Inversion during seismic land acquisition

RELATED APPLICATIONS

The present application relates to, claims priority to, and incorporates fully herein by reference, earlier filed United States Provisional Patent Application No. 62/359720 that was filed on July 8, 2016.

TECHNICAL FIELD

The present application relates to seismic surveys and long offset sensing with implementation of land nodes.

BACKGROUND

The following description is provided to assist the understanding of one skilled in the art, and is not meant in any way to unduly limit any claims in or relating to this specification.

Seismic surveying involves inputting a signal (e.g., vibration or impulsive) into the Earth so that the signal reflects and reverberates upon encountering various features of the Earth' s formation. Sensors are used to detect the reflections and reverberations, and those are measured and become traces and/or data that is recorded. The data can be processed and otherwise manipulated and analyzed to provide information about the Earth's formation. That information can indicate a presence or lack thereof of various mineral deposits such as hydrocarbons, and other features that are of interest.

The inputted signal is often created by large machines called land vibrators. They are mobile vehicles that have a large plate that moves up and down and impacts the earth to crate vibratory signals that travel into the earth. Of course, other sources are possible, such as explosives.

Seismic data acquisition can be conducted on land by placing sensors on the ground and/or near the surface of the ground (even buried or partially buried), those sensors being either physically unconnected from one another (thus being nodal), or being cabled together (thus being cabled). Most often, for surveying large areas of land, cabled sensors are economical and are used, and this is particularly the case of wide open and clear areas such as those in the Middle East and similar areas. Of course, nodal sensors can be used entirely, or partially. Nodal sensors can communicate wirelessly with a central recording station to communicate quality control information and/or seismic information. Data can also be stored on the nodes for later removal. The survey areas can be different sizes, but are often in the magnitude of square kilometers and can contain hundreds of thousands of individual sensors.

Full waveform inversion (FWI) is a seismic inversion method that can be used to derive an Earth model from seismic data. FWI technology can enable inversion for compressional velocities (Vp) and some anisotropic parameters, such as Thompson's parameters (epsilon and delta). Long offset data (such as more than 15 km) or ultra-long offset data (such as more than 20 km) and very low frequencies (such as less than 3 Hz) can be used for FWI to generate useable velocity models.

Acquisition of long and ultra-long offset seismic data can be problematic on land because of the distance from the survey area that the sensors should be placed. For example, even if the survey area is free and clear, it is not guaranteed that the surrounding area kilometers away is free and clear in the same way.

With those issues in mind, the present specification presents representative embodiments that are disclosed herein addressing some of those issues.

SUMMARY

The following summary of various combinations of embodied features in the present specification is meant to help the understanding of one skilled in the art, and is not meant in any way to unduly limit the interpreted scope of claims recited herein or any other claims in related application.

The present specification includes a method of performing a land seismic survey including placing a plurality of seismic receivers within an area that defines the receiver patch, placing a plurality of sources in an area that defines the source patch, and placing additional receivers outside the receiver and source patches, at a distance that defines the required long offset. These receivers (nodes) placed at the required long offset from the source path are called long offset nodes (LON).

During a seismic survey, several receiver patches and source patches can be deployed to cover the survey area. The LONs will be placed around each source and receiver patch at the same distance to maintain the constant long offset during the survey. Small deviations can be accepted from the desired maximum offset to accommodate the obstacle and restricted areas.

The present specification also includes a long offset land seismic survey spread having a number of seismic sensors within an area thereby defining a sensor receiver patch, a number of long offset seismic sensors outside of the receiver patch thereby surrounding the receiver patch and defining a long offset sensor area, wherein the area between the long offset sensor area and the receiver patch define therebetween an area that is fee from seismic sensors and also that defines a long offset distance.

BRIEF DESCRIPTION OF THE DRAWINGS

The following brief description of the drawings is meant to help the understanding of one skilled in the art when reviewing the present specification and is not meant in any way to unduly limit or affect claims in the present specification or other subsequent related specifications.

Figure 1 is a top view schematic of a seismic sensor patch and source patch with LONs around the patches.

Figure 2 is a top view schematic of multiple source and receiver patches with LONs around each source and receiver patch.

Figure 3 A is a schematic view of a drone for delivery of sensors and use in connection with a LON seismic survey.

Figure 3B is a schematic view of a drone for delivery and use in connection with a LON seismic survey. DESCRIPTION

The following description is intended to help the understanding of one skilled in the art regarding various combinations of embodied features described herein the present specification, but is not intended in any way to unduly limit the claims in the present specification, or any other related application specifications.

Full Waveform Inversion (FWI) is a seismic inversion method used to derive an earth model from seismic data. FWI technology allows inverting for compressional velocities (Vp), shear wave velocities (Vs), density (rho) and some anisotropic parameters, for instance, Thompson's parameters (epsilon and delta). Long offset data (more than 10 km or ultra-long offset data (more than 15 km) can be used for FWI to generate accurate velocity models with less constrain on the initial velocity model and on the low frequency content. Acquisition of long offset land seismic data can be expensive, as it requires very large shot patches around the receiver patches. Per various embodiments in the present specification, land nodes are used that record seismic data during a traditional land acquisition. The land survey can be acquired with a cable system or with a node system. To differentiate between regular nodes that are part of the receiver spread and the nodes used for long offset acquisition, herein the nodes used for long offset acquisition are called long-offset nodes (LON) or sometimes long offset seismic sensors and/or receivers.

A number of LON (reduced compared to the sensors in the receiver patch) can be deployed adjacent to and/or around or surrounding the shot patch at desired minimum offset distances, as shown in Figure 1, and record the generated shots. After one shot patch is acquired, the LON may be picked up and moved to the location of a second shot patch and deployed at desired minimum offsets, as shown in Figure 2.

