and N = the number of sweeps^[2]. Using an array of trucks will increase the force applied, therefore enhancing the Signal to noise Ratio (SNR). Generally, Vibroseis trucks generally only produce P-waves as they are designed to vibrate the mass vertically. Vibroseis trucks that produce S-waves exist, but they are rare and infrequently used. Vibroseis trucks are typically used when the acquisition region features no extreme topography, densely populated areas, and a relatively dry climate. Vibroseis trucks do not do well in wet climates, as they are very heavy and tend to get stuck and leave high amounts of property damage in wet terrain.

Weight Drops

Weight drops are another type of source. These are impulse sources which are generally used for shallow subsurface due to being much lower energy than dynamite or vibroseis. Examples of weight drops are sledgehammers hitting a metal plate on the ground and weights dropped heights of at least two meters. Accelerated weight drops (AWD) also fall in this category. AWD work by using a hydraulic system to lift a heavy steel hammer up, and a gas-charged piston forces the piston down. These have been proven as viable sources for VSP's and tool-orientation for micro-seismic surveys. ^[3]

File:Geophone.png

Figure 2: A geophone. The spike is designed to create a strong coupling between the geophone and the ground.

Receivers

For land surveys, geophones are used as the receivers. The instrument must have a solid connection to the ground, so they commonly feature spikes to help connect to the ground as seen in figure 2. Inside the geophone, a magnet is attached to the sides with a coil of wire suspended inside the magnet. As reflected waves return, the body of the geophone vibrates with the ground caused by the up-going energy. The coil vibrates at a different rate than the body, so the coil moves in and out of the magnetic field which induces an electrical current. The produced electrical current is recorded and is called the seismic trace, which is a representation of the subsurface's response to the inputted energy from the source. 3C geophones are designed to record 3 components of the wavefield: the P, SH, and SV waves. Another option to use for land receivers are land streamers that are towed behind a vibroseis truck ^[4].

Survey Design

The design of land surveys need to account for several factors:

1) Depth of target: The target horizon's approximate depth needs to be known along with the regional geological structure. Steeply dipping strata can be difficult to image with seismic, and there needs to be enough offset to image the target depth. In order to properly sample a waveform, there must be at least two samples per cycle for the highest frequency, or else aliasing occurs. a 3-D survey must be designed so that aliasing does not occur. As we usually do not know the subsurface geometry when designing a survey, a generous safety margin is usually built into the survey design (3 samples per wavelength is ideal).

Figure 3: Cartoon depiction that illustrates how the acquisition area must be greater than the size of the target of interest^[5].

2) Logistics: Permitting (getting permission from land owners), weather, and equipment availability will determine where and when a survey can be done.

3)Trace/Bin Spacing: The trace spacing in 2-D seismic data helps determine lateral resolution of the data. The trace spacing must be close enough to identify true reflection dip, or else spatial aliasing will occur. For 3-D data, the bin is a subdivision of the seismic survey that typically has equal dimensions. Bins are commonly assigned by Common Midpoints (CMP). The number of traces is described as the fold, and the traces within each bin are stacked to increase data quality. Interpreters only use full-fold data, as that data has been fully sampled by all of the traces for each shot. Figure 3 illustrates how in order to fully sample a subsurface target, the acquisition area will be much larger than the actual subsurface target. This is necessary in order to sample the target with far-offset traces. Bin size also helps determines lateral resolution. A rule of thumb is to have 3 to 4 bins for the smallest feature the survey is trying to image. However, the Fresnel zone also must be considered, and whichever is larger (bin or fresnel zone size) will determine the lateral resolution of the data.

4) Survey size: Ideally, the area of interest should be covered by full-fold data. In order to properly image the target of interest, the acquisition survey must be big enough to cover the entire target in the "Full Fold" region as pictured in Figure 3.^[5]

File:Layout of a Seismic Survey.png

Figure 4: An example showing the goal of seismic surveys. The goal is to hit the same midpoint at a range of offsets to properly sample the target horizon.

For 2-D seismic acquisition, the source and receivers are arranged in a line with the goal of sampling the same event with multiple shots at varying offset as shown in figure 4. These traces can then be displayed together in a CMP gather and then stacked to produce a single trace that enhances the strength of the signal.

In 3-D seismic surveys, the goal is the same is in 2-D surveys. Surveys aim to sample the same location at a range of offsets. Receivers are generally laid out in parallel lines (called the inline direction) with shot points perpendicular to the receiver directions (crossline direction). This creates a grid where shots are conducted along the crosslines, and as line of shots are completed, the receivers farthest behind the shot points is moved to the front of the survey.

Vertical Seismic Profiles

For Vertical Seismic Profiles (VSPs), receivers are lowered into a well via a wireline to selected depths as shown below in Figure 5. A source is placed close the wellhead, and several shots are performed. Generally 75 to 100 receivers are used with a spacing around 50 feet. One of the benefits of VSPs is that the full wavefied (both up and down going waves) are recorded and we know the exact depth of the geophones. Offset VSPs are designed similar to normal VSPs, however the source is set a certain distance away from the well, as shown in figure 6. "Walk-away" VSPs are also used to constrain seismic data and anisotropy. These are acquired by keeping the down-hole receivers at a constant depth while conducting shots at different distances from the well.

File:Vertical Seismic Proflie.jpg

Figure 5: A schematic showing the ray-paths of a VSP. Note we record both down and up-going waves.Spacing between the source and well has been exaggerated for clarity.

File:Offset VSP.jpg

Figure 6: An illustration of an offset VSP.

For further information, see the Land acquisition geometry page.

Downfalls of Land Acquisition

Terrain: Land acquisition can prove to be very dangerous due to terrain. In mountainous regions, helicopters must be used to transport crew and equipment. This results in dangerous working conditions and is costly for the acquisition company.
Permitting: Seismic data can only be acquired on land that the acquisition company is permitted to access. Landowners who elect not to allow access can result in areas where no seismic data is acquired.
Property damage: As stated above, vibroseis trucks can cause property and environmental damage which must be paid for by the acquisition company.
Dynamite: For dynamite-sourced surveys, drilling shot holes is costly and dynamite is dangerous to handle in general.The signal of dynamite can also be inconsistent due to variations in charge, hole depth, and what the shot hole is drilled in
Acquisition Footprint: Spatial variations/irregularities in the data that is not geological in origin, but effects of acquisition and/or processing^[6]. Data gaps due to permitting issues can lead to an irregular acquisition footprint, along with varying of any recording parameters during acquisition.

Marine Acquisition

Figure 7: A marine seismic vessel towing an array of streamers.^[7]

Marine Acquisition is accomplished by using large vessels outfitted with sources and streamers that are towed behind the ship (Figure 7). During marine acquisition, the vessels continuously sail from shot to shot, and no time is spent moving geophones as in land acquisition. This makes marine acquisition generally faster and less expensive than land acquisition.