Figure 1 shows long offset nodes (LON) 13 deployed around the shot patch 12 at the required minimum offset 14. Inside the shot patch 12 is the receiver patch 11 that is defined by the seismic sensors (receivers) being placed in that area. In Figure 1, the areas are shown as being rectangular, but other shapes are possible such as squares, round, oval or other shapes. Figure 2 shows a schematic of four acquisitions as that described in Figure 1. The long offset nodes (LON) 13 are deployed around each shot patchl2 at the required minimum offset 14. Figure 2 illustrates four successive patterns of acquisition with shot patches 12, receiver patches 1 1, and the associated long offset nodes 13. In practice, one survey is completed, then the receiver patche 11, shot patche 12, and LON 13 are moved to an adjacent position from the prior acquisition and another acquisition is performed.

To determine the locations and the number of LON 13 needed to perform an acquisition, the following information or combinations thereof may be used or generated: generate a 3D velocity model and a density model using available information including existent seismic data, VSP and log data, information about the target horizons in the area (shallow and deep horizons), positioning information for the proposed land acquisition; 3D ray tracing information, illumination of the target horizons for different offset ranges, 3D finite difference modeling using all shots and optimum location of the LON 13 derived from ray tracing, use reciprocity in modeling to reduce the computational time for 3D finite difference modeling (common receiver gathers ), and FWI on common receiver gathers data and determine if the long and ultra-long offset provides the information required to derive an accurate velocity model.

The number of LONs should be large enough to assure that the long offset data could generate a good velocity model and a good subsurface image. The number of LONs is determined based on seismic simulation, as was described above.

Deployment of LON 13 is of concern. A scenario can be if the survey area is free and clear of obstructions and/or is easily accessible. However, the surrounding area may not share that trait. Such can be where a field is measured, but the surrounding area is forest or other difficult terain. Or, where the survey area is surrounded by suburban areas where laying cabled systems is not practical, and even walking or driving into those survey areas is not practical.

The present specification disclosues a combination of features where the LON (nodes that are placed outside the survey area in a long offset position) are delivered to the long offset locations by way of autonomous flying systems, sometimes referred to as drones. One such embodiment is where a drone can have propellors that lift the flyign device, and the seismic sensor is either carried by the fluing device, or is incorported thereinto. A usefull design for the vehicle is shown and discussed in co-owned patent application publication no. US 2014-0078865 entitled Seismic Acquisition System-Based Unmanned Airborne Vehicle, which is referenced here and hereby incorporated by reference in its entirety for providing support for disclosure in the present specification.

Per embodiments of the present application, the LON 13 nodes can be incorporated into or be unmanned airborne vehicles that travel from a starting location to their long offset positions so that the area near the long offset positions does not need to be traversed by human operators on foot or by vehicle. The unmanned airborne vehicles with the sensors land on the ground thereby placing the sensor on the ground, detect the signals, and then the sensors are moved from the long offset locations. The unmanned airborne vehicles can also drop the LON from the vehicle thereby impacting and coupling to the Earth's surface.

Referring to FIG. 3 A, the unmanned airborne vehicle 150 may contain multiple propellers 310 that are driven by corresponding engines 311 (electric motors, for example) of the vehicle 150 for purposes of controlling the vertical lift and/or lateral movement of the vehicle 150. In general, the unmanned airborne vehicle 150 may include a body 307 and extensions 308 from the body 307 to position the engines 311 and propellers 310, as shown in FIG. 3 A. The unmanned airborne vehicle 150 may further include control electronics 320, which may, for example, autonomously control guiding the vehicle 150 along a predetermined flight path; harvesting seismic sensor data; acquiring seismic sensor data; wireless communicating of data to and from the vehicle 150, such as wireless communication to and from the vehicle 150 using Wi-Fi communication; controlling video or still image capture using high definition (HD) cameras 340 (two HD cameras 340-1 and 340-2, for example) and their associated motors; and so forth. As depicted in FIG. 3 A, the control electronics 230 may be coupled to a GNSS antenna 330 for purposes of receiving GNSS signal that are encoded with data indicative of the vehicle's location as well as a Wi- Fi antenna 332 for Wi-Fi communication.

In accordance with example implementations, the unmanned airborne vehicle 150 may be a vehicle similar to the AR Drone, which is available from Parrot™. The unmanned airborne vehicle may take on other forms, in accordance with further implementations. For example, FIG. 3B depicts an unmanned airborne vehicle 360, which achieves its vertical lift using buoyancy achieved from a lighter than air gas that is stored inside a body 362 of the vehicle 362. In this regard, the body 362 may contain a bladder or balloon that is filled with helium, in accordance with an example implementation. In general, lateral maneuvering of the unmanned airborne vehicle 360 as well as controlled vertical positioning may be achieved using, for example, propeller-based thrusters 364 (distributed below and on both sides of the body 362, for example) and wings/rudders (not shown), which are controlled by control electronics 370 of the vehicle 360. Similar to the unmanned airborne vehicle 150 of FIG. 3 A, control electronics 370 may control operation of the unmanned airborne vehicle 360 and its communication. Among its other features, as depicted in FIG. 3B, the unmanned airborne vehicle 360 may contain a GNSS antenna 380, as well as a Wi-Fi antenna 382.

The preceding description is covers a non-exhaustive number of combinations of embodiments intended to be within the scope of the present specificaiton, and with that in mind, is not meant to unduly limit the scope of claims in this specificiaton, or other subsequent and related claims in specifications.